fish2000 fish2000 - 6 months ago 16
Python Question

How can one perform color transforms with ICC profiles on a set of arbitrary pixel values (not on an image data structure)?

I'd like to convert a set of pixel values from one profiled colorspace to another, without these values residing in an image file, such as (say) a list of RGB/RGBA/CMYK/etc data structures.

I have Python and PIL at my disposal, but I'm interested in solutions in related environments if that's what it takes.

The latest PIL has very nice support for LittleCMS -- but no way to hand it anything other than a PIL image (or a legacy pyCMS object) for it to act upon.

As far as I can ascertain, the command-line tool

icctrans
that's included with LittleCMS does something of this sort, but I can't seem to find any non-skeletal documentation on it, and the documentation refers to it as a demonstration tool.

Answer

In order to use the current 2.3 version of Little CMS with Python, I translated lcms2.h to lcms2consts.py with the h2py.py script that comes in the Python distribution. The script does not translate struct declarations, but the constants are enough to do basic color transformations with ctypes and lcms2 as a dynamic library.

This example transforms a single colour from double precision Lab to 8-bit sRGB using built-in profiles. Use cmsOpenProfileFromFile(filename, 'r') instead for files.

import ctypes
from ctypes import byref
from lcms2consts import *

lcms = ctypes.windll.lcms2

inprof = lcms.cmsCreateLab4Profile(0)
outprof = lcms.cmsCreate_sRGBProfile()
xform = lcms.cmsCreateTransform(inprof, TYPE_Lab_DBL, 
    outprof, TYPE_RGB_8,
    INTENT_PERCEPTUAL, 0)
lcms.cmsCloseProfile(inprof)
lcms.cmsCloseProfile(outprof)

DblTriplet = ctypes.c_double * 3
ByteTriplet = ctypes.c_ubyte * 3
inbuf = DblTriplet(60.1,20.2,0.5)
outbuf = ByteTriplet()
lcms.cmsDoTransform(xform, byref(inbuf), byref(outbuf), 1)
print list(outbuf)

lcms.cmsDeleteTransform(xform)