Help with kCGImageAlphaPremultipliedLast raw data image access & change...

[cutmaster]

Hi,

I'm trying to create a method that must change (for this example) every black pixel with an alpha value > 0 to another color (lets say a blue one) but keeping the same alpha value.

Here's my code :

- (CGImageRef)ChangeColorOfImage:(CGImageRef)inImage {
    CGContextRef ctx;
    
    CFDataRef m_DataRef; 
    m_DataRef = CGDataProviderCopyData(CGImageGetDataProvider(inImage)); 
    UInt8 *pixels = (UInt8 * ) CFDataGetBytePtr(m_DataRef); 
    int length = CFDataGetLength(m_DataRef); 

    int r,g,b,a;
    for (int index = 0; index < length; index += 4) 
    { 
        r = pixels[index+0];
        g = pixels[index+1];
        b = pixels[index+2];
        a = pixels[index+3];
        
        if (r==0 && g==0 && b==0 && a>0) {
            pixels[index+0] = 0;
            pixels[index+1] = 128;
            pixels[index+2] = 255;
        }
    } 
    
    
    
    ctx = CGBitmapContextCreate(pixels, 
                                CGImageGetWidth(inImage), 
                                CGImageGetHeight(inImage), 
                                8, 
                                CGImageGetBytesPerRow(inImage), 
                                CGImageGetColorSpace(inImage), 
                                kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big); 


    CGImageRef imageRef = CGBitmapContextCreateImage (ctx); 
    CGContextRelease(ctx);
    
    return imageRef;
}

My problem is that, the resulted image is totally randomized in terms of color for each pixels !!



I presume that the keyword : kCGImageAlphaPremultipliedLast presume that the destination values must be computed in another way but I lose more than 3 hours searching the web without success

Can someone help me ?

Thanks a lot...

Laurent.

[cutmaster]

Okay, if it can help someone, here is the reason and its solution.
Importing PNG from Photoshop make them automatically using AlphaPreMultiplied mode...
To convert an ARGB value to the good expected premultiplied values, here's the part of code to change :

after the line
int r,g,b,a;
add :
float multiplier;

Then change the content of the portion code running inside the "if" with this :

        if (r==0 && g==0 && b==0 && a>0) {
            multiplier = (float)a/255.0f;
            pixels[index+0] = MAX(0,MIN(255,(int)(toRed*multiplier)));
            pixels[index+1] = MAX(0,MIN(255,(int)(toGreen*multiplier)));
            pixels[index+2] = MAX(0,MIN(255,(int)(toBlue*multiplier)));
        }

So if you understand, the multiplier value is the alpha value divided by 255 (in float), then you must multiply your color with this value, and ensure it'll not goes beyond the limits (0-255).

Bests.

Laurent.