Some png files when resized with png.quality results as empty/transparent png file

[tvatavuk]

We need to resize this demo.png image.

Issue happens with parameters: demo.png?w=1230&h=760&png.quality=75&mode=crop&scale=both

If we append png.lossless=true than resized image looks OK,
except that we can not control quality demo.png?w=1230&h=760&png.quality=75&mode=crop&scale=both&png.lossless=true

Issue happens with code that is using following dependencies:

• Imageflow.Net (0.8.5)
• Imazen.HybridCache (0.7.7)
• Imazen.Common (0.7.7)
• Imageflow.NativeRuntime.win-x86 (1.7.3-rc67)
• Imageflow.NativeRuntime.win-x86_64 (1.7.3-rc67)

Same code works as expected with older version of dependencies:

• Imageflow.Net (0.7.24)
• Imazen.HybridCache (0.5.10)
• Imazen.Common (0.5.10)
• Imageflow.NativeRuntime.win-x86 (1.5.8-rc62)
• Imageflow.NativeRuntime.win-x86_64 (1.5.8-rc62)

Same issue is tracked here: 2sic/2sxc#2932

Please check 2sic/2sxc#2932 (comment)

[lilith]

So, I created an integration test on the latest version - but it works on my machine. Can you verify this is the correct input file? Maybe GitHub reencoded it?
https://imageflow-resources.s3.us-west-2.amazonaws.com/test_inputs/png_turns_empty.png

Using w=1230&h=760&png.quality=75&mode=crop&scale=both, I get https://imageflow-resources.s3.us-west-2.amazonaws.com/visual_test_checksums/0E5E63543D35A149D.png

[lilith]

Hmm, this is failing on windows but not linux.

[lilith]

And yet even on windows the result looks correct, despite being compressed differently. Can the problem image be emailed to lilith@imazen.io to make sure GitHub isn't 'repairing' it? Also, can you try with the very latest Imageflow release? This was listed as being in an older version.

[lilith]

So, GitHub apparently re-compresses images. The image you e-mailed me is a 24-bit PNG, 18.4KB, and the image GitHub lets me access is a 32-bit PNG, 27.5KB

[lilith]

This was specific to PNG files with a zero alpha channel, encoded when using lossy PNG, and when no transparency-inducing operations are performed. Thank you for e-mailing the problematic file, that allowed me to diagnose the issue.