Started restoring some old images & posts; changed themes to notepadium

hugo_blag/content/posts/2011-11-24-obscure-features-of-jpeg/jpeg_split.c

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+// 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);
+}