libpng warning: Ignoring incorrect gAMA value when sRGB is also present

[cbelth]

I have run the command:

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convert -flop image.png mirrored_image.png

to mirror an image. It worked, and examining the result, it is correct. However, I then went to resize the result with the following:

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convert mirrored_image.png -resize 256x256 resize_image.png

and it throws the following error:
libpng warning: Ignoring incorrect gAMA value when sRGB is also present
Anyone know how to solve this?

[fmw42]

What is your IM version and platform? Please always provide that. See viewtopic.php?f=1&t=9620

Also what is your version of libpng? It might be old and need an update.You can find it from

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convert -list format

[snibgo]

The problem is probably this:

In some versions of IM (or perhaps some versions of linpng), using "-gamma XX" to adjust mid-tones of pixels would also adjust the metadata gamma setting. But some versions of IM or libpng expect the gamma metadata to always be 0.454545 when a sRGB profile is present.

The cure is not to use "-gamma XX", but "-evaluate Pow YY" where YY = 1/XX.

When a file shows this warning, the workaround is "-set gamma 0.454545". This changes the metadata without changing the pixels.