Problem with setting color in NSBitmapImageRep

FROM : Christian Kaiser
DATE : Mon Mar 31 21:30:43 2008

I'm struggling with setting color in a NSBitmapImageRep. I'm computing
the components of a NSColor according to a numeric value; its basically
a linear interpolation between all components of two NSColors. I've got
a function which is computing the new NSColor based on a given value. If
I call this function repeatedly using the same value and setting the
color using NSBitmapImageRep setColor:atX:y:, I expect to have a uniform
image if applying to all pixels. But this is not the case...! What am I
missing ?

As I'm probably a little bit confusing, here the code producing my
strange image :



    // Creating two colors which define a gradient. The color for our image
    // will be computed using this gradient.

    NSColor *lightYellow =
        [NSColor colorWithCalibratedRed:0.97 green:0.93 blue:0.65
alpha:1.0];
 
    NSColor *darkRed =
        [NSColor colorWithCalibratedRed:0.69 green:0.09 blue:0.11
alpha:1.0];


    // Creating an empty image with the same color space.

    NSBitmapImageRep *img = [[NSBitmapImageRep alloc]
        initWithBitmapDataPlanes:NULL
        pixelsWide:10
        pixelsHigh:10
        bitsPerSample:8
        samplesPerPixel:3
        hasAlpha:NO
        isPlanar:YES
        colorSpaceName:[lightYellow colorSpaceName]
        bitmapFormat:0
        bytesPerRow:0
        bitsPerPixel:0];
 

    // Assigning the same (???) color to all pixels.
    int x, y;
    for (y = 0; y < 10; y++)
    {
        for (x = 0; x < 10; x++)
        {
            NSColor *imgColor = [GradientColor gradientColorWithValue:40.0
                minColor:lightYellow maxColor:darkRed
                minValue:30.0 maxValue:60.0];
         
            [img setColor:imgColor atX:x y:y];
        }
    }
 
 
    [[img TIFFRepresentation]
        writeToFile:@"/Users/chkaiser/Desktop/img.tif" atomically:YES];


And the function computing the gradient color based on our values:

+(NSColor*)gradientColorWithValue:(double)value minColor:(NSColor*)c1
    maxColor:(NSColor*)c2 minValue:(double)v1 maxValue:(double)v2
{
 
    // Color components.
    CGFloat comps1[5];
    [c1 getComponents:comps1];
    CGFloat comps2[5];
    [c2 getComponents:comps2];
 
    // Compute the multiplication factor for the value. This factor has
    // a value between 0 and 1.
    double factor = (value - v1) / (v2 - v1);
 
    // Compute the new components.
    int i;
    CGFloat newComps[5];
    for (i = 0; i < [c1 numberOfComponents]; i++)
    {
        newComps[i] =
            (factor * (comps2[i] - comps1[i])) +
                comps1[i];
    }
 
    // Create the new color with the computed components.
    NSColor *newColor = [NSColor colorWithColorSpace:[c1 colorSpace]
        components:newComps count:[c1 numberOfComponents]];
 
    return newColor;
 
}


What is going wrong here ?

I have not found any similar topic on this list until now, even if it
seems to me quite strange. Is there something obvious I am missing ?

--Christian Kaiser