First draft of 3rd post on Pi pan-tilt rig
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posts/2016-10-12-pi-pan-tilt-3.md
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title: Pi pan-tilt for huge images, part 3: Image processing & raw workflow
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author: Chris Hodapp
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date: October 4, 2016
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tags: photography, electronics, raspberrypi
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---
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This is the third part in this series, continuing on from
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[part 1][part1] and [part 2][]. The last post was about integrating
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the hardware with Hugin and PanoTools. This post still involves those
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tools somewhat, but is more about the image processing pipeline I've
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been using.
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Thus far (aside from my first stitched image) I've been using a raw
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workflow where possible. That is, all images arrive from the camera
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in a lossless format, and every intermediate step works in a lossless
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format. To list out some typical steps in this:
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- Acquire raw images from camera with [raspistill][].
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- Convert these to (lossless) TIFFs with [dcraw][].
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- Process these into a composite image with [Hugin][] & [PanoTools][],
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producing another lossless TIFF file (for low dynamic range) or
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[OpenEXR][] file (for [high dynamic range][hdr]).
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- Import into something like [darktable][] for postprocessing.
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This is possibly overkill. I do it anyway.
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# Acquiring Images
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I may have mentioned in the first post that I'm using
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[ArduCam's Raspberry Pi camera][]. This board uses a 5-megapixel
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[OmniVision OV5647][ov5647]. (I believe they have
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[another][arducam_omx219] that uses the 8-megapixel Sony OMX 219, but
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I haven't gotten my hands on one yet.)
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If you are expecting the quality of sensors and exposure control that
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even an old entry-level DSLR camera provides, this board will probably
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disappoint you. The sensor is tiny, it's noisy, and capturing still
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images is quite slow (6-7 seconds, in my tests).
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However, if you are accustomed to basically every other camera that is
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within 2-3 times the price and is meant to interface directly with a
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computer of some kind (USB webcams and the like), I think you'll find
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it quite impressive:
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- It comes in versions with CS lens mount, C mount, and M12 mount.
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CS-mount and C-mount lenses are plentiful from their existing use in
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security cameras, generally inexpensive, and generally good-quality
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(and for a bit extra, ones are available with
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electrically-controllable apertures and focus). M12 lenses
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are... plentiful and inexpensive, at least. I'll probably go into
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more detail on optics in a later post.
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- [raspistill][] will provide 10-bit raw images straight off the
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sensor (see its `--raw` option). Thus, we can bypass all of the
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automatic brightness, saturation, contrast, and whitebalance which
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are great for snapshots and video, but really annoying for composite
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images.
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- Likewise via [raspistill][], we may directly set the ISO speed and
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the shutter time in microseconds, bypassing all automatic exposure
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control.
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I'm mostly using the CS-mount version, which came with a lens that is
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surprisingly sharp. (TODO: Put example images here.) If anyone can
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tell me how to surpass all this for $30 (perhaps with those GoPro
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knockoffs that are emerging), please tell me.
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Reading raw images from the Raspberry Pi cameras is a little more
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convoluted, and I suspect that this is just how the CSI-2 pathway for
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imaging works on the Raspberry Pi. In short: pass the `--raw` option
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to raspistill, and it will produce a JPEG file which contains a
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normal, lossy image, followed by a binary dump of 10-bit raw Bayer
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data from the sensor.
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# Converting Raw Images
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People have already put considerable work into converting these rather
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strange raw image files into something more sane (as the Raspberry Pi
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forums document [here][forum1] and [here][forum2]) - like the
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[color tests][beale] by John Beale, and 6by9's patches to dcraw, some
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of which have made it into Dave Coffin's official [dcraw][].
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I've had to use 6by9's version of dcraw, which is at
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<https://github.com/6by9/RPiTest/tree/master/dcraw>. As I understand
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it, he's trying to get the rest of this included into official dcraw.
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On an older-revision ArduCam board, I ran into problems getting 6by9's
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dcraw to read the resultant raw images, which I fixed with a
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[trivial patch][dcraw-pr]. However, that board had other problems, so
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I'm no longer using it. (TODO: Explain those problems.)
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- TODO: Give dcraw options and why
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- TODO: 8-bit vs. 16-bit
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- TODO: Example of 10-bit raw (does it get anything?)
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# Hugin & PanoTools Stitching
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- Alongside exposure fusion, Hugin supports merging exposures into
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HDR. Interestingly, this doesn't require bracketing. If the scene
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contains a wide range of lighting across a larger area, and
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individual shots are at different exposures to account for this,
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this may still require HDR. (TODO: Example of this)
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- Bracketing might still be a requirement, however, if a single shot
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spans too wide an exposure range. Do keep in mind that this sensor
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provides only 10 bits and is noisy.
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# Postprocessing
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- darktable already uses floating-point internally for everything, so
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it gladly handles [OpenEXR][] files (which store pixel values in
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floating point).
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[part1]: ./2016-09-25-pi-pan-tilt-1.html
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[part2]: ./2016-10-04-pi-pan-tilt-2.html
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[raspistill]: https://www.raspberrypi.org/documentation/raspbian/applications/camera.md
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[dcraw]: https://www.cybercom.net/~dcoffin/dcraw/
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[Hugin]: http://wiki.panotools.org/Hugin
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[PanoTools]: http://wiki.panotools.org/Main_Page
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[OpenEXR]: http://www.openexr.com/
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[hdr]: https://en.wikipedia.org/wiki/High-dynamic-range_imaging
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[darktable]: http://www.darktable.org/
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[ArduCam]: http://www.arducam.com/camera-modules/raspberrypi-camera/
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[ov5647]: http://www.ovt.com/uploads/parts/OV5647.pdf
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[arducam_omx219]: http://www.arducam.com/8mp-sony-imx219-camera-raspberry-pi/
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[beale]: http://bealecorner.org/best/RPi/
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[forum1]: https://www.raspberrypi.org/forums/viewtopic.php?f=43&t=44918
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[forum2]: https://www.raspberrypi.org/forums/viewtopic.php?f=43&t=92562
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[dcraw-6by9]: https://github.com/6by9/RPiTest/tree/master/dcraw
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[dcraw-pr]: https://github.com/6by9/RPiTest/pull/1
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