Mac Forum / Programming / Mac Programming / December 2006

# > So how the heck do you convert CMYK to RGB to make CoreImage happy?
#
# Naively, you do:
#    r = 1.0 - (c + k);
#    g = 1.0 - (m + k);
#    b = 1.0 - (y + k);

I already do that with setColorSpaceName.

# The better answer is to use ColorSync, which has all sorts of routines
# for converting pixels between different color spaces.

I've looking for an answer for quite some time, and I find
answers like 'that's a complicated subject discussed on
the web' or 'use Colorsync'. Problem I can never find actual
code to do this.

And since CoreImage can only operate on a fraction of the
images out there (especially small fraction with printers),
you would think Apple would somewhere give people a clear
code example that can convert planar or CMYK or indexed or
6/6/4 or whatever else into the 8 bit RGB(A) chunky that
CoreImage requires. The code I've managed to work out by
myself that discarded profiles and works on everything but
1 bps is

 NSSize imagesize = [rep size];
 NSRect imagerect = NSMakeRect(0,0,imagesize.width,imagesize.height);
 NSImage *contextimage = [[NSImage alloc] initWithSize: imagesize];
 [contextimage lockFocus];
   if ([[rep colorSpaceName] rangeOfString: @"RGB"].location==NSNotFound) {
     [rep setColorSpaceName: NSDeviceRGBColorSpace];
   }
   if ([rep hasAlpha]) [[NSColor clearColor] set]; else [[NSColor whiteColor] set];
   NSRectFill(imagerect);
   [rep drawInRect: imagerect];
   rep = [[NSBitmapImageRep alloc] initWithFocusedViewRect: imagerect];
 [contextimage unlockFocus];

So if anybody can point out the Colorsync calls that convert
to RGB with the image profile, I'd appreciate it.

# ColorSync is hardly something that Apple is trying to hide...

So where is it?

It's sort of like should I use this fancy new CoreVideo instead of
the old Quicktime to pick out PICT frames and convert to GIF? And the
guide points out this is one of the more frequent tasks. Which they
then follow with a hundred lines or so of code that I really don't
want to decipher.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
You face forward, or you face the possibility of shock and damage.