OT: Musings about image compression of Canon 7D

Patryk Rebisz wrote on 11/12/2009, 11:16 PM
Images in the link:
http://patrykrebisz.wordpress.com/2009/11/13/34/

I spend whole day running Canon 7D through its hoops. When picking up the camera from the rental place I asked about the possibility of HD from HDMI out while recording. The tech confirmed that it outputs 720p during both preview and recording (as might now from my previous post it was my biggest concern with Canon 5D MII). When I hooked up a small 8" HDMI monitor to the camera and recorded some random stuff using the monitor as the focusing reference it was right on the money. So the run&gun capability of this camera got solved as far as focus is concerned.

However what I also noticed is a significant amount of the "jello" effects (when the CMOS chip takes its time to dump the content of each frame and thus by the time it finishes the vertical lines in the frame appear skewed). The effect is mostly apparent when you do fast pans. Now, don't get too pissed off as all CMOS based cameras exhibit this effect (including Panavision's Genesis) it just happens that the effect is quite noticeable with 7D (and probably 5D as well). That said though when I started running the camera through the regular shooting scenarios it became less of an issue.

I shot using a whole range of exposure settings with my lens at f2, f2.8 (the f-stop I used the most often as some of my lenses don't go below that) and f4 and the ISO at 160, 320, 400, 640, 800, 1250, 2500, 3200, 6400. No matter which ISO I chose the footage exhibited a significant amount of compression artifacts. Even at ISO160 they were there. Out of curiosity I shot a scene stacking the 7D against Canon's consumer HV20 camcorder. The results were surprising. Both cameras delivered sharp image (HV20 being just a tad sharper but it could be because of the small sensor size everything was in focus while with 7D only the face of the subjects was suppose to be, thus optically the HV20's footage was perceived as having more sharpness). The latitude of both cameras was almost identical (if not exactly the same). Of course as HV20 used HDV compression it started to loose color information in dark areas while 7D held all what its compressor allowed it. That said though HV20 produced... more natural images. Because the HDV compression introduces "mosquito noise" artifacts that actually when stacked against clinically clean (but blocky due to the compression) 7D the HV20 looked a bit better. Now the whole comparison seems a bit nonsensical as obviously those are 2 different tools, but it reveals something about the heaviness of the 7D compression. Now what kind of real-life use has that "scientific" mumbo jumbo? Well, if it was up to me and I knew that the project i'm shooting will end up on YouTube or even high quality SD DVD then I would have no problem shooting with either camera but. If you are thinking of projecting your project in a theater or even on a 40" and larger flat screen TV then it's possible that you will find better tools.

Comments

farss wrote on 11/13/2009, 1:44 AM
I've noticed the same thing on low bitrate H.264 out of iMovie. Fast motion with lots of detail turns to mush, kind of like a bad oil painting The client who'd bought the file to me hadn't noticed it until I went to it frame by frame. The perils of working too long in this game. I don't watch movies for the story anymore, I watch the compression :)

How can you tell the DPs in a cinema?
They'll sit in the front row even if the cinema is empty.

Bob.

Patryk Rebisz wrote on 11/13/2009, 10:02 AM
Yes, though in this case there is barely any motion as the camera is locked off and yet the low bitrate (not that ow in the first place) does turn to mush.
Yoyodyne wrote on 11/13/2009, 1:00 PM
Hey Patryk, really appreciate your reports and info about the 7D. I've just picked one up and so far have been pretty pleased with the results. To my eye it seems less noisy than my JVC GY-HD100 which is a pretty clean looking camera. So far the rolling shutter has been a non issue for me but the aliasing has been.

I was shooting a bunch of stuff for Western Star trucks, pretty much a worse case scenario for aliasing - all those big grills - and it was noticable in some cases. Using the Phil Bloom 7D settings (low contrast/sharpening all the way down/low chroma) seems to help a fair bit. I'll learn more when I start working more with the footage.

Any thoughts/tips about aliasing?
Coursedesign wrote on 11/13/2009, 2:27 PM
Patryk,

Many thanks for that field report, that is very helpful (including the workarounds).

I want an AJA KiPro...

Fits under the camera, records 1080p for hours to a virtually uncompressed format, time code, ...

Mmmm.

Patryk Rebisz wrote on 11/13/2009, 2:57 PM
Yoyodyne, JVC is a noisy camera. Check out this comparison i made some time ago:
http://patrykrebisz.wordpress.com/2009/09/23/canon-hv30-v-high-end-jvc-gy-hd250u-comparison/

As far as anti-ailising -- again it becomes a problem mostly because of the way the camera compresses images. In this image:
http://patrykrebisz.com/7D_test/blur.jpg

Look at the bluish line in the window molding right above the night light on the level of the stand... HV20's compression has no problem resolving this thin horizontal line while 7D ends up with a blob of "something."
Yoyodyne wrote on 11/13/2009, 3:36 PM
Oh, I agree that the 200/250 series are very noisy but the 100/110 are pretty clean. A lot of people were pretty disappointed with the high level of noise in the 200/250 cams. It kept me from moving to that camera series.

Those little HV cams are real impressive though!