PSNR is different for the same images, different formats

[ritave]

OS: Windows 8 Enterprise
Magick Version: 6.8.9
Bug: When calculating PSNR between two images, the PSNR differs depending on the format of the files (but the same content).
Steps to reproduce:
Download reference images: http://www.filedropper.com/refimages

Code: Select all

UTIL_DB_compare_.exe -metric PSNR f00140_ref.png f00140_ref.bmp asd.png
1.#INF // Images reported the same

UTIL_DB_compare_.exe -metric PSNR f00140_ref.png f00140.png p.png
29.6955

UTIL_DB_compare_.exe -metric PSNRf00140_ref.bmp f00140.bmp b.png
28.4461 // But the PSNR differs

UTIL_DB_compare_.exe -metric PSNR p.png b.png total.png
1.#INF // And yet the resulting images dont

[ritave]

I got some more debug info:
This is due to the 1 bit alpha channel in png files.
GetMeanSquaredDistortion@compare.c might be calculated wrongly in some way. When I commented out the calculations for alpha channel as well as harcoded

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distortion[CompositeChannels]/=(double) GetNumberChannels(image,channel);

to

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distortion[CompositeChannels]/=(double) 3;

the results are the same in both file formats

[magick]

We can reproduce the problem you posted and have a patch in ImageMagick 6.8.9-6 Beta, available by sometime tomorrow. Thanks.

[jesse1]

Can you please explain what this fix means?
I compared images successfully with 6.8.9.5, but with latest version it returns error "image morphology differs". What should I do about it?

update: I updated not to latest but to 6.8.9.7 since it is last available version via brew for mac osx.
and a full error code is: compare: image morphology differs `<path to image>' @ error/compare.c/CompareImageChannels/173

[jesse1]

I found it, it was because of alpha channel which was present in one image but absent in another. After adding option "-alpha Off", it started working again.