Canon DPP4 gives extra pixels on raw conversion?

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rjlittlefield
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Canon DPP4 gives extra pixels on raw conversion?

Post by rjlittlefield »

A user of Zerene Stacker has reported to me that raw converting with Canon DPP4, from his 7D Mk II camera, gives different size images from one frame to another.

In four frames shot as a focus stack, he saw

Code: Select all

Image 1:  57.1 M      5475, 3649
Image 2:  57.2 M      5475, 3650
Image 3:  57.2 M      5475, 3650
Image 4:  57.1 M.     5474, 3649
The nominal specification for 7D Mk II is 5472 x 3648, so it seems that DPP4 is giving a couple of extra pixels, but not always the same number.

I find this quite strange, so I am curious.

Does anyone know why this happens? Especially, why is it different from one frame to another?

--Rik

Pau
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Post by Pau »

Just a possibility, but could it be due to a compressed raw format?
Pau

rjlittlefield
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Post by rjlittlefield »

could it be due to a compressed raw format?
Does not seem likely. Compression on raw files is normally completely lossless. I've heard of a few raw formats that do lossy compression in some circumstances, but if I recall correctly that was described as slightly altering pixel values, not image sizes.

Lightroom converts all four of these raw files to the same size output, 5472 x 3648.

--Rik

apt403
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Post by apt403 »

Total shot in the dark without having access to the files, but perhaps it's some weirdness going on with the way DPP4 handles RAW demosaicing, specifically where it chooses to crop the edges.

I'm seeing 5472 × 3648 quoted as the max effective resolution for the 7D Mk II from various sources, smaller than the sizes reported by the user. Perhaps DPP4 is leaving some 'extra' pixels in the image that would normally only be used for interpolation. It'd be interesting to run the raw files through a different raw processor, and maybe view the raw bayer output before demosaicing.

brentbristol
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Post by brentbristol »

I hope that this thread is not too stale. I just ran into this issue as well. I believe that it is only a problem if one turns on distortion correction in DPP4 and varies focus distance with the lens rather than moving the camera distance. There must be some small change in distortion correction with focus distance and DPP4 crops the resulting frame accordingly. At any rate turning off distortion correction made the problem go away completely. I was able to shoot the same stack moving the camera instead of changing the lens focus distance and leave distortion correction on. For much of what I do distortion correction does not offer any real advantage and is simply not worth the complications caused by slight variations in file sizes. If one absolutely needs the distortion correction then move the camera distance instead of change the focus distance of the lens or manually crop the photos in the stack to be exactly the same size after processing.
The trouble with quick and dirty is that the dirty remains after the quick is gone.

rjlittlefield
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Post by rjlittlefield »

Nope, not stale at all. I'm delighted to hear a good explanation for this one!

In fact, I'm now scurrying off to update the Zerene Stacker website where this applies.

--Rik

rjlittlefield
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Post by rjlittlefield »

brentbristol wrote:If one absolutely needs the distortion correction then ... manually crop the photos in the stack to be exactly the same size after processing.
In Zerene Stacker, you can also set Options > Preprocessing > Image Pre-cropping. That causes the cropping to be done on the fly, as part of reading the image from file.

--Rik

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