Canon Line Offset Noise

Started Oct 20, 2009 | Discussions thread
Steen Bay Veteran Member • Posts: 7,418
Re: The look of sharpened noise

ejmartin wrote:

Steen Bay wrote:

ejmartin wrote:

Steen Bay wrote:

GaborSch wrote:

Steen Bay wrote:

What I meant is, that normal random noise doesn't appear as maze if sharpened

Well, here is an example, from one of the 7D raw files posted by bronxbombers, NR off, strong sharpening by DPP, 200% view:

Yes, it ain't pretty, I think we can agree about that, but Bronxbombers' 7D had this additional issue with unbalanced green gains, which confuses things a bit. I started this subthread wondering why mazing was so visible in Hulkster's 7D and (especially) 50D DPP test pics (sharpening 3, which isn't strong) at 100%, from ISO 400 and up, and I think it's (more or less) clear that the mazing must be created by the demosaicing algorytm in its attempt to maximize the amount of detail.

I agree.

Is the maze a real problem? Maybe not, it's probably just the visible effect of the compromises made in the demosaicing process, but it certainly isn't pretty at 100% view!

It's a problem in that the gain imbalance violates assumptions on which the demosaic algorithm is based. A checkerboard pattern of G values does not occur in real world images (it could only be caused by a texture at or beyond Nyquist in both vertical and horizontal directions , something quite rare in real world scenes, and would be suppressed by the AA filter anyway), so the algorithm should be allowed to assume that it will not be presented with such data. It would be nice if the demosaic algorithm is robust enough not to generate mazes, but let's put the blame for mazing where it belongs -- on the camera that generates RAW data not corresponding to any real world image.

Thanks, but I'm not quite sure what it is you're saying here. Are you suggesting that the very visible maze artifacts in Hulkster's 7D and 50D test pics also is caused by an imbalance in gains, on both cameras? (That would be pretty disturbing!) Or is the maze a 'normal' thing, caused by the compromises made in the demosaicing, also on cameras with perfectly balanced gains?

(The maze pattern is very easy to see in the DPP 3.7 "sharpen=3" and "cropped by clock" pics, at ISO 400 and above. Link to the test in the post below)

http://forums.dpreview.com/forums/read.asp?forum=1019&message=33411995

The maze artifacts of sharpened high ISO images are the same sort of thing as would be caused by demosaic of low ISO images with gain imbalance of greens. The demosaic is making straight line vertical or horizontal interpolations of RAW data that has a substantial (random, in the case of high ISO, due to noise) variation at Nyquist. The converter then sharpens those vertical or horiztontal bars. Now, low ISO data should hot have those sorts of bars, and indeed hulkster's low ISO images don't show any, even with sharpening. bronxbomber's 7D images did have strong variation at Nyquist, even at base ISO, due to the gain imbalance; thus he got the same sort of mazing pattern.

I have been wondering whether those who are disappointed with the low ISO behavior of their 7D in regard to noise, might have a copy with some gain imbalance, though perhaps not so strong as in BB's copy that would cause strong mazing but rather just an overall increase in artifacting in the conversions.

Think I've got it now, thanks. Sorry for being a bit slow

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