Compression artifact - Quilting at DVinfo.net

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Old July 16th, 2003, 03:24 AM   #1
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Compression artifact - Quilting

I'm new to this forum, but I've looked back and can't find this issue covered in your archives. Apologies if it has been.

I find that my XM1 often ends up giving me images with very severe "quilting" artifacts - that is, where the compression has ended up producing very obvious discontinuities between 8x8 pixel blocks. Obviuosly that ends up making the image look very blocky, almost as if it's a 90x72 image rather than 720x576. Some frames look really bad, sometimes it's not apparent at all.

I've put some images up here (they're 200 pixel segments from the original DV AVI, PAL, enlarged in Photoshop). They were "normal" mode (ie interlaced) and hand-held (so there's a certain amount of camera-shake..)

http://www.wjm.clara.net/dv/test1b.jpg
http://www.wjm.clara.net/dv/test2b.jpg

I'd be grateful for any comments. My question really is "what causes this, and is there anything I can do about it?" Is it just a constraint of the codec in the camera - or is there some defect which I could get fixed? And if it is just a constraint, is it exacerbated by camera movement and interlacing (because the image to be compressed is just that much busier..)? And what would I have to pay for a camera that implements a better codec?

Thanks..
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Old July 16th, 2003, 05:25 AM   #2
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I don't see "quilting" effects in the images (8x8 fied pixel structures). Your pics show the standard shortcommings (mousquito noise) visible in all DCT based compression concepts. They are the result of missing high frequency components during the quantization proces. Some DV codecs are somewhat more "intelligent" in handling these limits and/or are more affected by image business (noise..). The good news is that in moving images these artifacts are somewhat "smoothed" by temporal dithering and end up in a kind of edge business, rather than the pixelisation effects which are visible in your jpeg images
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Old July 16th, 2003, 05:31 AM   #3
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It is hard to see without seeing the full image unaltered. Now
I guess with your particular picture (interesting shooting subject)
we will never get to see it, but enlarging a specific section only
will do you good if we can see the full picture at the full resolution
unlatered (preferred in a LOSSLESS compression format like
BMP or TIFF) to better judge it....
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Old July 16th, 2003, 05:52 AM   #4
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Perhaps "quilting" was the wrong word - the discontinuities are certainly far more apparent in the vertical plane than the horizontal. The effect is of discrete 8 pixel bands across the screen rather than 8x8 squares - look at the hair, for example - one band with fussy detail followed by one band which is blurred .. I don't think this is what you mean by "mosquito" effects, is it?

But thanks for taking a look - it's interesting at least that you don't think it's particularly abnormal.

But the effect is more obvious in some circumstances than in others, and I'd like to get to the bottom of what these circumstances might be. Your reference to "high frequency" components makes me wonder whether the effect will be more obvious where the image has lots of bright or over-exposed areas - or do I misunderstand what you mean by "high frequency"?
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Old July 16th, 2003, 07:50 AM   #5
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Agree Walter, on the hair edge, just above the hand there is maybe some quilting too (3 blocks)
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Old July 16th, 2003, 08:25 AM   #6
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You'll have to post the full image. Otherwise we cannot help you. <g>
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Old July 16th, 2003, 08:42 AM   #7
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High freq components are the equivalent of image details. Some steps in the DV compression sequence make that all the details together can not be compressed (in order to keep the datarate within the 25Mbit/s) and are thus left out
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Old July 16th, 2003, 09:15 AM   #8
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For Rob, and others, who want an uncompressed full frame -

** Rob Lohman: link removed because it contained nudity
Walter, you can send the link to persons who request it private
through e-mail, but please not publically on the board. Thanks **

She's sweet and innocent - but it weighs in at 750Kb (actually LZW compression, but that's lossless..)

You'll note from the interlacing that it's from a zoom - which probably adds to the difficulty?

But really, what I want to know is whether the Canon is doing all I can expect of it, or whether there's something I could and should fix - or must I just work within the constaints (and improve my technique!)?

Thanks.
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Old July 16th, 2003, 09:30 AM   #9
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I've looked at the full resolution image and it looks fine. The
problem most likely is due to what Andre has been describing.

The picture has quite a lot of detail and there is only so much
that will fit in 25 mbps.
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Old July 16th, 2003, 12:18 PM   #10
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Thanks for the thoughts.

Apologies for transgressing with the link !!
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