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Messages - Yoshiyuki Blade

#101
General Chat / Re: LQ video compression in Canon DSLRs
January 10, 2013, 08:05:54 PM
Yeah, I grabbed the same video sample and it seems your screenshots were taken at the full range (unclipped). I opened it up in MPC-HC and it clipped the bushes in the shadows completely, but it looks "as intended" when loaded into AVISynth and converted to RGB with the proper matrix. Anyway, the contrast looks like what I'd expect from one of the default picture styles though; there's nothing out of the ordinary with how it looks. If the photographer took a RAW still shot and loaded it into Canon's bundled software (Digital Photo Professional) working in the sRGB space, it'll look very close to that, minus the obvious resolution and quality difference.

It does look rather mushy though, since there's a ton of high frequency detail, high motion and deep DoF at a (relatively) low bitrate, and at 30 fps to boot. If other cameras can handle those conditions with higher AVC profiles/features, then that would be pretty impressive. I'm not sure if CABAC (a 10-15% quality improvement) would make or break the quality in this case though.
#102
General Chat / Re: LQ video compression in Canon DSLRs
January 09, 2013, 05:40:29 PM
The efficiency of compression shouldn't affect the contrast of the video (looking washed-out, etc). Canon's video clips should look pretty close to the "picture style" chosen for recording. If the samples you downloaded were recorded with, for example, a Neutral "picture style" with -4 contrast, it *should* look pretty washed out lol. People intentionally choose "flat" styles for a easier post processing.

There's also a problem with how videos are encoded (with the 5D2 at least). Normally, images take up the full range (0-255) on the histogram, but in video it's recorded in the YV12 color space using rec.601 coefficients. If you play it back as-is, it'll look wrong for a couple reasons, but biggest one is that the shadows and highlights will be clipped. The YV12 space assumes limited range (aka TV range, 16-235) and will ignore everything else. To be honest, this actually makes the image look *less* washed out, but it's incorrect nonetheless. However, this is also a blessing because the camera didn't just throw away the full range of detail or downscale it carelessly. Some post processing has to be done to properly bring the full range into limited range.
#103
First post on the "new" forums, but I've been following ML for a while and donated a little a while back. Thanks for all your hard work, everyone! I've only recently found this thread and it seems very exciting. I'm curious about where you guys are headed with this now.

The most recent posts seem to focus more on the h.264 side of things, but a couple months ago, there was crazy talk about implementing an mjpeg encoder directly with the raw 422 data, which was wonderful to imagine. It had me thinking about how much goes into the pipeline of image processing at the moment you press that record button. Where do you suppose would be the best place to min/max for video quality?

With my 5D2, I found that the silent pics written while recording (1872x1080) don't really represent the actual resolution being recorded. I've done subjective tests with various resolutions listed in the 422toimage converter xml files and it's probably 1720x974. If bypassing the upscale process to 1080p is possible, that alone would probably help a lot in increasing the overall quality of the video. Less bits is required for the same quality and less resolution to encode frees up more cpu performance. And boy does audio take up a ton of overhead! With audio on, the video bitrate can only go up to the 60 megabits before crashing, but can go up to the 120s with it off. I can record at Q-16 constantly in fairly busy scenes that way. What's the deal with that lol.

I also noticed that the difference between the (converted) raw 422 image and when I downsample it to 420 is very negligible. However, I haven't tested it on bright vivid reds yet, where perceiving the difference in chroma resolution is most apparent. I wonder if trading performance for chroma resolution is worth the extra cost. Anyway, thanks again guys. I'm not literate in programming or understanding the low level stuff that goes on, but I do like to get into the nitty gritty technical details of video as much as possible.