No function to retrieve cmsProfileID

[haasn]

I see a definition for cmsProfileID in lcms2.h, and it's referenced inside cmsICCHeader, but I cannot find any way to retrieve this information through the actual LittleCMS 2 API.

Is it possible to get the profile ID somehow? If not, could it be added?

My use case: I want to quickly figure out if an ICC profile has changed, as that invalidates my cache. Currently I am comparing the entire file against a reference copy, but that doesn't play nicely with profiles loaded from memory (or generated from scratch), and it's also less efficient to store a full copy of a potentially large ICC profile rather than just a 128-bit hash.

[mm2]

Hi, the functions to get/set/compute profileID are available since lcms2-2.0
cmsGetHeaderProfileID()
cmsSetHeaderProfileID()
cmsMD5computeID()

There is documentation in the API reference manual
Regards
Marti

[haasn]

Thanks, I didn't find the section for them since it was so far at the bottom.

On a related note, do you know why identically generated profiles end up with different profile IDs? eg. see this file

#include <stdio.h>
#include <lcms2.h>

int main() {
    cmsCIExyY d65 = {0.3127, 0.3290, 1.0};
    cmsCIExyYTRIPLE prim_rec709 = {
        .Red   = {0.64, 0.33, 1.0},
        .Green = {0.30, 0.60, 1.0},
        .Blue  = {0.15, 0.06, 1.0},
    };
    cmsToneCurve *tc = cmsBuildGamma(NULL, 2.2);

    cmsHPROFILE p = cmsCreateRGBProfile( &d65, &prim_rec709, (cmsToneCurve*[3]) { tc, tc, tc });
    cmsSetProfileVersion(p, 2.0);
    cmsMD5computeID(p);

    cmsProfileID id;
    cmsGetHeaderProfileID(p, id.ID8);
    printf("id: 0x%x%x%x%x\n", id.ID32[0], id.ID32[1], id.ID32[2], id.ID32[3]);

    cmsCloseProfile(p);
    cmsFreeToneCurve(tc);

    return 0;
}

Building the program and running it a few times gives me results like this:

id: 0x655d952fffec2bbbb3ae6ed256b0d281
id: 0x5afd5f96bb3c3b6ef906c050c32a80
id: 0x5afd5f96bb3c3b6ef906c050c32a80
id: 0xcc05703d8c8af55cf7aa321640112c6b
id: 0xb2c8a0b3ef6d3c431b962e1383eb11e0
id: 0xb304d4397782884e92c6d7716e72693
id: 0x1e59381940a058dc36ed82c9384722fc
id: 0x1e59381940a058dc36ed82c9384722fc

Do you know what causes that?

[mm2]

On a related note, do you know why identically generated profiles
end up with different profile IDs? eg. see this file
Do you know what causes that?

ICC profiles holds a creation time stamp that contributes to the MD5
profile ID, strictly speaking each time you create a new profile it is
indeed different. This is not a lcms behavior but something imposed by
the ICC spec. The only way to obtain exact ID is by copying the file
or memory block that holds the profile.

Regards
Marti