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jsummers / bmpsuite

A set of Windows BMP image files, for testing purposes
https://entropymine.com/jason/bmpsuite/
GNU General Public License v3.0
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Add q/rgb24v4.bmp #4

Closed mayeut closed 7 years ago

mayeut commented 7 years ago

This image has red & green channels swapped. The color calibration from v4 header tries to swap them back. Classified as questionnable given the comment regarding gamma & chromaticity.

Firefox, Safari & Chrome are failing to display this correctly. When writing calibration information from this BMP in a PNG file (cHRM & gAMA chunks), channels are swapped & displayed properly.

jsummers commented 7 years ago

Thanks for this, but I haven't decided what to do about it. The equivalent PNG file does seem to work, but I'm not sure I understand chromaticities well enough to know if this is a legitimate thing to do. And I wonder if swapping red/blue might be safer than red/green.

Another issue is that I'd rather not create any more BMPv4 files until I know that I have the gamma fields set correctly. In fact, I just found something that suggests I guessed wrong, and the gamma should be 2.2 instead of 1/2.2.

mayeut commented 7 years ago

The only "information" I found is ImageMagick implementation which suggest 1/2.2

https://github.com/ImageMagick/ImageMagick/blob/4cc6ec8a4197d4c008577127736bf7985d632323/coders/bmp.c#L759 Compared to their PNG implementation https://github.com/ImageMagick/ImageMagick/blob/4cc6ec8a4197d4c008577127736bf7985d632323/coders/png.c#L2589

But this is just one implementation...

jsummers commented 7 years ago

There's a Petzold book that suggests 0x23333 (2.2) as a possible gamma value. Try a web search for the terms: BITMAPV4HEADER 0x23333

jsummers commented 7 years ago

I decided against this, for now. I'd want to get expert advice first.