CocoaDev

Is it possible to sample a color in, for example, a tiff file, and get it’s Lab value? Specifically, I have an Lab tiff that contains an array of color patches. What I would like to be able to do is read in the tiff and obtain the L, a and b* values of each of the color squares. I can’t see how to do that with the NSColor methods I’m familiar with. I’m sure this is possible but I’m equally sure that I don’t know how to do it. Can anyone shed any light for me?

targetImage is an     NSImage.  I find that when I open an 8 bit Lab tiff (with the following code) the program exits (with the following log entry).

• (void)didEnd: (NSOpenPanel *)sheet returnCode: (int)code contextInfo: (void *)contextInfo { NSString *filename;

  if (code == NSOKButton) { NSLog(@”%@”, [sheet filename]); [targetImage initWithContentsOfFile: [sheet filename]]; NSLog(@”representations: %@”, [targetImage representations]); NSLog(@”targetImage = %@”, targetImage); if ([targetImage isValid]) { NSLog(@”%@ is valid”, [sheet filename]); } }

2006-06-04 05:32:13.440 LabTargetReader[1970] myOpen 2006-06-04 05:32:27.526 LabTargetReader[1970] /Users/samlowry/Desktop/DJ130 profile testing/ECI2002CMYK i1(A3)_1_2 LAB.tif 2006-06-04 05:32:28.018 LabTargetReader[1970] Bad colorspace name NSCustomColorSpace 2006-06-04 05:32:28.018 LabTargetReader[1970] Bad colorspace name NSCustomColorSpace 2006-06-04 05:32:28.018 LabTargetReader[1970] Image 0 has invalid parameters

LabTargetReader has exited with status -1.

From that code alone, you’re using NSImage completely wrong. It’s failing because your targetImage isn’t being initalized right. Try this instead:

targetImage = [[NSImage alloc] initWithContentsOfFile: [sheet filename]];

Thanks for your response. targetImage has been allocated earlier, in my awakeFromNib method. Is that not a good practice? In any case, it works just fine for RGB images.

My chief concern, however, is how to then convert the RGB data to Lab. Anyone have any insight?

Alloc/init must always be chained as above. It is acceptable to do the equivalent on separate lines, such as:

image = [NSImage alloc]; image = [image init…];

But there is no point to this and nobody does it. Widely separating your alloc/init is a bad idea, and failing to assign the value of init to the variable is an error.

So, is there no way to open a file in Lab color and sample it? I can now open an RGB file, scan it and sample the color squares. To get Lab values do I have to go through the Colorsync carbon APIs to make the conversion?

Is anyone here familiar with this sort of thing?

Thank you.