Fixed obscure-JPEG post
This commit is contained in:
Chris Hodapp
parent
b44b667dd3
commit
fdb046baf6
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images/obscure_jpeg_features/20100713-0107-interleave.jpg
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images/obscure_jpeg_features/20100713-0107-interleave.jpg
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images/obscure_jpeg_features/jpeg_split.c
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images/obscure_jpeg_features/jpeg_split.c
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// jpeg_split.c: Write each scan from a multi-scan/progressive JPEG.
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// This is based loosely on example.c from libjpeg, and should require only
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// libjpeg as a dependency (e.g. gcc -ljpeg -o jpeg_split.o jpeg_split.c).
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#include <stdio.h>
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#include <jerror.h>
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#include "jpeglib.h"
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#include <setjmp.h>
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#include <string.h>
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void read_scan(struct jpeg_decompress_struct * cinfo,
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JSAMPARRAY buffer,
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char * base_output);
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int read_JPEG_file (char * filename, int scanNumber, char * base_output);
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int main(int argc, char **argv) {
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if (argc < 3) {
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printf("Usage: %s <Input JPEG> <Output base name>\n", argv[0]);
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printf("This reads in the progressive/multi-scan JPEG given and writes out each scan\n");
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printf("to a separate PPM file, named with the scan number.\n");
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return 1;
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}
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char * fname = argv[1];
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char * out_base = argv[2];
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read_JPEG_file(fname, 1, out_base);
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return 0;
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}
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struct error_mgr {
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struct jpeg_error_mgr pub;
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jmp_buf setjmp_buffer;
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};
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METHODDEF(void) error_exit (j_common_ptr cinfo) {
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struct error_mgr * err = (struct error_mgr *) cinfo->err;
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(*cinfo->err->output_message) (cinfo);
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longjmp(err->setjmp_buffer, 1);
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}
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int read_JPEG_file (char * filename, int scanNumber, char * base_output) {
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struct jpeg_decompress_struct cinfo;
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struct error_mgr jerr;
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FILE * infile; /* source file */
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JSAMPARRAY buffer; /* Output row buffer */
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int row_stride; /* physical row width in output buffer */
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if ((infile = fopen(filename, "rb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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return 0;
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}
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// Set up the normal JPEG error routines, then override error_exit.
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cinfo.err = jpeg_std_error(&jerr.pub);
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jerr.pub.error_exit = error_exit;
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// Establish the setjmp return context for error_exit to use:
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if (setjmp(jerr.setjmp_buffer)) {
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jpeg_destroy_decompress(&cinfo);
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fclose(infile);
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return 0;
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}
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jpeg_create_decompress(&cinfo);
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jpeg_stdio_src(&cinfo, infile);
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(void) jpeg_read_header(&cinfo, TRUE);
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// Set some decompression parameters
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// Incremental reading requires this flag:
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cinfo.buffered_image = TRUE;
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// To perform fast scaling in the output, set these:
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cinfo.scale_num = 1;
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cinfo.scale_denom = 1;
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// Decompression begins...
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(void) jpeg_start_decompress(&cinfo);
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printf("JPEG is %s-scan\n", jpeg_has_multiple_scans(&cinfo) ? "multi" : "single");
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printf("Outputting %ix%i\n", cinfo.output_width, cinfo.output_height);
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// row_stride = JSAMPLEs per row in output buffer
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row_stride = cinfo.output_width * cinfo.output_components;
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// Make a one-row-high sample array that will go away when done with image
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buffer = (*cinfo.mem->alloc_sarray)
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((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
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// Start actually handling image data!
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while(!jpeg_input_complete(&cinfo)) {
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read_scan(&cinfo, buffer, base_output);
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}
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// Clean up.
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(void) jpeg_finish_decompress(&cinfo);
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jpeg_destroy_decompress(&cinfo);
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fclose(infile);
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if (jerr.pub.num_warnings) {
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printf("libjpeg indicates %i warnings\n", jerr.pub.num_warnings);
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}
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}
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void read_scan(struct jpeg_decompress_struct * cinfo,
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JSAMPARRAY buffer,
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char * base_output)
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{
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char out_name[1024];
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FILE * outfile = NULL;
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int scan_num = 0;
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scan_num = cinfo->input_scan_number;
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jpeg_start_output(cinfo, scan_num);
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// Read up to the next scan.
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int status;
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do {
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status = jpeg_consume_input(cinfo);
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} while (status != JPEG_REACHED_SOS && status != JPEG_REACHED_EOI);
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// Construct a filename & write PPM image header.
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snprintf(out_name, 1024, "%s%i.ppm", base_output, scan_num);
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if ((outfile = fopen(out_name, "wb")) == NULL) {
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fprintf(stderr, "Can't open %s for writing!\n", out_name);
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return;
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}
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fprintf(outfile, "P6\n%d %d\n255\n", cinfo->output_width, cinfo->output_height);
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// Read each scanline into 'buffer' and write it to the PPM.
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// (Note that libjpeg updates cinfo->output_scanline automatically)
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while (cinfo->output_scanline < cinfo->output_height) {
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jpeg_read_scanlines(cinfo, buffer, 1);
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fwrite(buffer[0], cinfo->output_components, cinfo->output_width, outfile);
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}
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jpeg_finish_output(cinfo);
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fclose(outfile);
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}
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211
images/obscure_jpeg_features/libjpeg-wizard.txt
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images/obscure_jpeg_features/libjpeg-wizard.txt
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Advanced usage instructions for the Independent JPEG Group's JPEG software
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==========================================================================
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This file describes cjpeg's "switches for wizards".
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The "wizard" switches are intended for experimentation with JPEG by persons
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who are reasonably knowledgeable about the JPEG standard. If you don't know
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what you are doing, DON'T USE THESE SWITCHES. You'll likely produce files
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with worse image quality and/or poorer compression than you'd get from the
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default settings. Furthermore, these switches must be used with caution
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when making files intended for general use, because not all JPEG decoders
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will support unusual JPEG parameter settings.
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Quantization Table Adjustment
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-----------------------------
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Ordinarily, cjpeg starts with a default set of tables (the same ones given
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as examples in the JPEG standard) and scales them up or down according to
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the -quality setting. The details of the scaling algorithm can be found in
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jcparam.c. At very low quality settings, some quantization table entries
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can get scaled up to values exceeding 255. Although 2-byte quantization
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values are supported by the IJG software, this feature is not in baseline
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JPEG and is not supported by all implementations. If you need to ensure
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wide compatibility of low-quality files, you can constrain the scaled
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quantization values to no more than 255 by giving the -baseline switch.
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Note that use of -baseline will result in poorer quality for the same file
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size, since more bits than necessary are expended on higher AC coefficients.
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You can substitute a different set of quantization values by using the
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-qtables switch:
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-qtables file Use the quantization tables given in the named file.
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The specified file should be a text file containing decimal quantization
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values. The file should contain one to four tables, each of 64 elements.
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The tables are implicitly numbered 0,1,etc. in order of appearance. Table
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entries appear in normal array order (NOT in the zigzag order in which they
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will be stored in the JPEG file).
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Quantization table files are free format, in that arbitrary whitespace can
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appear between numbers. Also, comments can be included: a comment starts
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with '#' and extends to the end of the line. Here is an example file that
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duplicates the default quantization tables:
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# Quantization tables given in JPEG spec, section K.1
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# This is table 0 (the luminance table):
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16 11 10 16 24 40 51 61
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12 12 14 19 26 58 60 55
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14 13 16 24 40 57 69 56
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14 17 22 29 51 87 80 62
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18 22 37 56 68 109 103 77
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24 35 55 64 81 104 113 92
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49 64 78 87 103 121 120 101
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72 92 95 98 112 100 103 99
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# This is table 1 (the chrominance table):
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17 18 24 47 99 99 99 99
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18 21 26 66 99 99 99 99
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24 26 56 99 99 99 99 99
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47 66 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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If the -qtables switch is used without -quality, then the specified tables
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are used exactly as-is. If both -qtables and -quality are used, then the
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tables taken from the file are scaled in the same fashion that the default
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tables would be scaled for that quality setting. If -baseline appears, then
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the quantization values are constrained to the range 1-255.
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By default, cjpeg will use quantization table 0 for luminance components and
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table 1 for chrominance components. To override this choice, use the -qslots
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switch:
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-qslots N[,...] Select which quantization table to use for
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each color component.
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The -qslots switch specifies a quantization table number for each color
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component, in the order in which the components appear in the JPEG SOF marker.
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For example, to create a separate table for each of Y,Cb,Cr, you could
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provide a -qtables file that defines three quantization tables and say
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"-qslots 0,1,2". If -qslots gives fewer table numbers than there are color
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components, then the last table number is repeated as necessary.
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Sampling Factor Adjustment
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--------------------------
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By default, cjpeg uses 2:1 horizontal and vertical downsampling when
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compressing YCbCr data, and no downsampling for all other color spaces.
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You can override this default with the -sample switch:
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-sample HxV[,...] Set JPEG sampling factors for each color
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component.
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The -sample switch specifies the JPEG sampling factors for each color
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component, in the order in which they appear in the JPEG SOF marker.
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If you specify fewer HxV pairs than there are components, the remaining
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components are set to 1x1 sampling. For example, the default YCbCr setting
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is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
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"-sample 2x2".
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There are still some JPEG decoders in existence that support only 2x1
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sampling (also called 4:2:2 sampling). Compatibility with such decoders can
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be achieved by specifying "-sample 2x1". This is not recommended unless
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really necessary, since it increases file size and encoding/decoding time
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with very little quality gain.
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Multiple Scan / Progression Control
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-----------------------------------
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By default, cjpeg emits a single-scan sequential JPEG file. The
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-progressive switch generates a progressive JPEG file using a default series
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of progression parameters. You can create multiple-scan sequential JPEG
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files or progressive JPEG files with custom progression parameters by using
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the -scans switch:
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-scans file Use the scan sequence given in the named file.
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The specified file should be a text file containing a "scan script".
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The script specifies the contents and ordering of the scans to be emitted.
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Each entry in the script defines one scan. A scan definition specifies
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the components to be included in the scan, and for progressive JPEG it also
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specifies the progression parameters Ss,Se,Ah,Al for the scan. Scan
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definitions are separated by semicolons (';'). A semicolon after the last
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scan definition is optional.
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Each scan definition contains one to four component indexes, optionally
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followed by a colon (':') and the four progressive-JPEG parameters. The
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component indexes denote which color component(s) are to be transmitted in
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the scan. Components are numbered in the order in which they appear in the
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JPEG SOF marker, with the first component being numbered 0. (Note that these
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indexes are not the "component ID" codes assigned to the components, just
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positional indexes.)
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The progression parameters for each scan are:
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Ss Zigzag index of first coefficient included in scan
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Se Zigzag index of last coefficient included in scan
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Ah Zero for first scan of a coefficient, else Al of prior scan
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Al Successive approximation low bit position for scan
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If the progression parameters are omitted, the values 0,63,0,0 are used,
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producing a sequential JPEG file. cjpeg automatically determines whether
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the script represents a progressive or sequential file, by observing whether
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Ss and Se values other than 0 and 63 appear. (The -progressive switch is
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not needed to specify this; in fact, it is ignored when -scans appears.)
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The scan script must meet the JPEG restrictions on progression sequences.
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(cjpeg checks that the spec's requirements are obeyed.)
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Scan script files are free format, in that arbitrary whitespace can appear
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between numbers and around punctuation. Also, comments can be included: a
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comment starts with '#' and extends to the end of the line. For additional
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legibility, commas or dashes can be placed between values. (Actually, any
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single punctuation character other than ':' or ';' can be inserted.) For
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example, the following two scan definitions are equivalent:
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0 1 2: 0 63 0 0;
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0,1,2 : 0-63, 0,0 ;
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Here is an example of a scan script that generates a partially interleaved
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sequential JPEG file:
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0; # Y only in first scan
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1 2; # Cb and Cr in second scan
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Here is an example of a progressive scan script using only spectral selection
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(no successive approximation):
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# Interleaved DC scan for Y,Cb,Cr:
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0,1,2: 0-0, 0, 0 ;
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# AC scans:
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0: 1-2, 0, 0 ; # First two Y AC coefficients
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0: 3-5, 0, 0 ; # Three more
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1: 1-63, 0, 0 ; # All AC coefficients for Cb
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2: 1-63, 0, 0 ; # All AC coefficients for Cr
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0: 6-9, 0, 0 ; # More Y coefficients
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0: 10-63, 0, 0 ; # Remaining Y coefficients
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Here is an example of a successive-approximation script. This is equivalent
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to the default script used by "cjpeg -progressive" for YCbCr images:
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# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
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0,1,2: 0-0, 0, 1 ;
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# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
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0: 1-5, 0, 2 ;
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# Send all Cr,Cb AC coefficients, minus lowest bit:
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# (chroma data is usually too small to be worth subdividing further;
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# but note we send Cr first since eye is least sensitive to Cb)
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2: 1-63, 0, 1 ;
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1: 1-63, 0, 1 ;
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# Send remaining Y AC coefficients, minus 2 lowest bits:
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0: 6-63, 0, 2 ;
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# Send next-to-lowest bit of all Y AC coefficients:
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0: 1-63, 2, 1 ;
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# At this point we've sent all but the lowest bit of all coefficients.
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# Send lowest bit of DC coefficients
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0,1,2: 0-0, 1, 0 ;
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# Send lowest bit of AC coefficients
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2: 1-63, 1, 0 ;
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1: 1-63, 1, 0 ;
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# Y AC lowest bit scan is last; it's usually the largest scan
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0: 1-63, 1, 0 ;
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It may be worth pointing out that this script is tuned for quality settings
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of around 50 to 75. For lower quality settings, you'd probably want to use
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a script with fewer stages of successive approximation (otherwise the
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initial scans will be really bad). For higher quality settings, you might
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want to use more stages of successive approximation (so that the initial
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scans are not too large).
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@ -1,58 +1,126 @@
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---
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layout: post
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title: Obscure features of JPEG
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tags: image compression, images, jpeg, Technobabble
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status: publish
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type: post
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published: true
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author: Chris Hodapp
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date: November 24, 2011
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---
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*(This is a modified version of what I wrote up at work when I saw that progressive JPEGs could be nearly a drop-in replacement that offered some additional functionality and ran some tests on this.)*
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*(This is a modified version of what I wrote up at work when I saw
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that progressive JPEGs could be nearly a drop-in replacement that
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offered some additional functionality and ran some tests on this.)*
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Introduction
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============
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The long-established JPEG standard contains a considerable number of features that are seldom-used and sometimes virtually unknown. This all is in spite of the widespread use of JPEG and the fact that every JPEG decoder I tested was compatible with all of the features I will discuss, probably because [IJG libjpeg](http://www.ijg.org/) (or [this](http://www.freedesktop.org/wiki/Software/libjpeg)) runs basically everywhere.
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The long-established JPEG standard contains a considerable number of
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features that are seldom-used and sometimes virtually unknown. This
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all is in spite of the widespread use of JPEG and the fact that every
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JPEG decoder I tested was compatible with all of the features I will
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discuss, probably because [IJG libjpeg](http://www.ijg.org/) (or
|
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[this](http://www.freedesktop.org/wiki/Software/libjpeg)) runs
|
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basically everywhere.
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Progressive JPEG
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================
|
||||
One of the better-known features, though still obscure, is that of progressive JPEGs. Progressive JPEGs contain the data in a different order than more standard (sequential) JPEGs, enabling the JPEG decoder to produce a full-sized image from just the beginning portion of a file (at a reduced detail level) and then refine those details as more of the file is available.
|
||||
|
||||
This was originally made for web usage over slow connections. While it is rarely-used, most modern browsers support this incremental display and refinement of the image, and even those applications that do not attempt this support still are able to read the full image.
|
||||
One of the better-known features, though still obscure, is that of
|
||||
progressive JPEGs. Progressive JPEGs contain the data in a different
|
||||
order than more standard (sequential) JPEGs, enabling the JPEG decoder
|
||||
to produce a full-sized image from just the beginning portion of a
|
||||
file (at a reduced detail level) and then refine those details as more
|
||||
of the file is available.
|
||||
|
||||
Interestingly, since the only real difference between a progressive JPEG and a sequential one is that the coefficients come in a different order, the conversion between progressive and sequential is lossless. Various lossless compression steps are applied to these coefficients and as this reordering may permit a more efficient encoding, a progressive JPEG often is smaller than a sequential JPEG expressing an identical image.
|
||||
This was originally made for web usage over slow connections. While it
|
||||
is rarely-used, most modern browsers support this incremental display
|
||||
and refinement of the image, and even those applications that do not
|
||||
attempt this support still are able to read the full image.
|
||||
|
||||
Interestingly, since the only real difference between a progressive
|
||||
JPEG and a sequential one is that the coefficients come in a different
|
||||
order, the conversion between progressive and sequential is
|
||||
lossless. Various lossless compression steps are applied to these
|
||||
coefficients and as this reordering may permit a more efficient
|
||||
encoding, a progressive JPEG often is smaller than a sequential JPEG
|
||||
expressing an identical image.
|
||||
|
||||
One command I've used pretty frequently before posting a large photo online is:
|
||||
|
||||
```bash
|
||||
jpegtran -optimize -progressive -copy all input.jpg > output.jpg
|
||||
```
|
||||
|
||||
This losslessly converts *input.jpg* to a progressive version and optimizes it as well. (*jpegtran* can do some other things losslessly as well - flipping, cropping, rotating, transposing, converting to greyscale.)
|
||||
This losslessly converts *input.jpg* to a progressive version and
|
||||
optimizes it as well. (*jpegtran* can do some other things losslessly
|
||||
as well - flipping, cropping, rotating, transposing, converting to
|
||||
greyscale.)
|
||||
|
||||
Multi-scan JPEG
|
||||
===============
|
||||
More obscure still is that progressive JPEG is a particular case of something more general: a **multi-scan JPEG**.
|
||||
|
||||
Standard JPEGs are single-scan sequential: All of the data is stored top-to-bottom, with all of the color components and coefficients together and in full. This includes, per **MCU** (minimum coded unit, an 8x8 pixel square or some small multiple of it), 64 coefficients each for each one of the 3 color components (typically Y,Cb,Cr). The coefficients are from an 8x8 DCT transform matrix, but they are stored in a zigzag order that preserves locality with regard to spatial frequency as this permits more efficient encoding. The first coefficient (0) is referred to as the DC coefficient; the others (1-63) are AC coefficients.
|
||||
More obscure still is that progressive JPEG is a particular case of
|
||||
something more general: a **multi-scan JPEG**.
|
||||
|
||||
Standard JPEGs are single-scan sequential: All of the data is stored
|
||||
top-to-bottom, with all of the color components and coefficients
|
||||
together and in full. This includes, per **MCU** (minimum coded unit,
|
||||
an 8x8 pixel square or some small multiple of it), 64 coefficients
|
||||
each for each one of the 3 color components (typically Y,Cb,Cr). The
|
||||
coefficients are from an 8x8 DCT transform matrix, but they are stored
|
||||
in a zigzag order that preserves locality with regard to spatial
|
||||
frequency as this permits more efficient encoding. The first
|
||||
coefficient (0) is referred to as the DC coefficient; the others
|
||||
(1-63) are AC coefficients.
|
||||
|
||||
Multi-scan JPEG permits this information to be packed in a fairly
|
||||
arbitrary way (though with some restrictions). While information is
|
||||
still stored top-to-bottom, it permits for only some of the data in
|
||||
each MCU to be given, with the intention being that later scans will
|
||||
provide other parts of this data (hence the name multi-scan). More
|
||||
specifically:
|
||||
|
||||
Multi-scan JPEG permits this information to be packed in a fairly arbitrary way (though with some restrictions). While information is still stored top-to-bottom, it permits for only some of the data in each MCU to be given, with the intention being that later scans will provide other parts of this data (hence the name multi-scan). More specifically:
|
||||
* The three color components (Y for grayscale, and Cb/Cr for color) may be split up between scans.
|
||||
* The 64 coefficients in each component may be split up. *(Two restrictions apply here for any given scan: the DC coefficient must always precede the AC coefficients, and if only AC coefficients are sent, then they may only be for one single color component.)*
|
||||
* Some bits of the coefficients may be split up. *(This, too, is subject to a restriction, not to a given scan but to the entire image: You must specify some of the DC bits. AC bits are all optional. Information on how many bits are actually used here is almost nonexistent.)*
|
||||
* The 64 coefficients in each component may be split up. *(Two
|
||||
restrictions apply here for any given scan: the DC coefficient must
|
||||
always precede the AC coefficients, and if only AC coefficients are
|
||||
sent, then they may only be for one single color component.)*
|
||||
* Some bits of the coefficients may be split up. *(This, too, is
|
||||
subject to a restriction, not to a given scan but to the entire
|
||||
image: You must specify some of the DC bits. AC bits are all
|
||||
optional. Information on how many bits are actually used here is
|
||||
almost nonexistent.)*
|
||||
|
||||
In other words:
|
||||
* You may leave color information out to be added later.
|
||||
* You may let spatial detail be only a low-frequency approximation to be refined later with higher-frequency coefficients. (As far as I can tell, you cannot consistently reduce grayscale detail beyond the 8x8 pixel MCU while still recovering that detail in later scans.)
|
||||
* You may leave grayscale and color values at a lower precision (i.e. coarsely quantized) to have more precision added later.
|
||||
* You may do all of the above in almost any order and almost any number of steps.
|
||||
|
||||
Your libjpeg distribution probably contains something called **wizard.txt** someplace (say, /usr/share/docs/libjpeg8a or /usr/share/doc/libjpeg-progs); I don't know if an online copy is readily available, but mine is [here]({{ site.baseurl }}/assets/obscure_jpeg_features/libjpeg-wizard.txt). I'll leave detailed explanation of a scan script to the "Multiple Scan / Progression Control" section of this document, but note that:
|
||||
* You may leave color information out to be added later.
|
||||
* You may let spatial detail be only a low-frequency approximation to
|
||||
be refined later with higher-frequency coefficients. (As far as I
|
||||
can tell, you cannot consistently reduce grayscale detail beyond the
|
||||
8x8 pixel MCU while still recovering that detail in later scans.)
|
||||
* You may leave grayscale and color values at a lower precision
|
||||
(i.e. coarsely quantized) to have more precision added later.
|
||||
* You may do all of the above in almost any order and almost any
|
||||
number of steps.
|
||||
|
||||
Your libjpeg distribution probably contains something called
|
||||
**wizard.txt** someplace (say, `/usr/share/docs/libjpeg8a` or
|
||||
`/usr/share/doc/libjpeg-progs`); I don't know if an online copy is
|
||||
readily available, but mine is
|
||||
[here](<../images/obscure_jpeg_features/libjpeg-wizard.txt>). I'll
|
||||
leave detailed explanation of a scan script to the "Multiple Scan /
|
||||
Progression Control" section of this document, but note that:
|
||||
|
||||
* Each non-commented line corresponds to one scan.
|
||||
* The first section, prior to the colon, specifies which plane to send, Y (0), Cb (1), or Cr (2).
|
||||
* The two fields immediately after the colon give the first and last indices of coefficients from that plane that should be in the scan. Those indices are from 0 to 63 in zigzag order; 0 = DC, 1-63 = AC in increasing frequency.
|
||||
* The two fields immediately after those specify which bits of those coefficients this scan contains.
|
||||
* The first section, prior to the colon, specifies which plane to
|
||||
send, Y (0), Cb (1), or Cr (2).
|
||||
* The two fields immediately after the colon give the first and last
|
||||
indices of coefficients from that plane that should be in the
|
||||
scan. Those indices are from 0 to 63 in zigzag order; 0 = DC, 1-63 =
|
||||
AC in increasing frequency.
|
||||
* The two fields immediately after those specify which bits of those
|
||||
coefficients this scan contains.
|
||||
|
||||
According to that document, the standard script for a progressive JPEG is this:
|
||||
|
||||
```bash
|
||||
# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
|
||||
0,1,2: 0-0, 0, 1 ;
|
||||
# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
|
||||
@ -74,47 +142,216 @@ According to that document, the standard script for a progressive JPEG is this:
|
||||
1: 1-63, 1, 0 ;
|
||||
# Y AC lowest bit scan is last; it's usually the largest scan
|
||||
0: 1-63, 1, 0 ;</pre>
|
||||
```
|
||||
|
||||
And for standard, sequential JPEG it is:
|
||||
|
||||
```bash
|
||||
0 1 2: 0 63 0 0;
|
||||
```
|
||||
|
||||
In [this image]({{ site.baseurl }}/assets/obscure_jpeg_features/20100713-0107-interleave.jpg) I used a custom scan script that sent all of the Y data, then all Cb, then all Cr. Its custom scan script was just this:
|
||||
In
|
||||
[this image](../images/obscure_jpeg_features/20100713-0107-interleave.jpg)
|
||||
I used a custom scan script that sent all of the Y data, then all Cb,
|
||||
then all Cr. Its custom scan script was just this:
|
||||
|
||||
```bash
|
||||
0;
|
||||
1;
|
||||
2;
|
||||
```
|
||||
|
||||
While not every browser may do this right, most browsers will render the greyscale as its comes in, then add color to it one plane at a time. It'll be more obvious over a slower connection; I purposely left the image fairly large so that the transfer would be slower. You'll note as well that the greyscale arrives much more slowly than the color.
|
||||
While not every browser may do this right, most browsers will render
|
||||
the greyscale as its comes in, then add color to it one plane at a
|
||||
time. It'll be more obvious over a slower connection; I purposely left
|
||||
the image fairly large so that the transfer would be slower. You'll
|
||||
note as well that the greyscale arrives much more slowly than the
|
||||
color.
|
||||
|
||||
Code & Utilities
|
||||
====================
|
||||
The **cjpeg** tool from libjpeg will (among other things) create a JPEG using a custom scan script. Combined with ImageMagick, I used a command like:
|
||||
Code & Utilities
|
||||
================
|
||||
|
||||
The **cjpeg** tool from libjpeg will (among other things) create a
|
||||
JPEG using a custom scan script. Combined with ImageMagick, I used a
|
||||
command like:
|
||||
|
||||
```bash
|
||||
convert input.png ppm:- | cjpeg -quality 95 -optimize -scans scan_script > output.jpg
|
||||
```
|
||||
|
||||
Or if the input is already a JPEG, **jpegtran** will do the same thing, losslessly (as it's merely reordering coefficients):
|
||||
Or if the input is already a JPEG, `jpegtran` will do the same
|
||||
thing, losslessly (as it's merely reordering coefficients):
|
||||
|
||||
```bash
|
||||
jpegtran -scans scan_script input.jpg > output.jpg
|
||||
```
|
||||
|
||||
libjpeg has some interesting features as well. Rather than decoding an entire full-resolution JPEG and then scaling it down, for instance (a common use case when generating thumbnails), you may set it up when decoding so that it will simply do the reduction for you while decoding. This takes less time and uses less memory compared with getting the full decompressed version and resampling afterward.
|
||||
libjpeg has some interesting features as well. Rather than decoding an
|
||||
entire full-resolution JPEG and then scaling it down, for instance (a
|
||||
common use case when generating thumbnails), you may set it up when
|
||||
decoding so that it will simply do the reduction for you while
|
||||
decoding. This takes less time and uses less memory compared with
|
||||
getting the full decompressed version and resampling afterward.
|
||||
|
||||
The following C code, based loosely on **example.c** from libjpeg, will split up a multi-scan JPEG into a series of numbered PPM files, each one containing a scan. Look for **cinfo.scale_num** (circa lines 67, 68) to use the fast scaling features mentioned in the last paragraph, and note that the code only processes as much input JPEG as it needs for the next scan. (It needs nothing special to build besides a functioning libjpeg. *gcc -ljpeg -o jpeg_split.o jpeg_split.c* works for me.)
|
||||
The C code below (or [here](../images/obscure_jpeg_features) or this
|
||||
[gist](https://gist.github.com/9220146)), based loosely on `example.c`
|
||||
from libjpeg, will split up a multi-scan JPEG into a series of
|
||||
numbered PPM files, each one containing a scan. Look for
|
||||
`cinfo.scale_num` (circa lines 67, 68) to use the fast scaling
|
||||
features mentioned in the last paragraph, and note that the code only
|
||||
processes as much input JPEG as it needs for the next scan. (It needs
|
||||
nothing special to build besides a functioning libjpeg. `gcc -ljpeg -o
|
||||
jpeg_split.o jpeg_split.c` works for me.)
|
||||
|
||||
{% gist 9220146 %}
|
||||
```c
|
||||
// jpeg_split.c: Write each scan from a multi-scan/progressive JPEG.
|
||||
// This is based loosely on example.c from libjpeg, and should require only
|
||||
// libjpeg as a dependency (e.g. gcc -ljpeg -o jpeg_split.o jpeg_split.c).
|
||||
#include <stdio.h>
|
||||
#include <jerror.h>
|
||||
#include "jpeglib.h"
|
||||
#include <setjmp.h>
|
||||
#include <string.h>
|
||||
|
||||
void read_scan(struct jpeg_decompress_struct * cinfo,
|
||||
JSAMPARRAY buffer,
|
||||
char * base_output);
|
||||
int read_JPEG_file (char * filename, int scanNumber, char * base_output);
|
||||
|
||||
int main(int argc, char **argv) {
|
||||
if (argc < 3) {
|
||||
printf("Usage: %s <Input JPEG> <Output base name>\n", argv[0]);
|
||||
printf("This reads in the progressive/multi-scan JPEG given and writes out each scan\n");
|
||||
printf("to a separate PPM file, named with the scan number.\n");
|
||||
return 1;
|
||||
}
|
||||
|
||||
char * fname = argv[1];
|
||||
char * out_base = argv[2];
|
||||
read_JPEG_file(fname, 1, out_base);
|
||||
return 0;
|
||||
}
|
||||
|
||||
struct error_mgr {
|
||||
struct jpeg_error_mgr pub;
|
||||
jmp_buf setjmp_buffer;
|
||||
};
|
||||
|
||||
METHODDEF(void) error_exit (j_common_ptr cinfo) {
|
||||
struct error_mgr * err = (struct error_mgr *) cinfo->err;
|
||||
(*cinfo->err->output_message) (cinfo);
|
||||
longjmp(err->setjmp_buffer, 1);
|
||||
}
|
||||
|
||||
int read_JPEG_file (char * filename, int scanNumber, char * base_output) {
|
||||
struct jpeg_decompress_struct cinfo;
|
||||
struct error_mgr jerr;
|
||||
FILE * infile; /* source file */
|
||||
JSAMPARRAY buffer; /* Output row buffer */
|
||||
int row_stride; /* physical row width in output buffer */
|
||||
|
||||
if ((infile = fopen(filename, "rb")) == NULL) {
|
||||
fprintf(stderr, "can't open %s\n", filename);
|
||||
return 0;
|
||||
}
|
||||
|
||||
// Set up the normal JPEG error routines, then override error_exit.
|
||||
cinfo.err = jpeg_std_error(&jerr.pub);
|
||||
jerr.pub.error_exit = error_exit;
|
||||
// Establish the setjmp return context for error_exit to use:
|
||||
if (setjmp(jerr.setjmp_buffer)) {
|
||||
jpeg_destroy_decompress(&cinfo);
|
||||
fclose(infile);
|
||||
return 0;
|
||||
}
|
||||
jpeg_create_decompress(&cinfo);
|
||||
jpeg_stdio_src(&cinfo, infile);
|
||||
(void) jpeg_read_header(&cinfo, TRUE);
|
||||
|
||||
// Set some decompression parameters
|
||||
|
||||
// Incremental reading requires this flag:
|
||||
cinfo.buffered_image = TRUE;
|
||||
// To perform fast scaling in the output, set these:
|
||||
cinfo.scale_num = 1;
|
||||
cinfo.scale_denom = 1;
|
||||
|
||||
// Decompression begins...
|
||||
(void) jpeg_start_decompress(&cinfo);
|
||||
|
||||
printf("JPEG is %s-scan\n", jpeg_has_multiple_scans(&cinfo) ? "multi" : "single");
|
||||
printf("Outputting %ix%i\n", cinfo.output_width, cinfo.output_height);
|
||||
|
||||
// row_stride = JSAMPLEs per row in output buffer
|
||||
row_stride = cinfo.output_width * cinfo.output_components;
|
||||
// Make a one-row-high sample array that will go away when done with image
|
||||
buffer = (*cinfo.mem->alloc_sarray)
|
||||
((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
|
||||
|
||||
// Start actually handling image data!
|
||||
while(!jpeg_input_complete(&cinfo)) {
|
||||
read_scan(&cinfo, buffer, base_output);
|
||||
}
|
||||
|
||||
// Clean up.
|
||||
(void) jpeg_finish_decompress(&cinfo);
|
||||
jpeg_destroy_decompress(&cinfo);
|
||||
fclose(infile);
|
||||
|
||||
if (jerr.pub.num_warnings) {
|
||||
printf("libjpeg indicates %i warnings\n", jerr.pub.num_warnings);
|
||||
}
|
||||
}
|
||||
|
||||
void read_scan(struct jpeg_decompress_struct * cinfo,
|
||||
JSAMPARRAY buffer,
|
||||
char * base_output)
|
||||
{
|
||||
char out_name[1024];
|
||||
FILE * outfile = NULL;
|
||||
int scan_num = 0;
|
||||
|
||||
scan_num = cinfo->input_scan_number;
|
||||
jpeg_start_output(cinfo, scan_num);
|
||||
|
||||
// Read up to the next scan.
|
||||
int status;
|
||||
do {
|
||||
status = jpeg_consume_input(cinfo);
|
||||
} while (status != JPEG_REACHED_SOS && status != JPEG_REACHED_EOI);
|
||||
|
||||
// Construct a filename & write PPM image header.
|
||||
snprintf(out_name, 1024, "%s%i.ppm", base_output, scan_num);
|
||||
if ((outfile = fopen(out_name, "wb")) == NULL) {
|
||||
fprintf(stderr, "Can't open %s for writing!\n", out_name);
|
||||
return;
|
||||
}
|
||||
fprintf(outfile, "P6\n%d %d\n255\n", cinfo->output_width, cinfo->output_height);
|
||||
|
||||
// Read each scanline into 'buffer' and write it to the PPM.
|
||||
// (Note that libjpeg updates cinfo->output_scanline automatically)
|
||||
while (cinfo->output_scanline < cinfo->output_height) {
|
||||
jpeg_read_scanlines(cinfo, buffer, 1);
|
||||
fwrite(buffer[0], cinfo->output_components, cinfo->output_width, outfile);
|
||||
}
|
||||
|
||||
jpeg_finish_output(cinfo);
|
||||
fclose(outfile);
|
||||
}
|
||||
```
|
||||
|
||||
Examples
|
||||
========
|
||||
|
||||
Here are all 10 scans from a standard progressive JPEG, separated out with the example code:
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user