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Friend gave me photos. TIFF with LZW compression. Parks, people walking, relaxing. People are doing fitness. I want to crop these TIFFs, resize. Then I will export it to JPEG. Then I will post these JPEGs on my blog.

I run Windows 10 Home 64-bit. GIMP 2.10.

I opening TIFF in GIMP. GIMP says:

The image has an embedded color profile sRGB IEC61966-2.1

Convert the image to the built-in sRGB color profile?

Rendering intent: Relative colorimetric.

Black Point Compensation. Flag is on.

Button Convert. Button Keep.

How do I respond to this GIMP question?

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  • \$\begingroup\$ "How do I respond to this GIMP question?" – Flip a coin. It makes no difference at all. \$\endgroup\$
    – xiota
    Oct 29, 2020 at 15:04

2 Answers 2

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Don't convert - at all.

At import tell it not to convert.
At export tell it not to convert.

sRGB IEC61966-2.1 is "industry standard" RGB used for the web. Any time you do colour-space conversion you potentially change the colour balance. If you don't have a fully calibrated system this can make changes you can't even see or know about. Preserving standard sRGB throughout means less margin for error.

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    \$\begingroup\$ Converting from sRGB to sRGB should make no difference. \$\endgroup\$
    – xiota
    Oct 28, 2020 at 13:38
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    \$\begingroup\$ Converting to "built-in sRGB color profile" could make a lot of difference, as we have no clue what that profile is. Leaving it completely alone is always going to be a better strategy. \$\endgroup\$
    – Tetsujin
    Oct 28, 2020 at 13:40
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    \$\begingroup\$ GIMP built-in sRGB is functionally identical to sRGB2014.icc from the International Color Consortium. Applying and converting each profile in sequence results in no change to the image data (identical md5 hash). \$\endgroup\$
    – xiota
    Oct 29, 2020 at 14:20
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    \$\begingroup\$ I'm not sure why you're building such a defence of "converting to the same thing" when the simplest way to be certain is to not convert if you don't have to. \$\endgroup\$
    – Tetsujin
    Oct 29, 2020 at 14:53
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    \$\begingroup\$ You claim "could make a lot of difference" – in fact, it makes no difference at all. Any potential change from sRGB to sRGB would be dwarfed by the resize operation OP intends to perform. \$\endgroup\$
    – xiota
    Oct 29, 2020 at 15:02
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In this case, it makes little to no difference, since you would be converting from sRGB to sRGB. At most, you would see a few insignificant-bit flips. (Note: insignificant bit is a technical term.)

If the input file had a different profile, like Adobe RGB, you should convert to sRGB since you plan to use the images on the web.

Do save copies of the unmodified original files.

To demonstrate there is no significant difference:

  • Since I do not have copies of your images, I obtained the sRGB v2 color profile from the International Color Consortium website (sRGB2014.icc).

    I chose this file because the profile you are using says -2.1. There are other versions that may give different results.

  • Then I generated a Hald-12 image with ImageMagick. Each pixel in a Hald image is a different color, arranged in a pattern that can used to apply color corrections via lookup table.

    convert hald:12 hald12-0.png

  • Using GIMP, I exported to the png to hald12-1.bmp, making sure to check "Do not write colorspace information".

  • Assigned the color profile sRGB2014.icc to the image.

  • Converted to the GIMP built-in sRGB profile.

  • Exported the result to hald12-2.bmp.

  • Compared results with md5sum. In this case, the conversion resulted in no difference.

    64869fef4a0acf95bea03369d5a89149  hald12-0.png
6a083813783d9f99a0918c3d97e2364b  hald12-1.bmp
6a083813783d9f99a0918c3d97e2364b  hald12-2.bmp
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    \$\begingroup\$ I'd still always recommend only ever converting at export. Personally I work at Adobe RGB right through from camera, which gives me the option of still having it available later if needed. One quick 'throwaway' conversion to sRGB at export & my original is safe. \$\endgroup\$
    – Tetsujin
    Oct 28, 2020 at 13:47
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    \$\begingroup\$ Adobe RGB has slightly wider gamut, so there is increased risk of banding. OP appears to be doing simple edits to prep for web, so conversion at import would be fine. \$\endgroup\$
    – xiota
    Oct 28, 2020 at 14:02
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    \$\begingroup\$ …but he's not converting from Adobe, he's converting from one defined sRGB to another ill-defined. Therefore, don't convert at all. \$\endgroup\$
    – Tetsujin
    Oct 28, 2020 at 14:20
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    \$\begingroup\$ That's why that sentence starts with If. It's possible there are images with other profiles in the collection that OP hasn't encountered yet. Also, GIMP is only reporting what the profile claims to be. Without examining the profile more closely, it's not certain that it really is what it says. You put more trust in the embedded profile... I think it doesn't matter... but if pressed, I'd trust the GIMP profile over one with unknown source. \$\endgroup\$
    – xiota
    Oct 28, 2020 at 14:45
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    \$\begingroup\$ Well… if the embedded profile is unreliable… & the built-in profile is an unknown… it would make even less sense to play with it. Worst-case scenario for converting to any profile at import is that the user, in a 'little knowledge is a dangerous thing' kind of way, had a screen profile set somewhere in colour prefs. I've seen plenty of people do it in Photoshop without knowing how much it messes up their colour workflow. \$\endgroup\$
    – Tetsujin
    Oct 28, 2020 at 14:49

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