Created attachment 353038 [details]
testsalmon images for RGB-CMYK-RGB

Using Gimp 2.8.22 on Windows 10. I have a very small, 8-bits per channel, solid-color PNG test image. Using other software, I convert it to CMYK, using color management One converstion is to JPG, one conversion is to TIF. The color profiles are stripped away after conversion. Even though GIMP cannot produce CMYK, it can open these files, re-converting to RGB.

I expect that the JPG and TIF files should look the same, even if there is some loss from the original PNG. But in GIMP, they look noticeable different. I have analyzed this using GraphicsMagick identify -verbose to get the channel values.

The original RGB png image:
R=56797, G=38550, B=38036.

Converted to CMYK JPG (highest quality):
C=1799, M=20817, Y=13364, K=1028.

Converted to CMYK TIF:
C=1799, M=20817, Y=13364, K=1028.

CMYK JPG imported to GIMP, exported as png:
R=57568, G=46517, B=45489.

CMYK TIF imported to GIMP, exported as png:
R=62708, G=43947, B=51143.

Thus, with GIMP, the re-converted images are noticeably different. The one via JPG is vaguely like the original, but the one via TIF is quite different.

CMYK JPG imported to Windows Paint, exported as png:
R=61680, G=46260, B=43947.

CMYK TIF imported to Windows Paint, exported as png:
R=61680, G=46260, B=43947.

Thus, with Windows Paint, the re-converted images are identical.

I conclude that the transfer function GIMP uses when importing CMYK, is different depending on whether the imported image is JPG or TIF. No color profiles embedded.

The discrepancy is quite noticeable in GIMP. A similar discrepancy happens in Irfan View, a Windows image viewer (without using color management).

However, there is no discrepancy in Windows Paint, or in Photoshop Elements 11, or in Adobe Acrobat Pro 9, or Krita 3. These four programs show both of the re-converted images at roughly the original color. Some loss is expected, since color profiles were not embedded.

Intriguingly, the GraphicsMagick viewer shows the re-converted images as identical, but both are like what GIMP gets from intermediate TIF. That is, they are noticeably different from the original. See attached testsalmon.zip for images.