Setting Device Independent RGB on iOS

I am detecting the rgb of a tapped pixel. Different iPads return slightly different RGB values. I maintain a plist of the different values returned per device and when the app opens it determines the device I am on and uses appropriate values. This is a terrible solution - but it does work.

I now want to fix this properly so I dived into colorspace on iOS. It seems I can use CGColorSpaceCreateCalibratedRGB to set a standard RGB regardless of device so the values returned are the same? Or do I need to convert the

However, I do not know any of the values needed to create a standard color space across devices so my pixel color return values are always the same - or if this is possible.

Some current example return values: iPad 2 r31 g0 b133 a1 iPad Air r30 g0 b132 a1

Can anyone help 'normalize' the pixel return value in a device independent way?

- (UIColor*) getPixelColorAtLocation:(CGPoint)point {
UIColor* color = nil;
CGImageRef inImage = self.image.CGImage;
// Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpha, Red, Green, Blue
CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
if (cgctx == NULL) { return nil; /* error */ }

size_t w = CGImageGetWidth(inImage);
size_t h = CGImageGetHeight(inImage);
CGRect rect = {{0,0},{w,h}}; 

// Draw the image to the bitmap context. Once we draw, the memory 
// allocated for the context for rendering will then contain the 
// raw image data in the specified color space.
CGContextDrawImage(cgctx, rect, inImage); 

// Now we can get a pointer to the image data associated with the bitmap
// context.
unsigned char* data = CGBitmapContextGetData (cgctx);
if (data != NULL) {
    //offset locates the pixel in the data from x,y. 
    //4 for 4 bytes of data per pixel, w is width of one row of data.
    int offset = 4*((w*round(point.y))+round(point.x));
    int alpha =  data[offset]; 
    int red = data[offset+1]; 
    int green = data[offset+2]; 
    int blue = data[offset+3]; 
    ////NSLog(@"offset: %i colors: RGB A %i %i %i  %i",offset,red,green,blue,alpha);
    //NSLog(@"colors: RGB A %i %i %i  %i",red,green,blue,alpha);
    color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
}


// When finished, release the context
CGContextRelease(cgctx); 
// Free image data memory for the context
if (data) { free(data); }

return color;

}

• (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {

  CGContextRef context = NULL; CGColorSpaceRef colorSpace; void * bitmapData; int bitmapByteCount; int bitmapBytesPerRow;

  // Get image width, height. We'll use the entire image. size_t pixelsWide = CGImageGetWidth(inImage); size_t pixelsHigh = CGImageGetHeight(inImage);

  // Declare the number of bytes per row. Each pixel in the bitmap in this // example is represented by 4 bytes; 8 bits each of red, green, blue, and // alpha. bitmapBytesPerRow = (pixelsWide * 4); bitmapByteCount = (bitmapBytesPerRow * pixelsHigh);

  // Use the generic RGB color space. colorSpace = CGColorSpaceCreateDeviceRGB();

  if (colorSpace == NULL) { fprintf(stderr, "Error allocating color space\n"); return NULL; }

  // Allocate memory for image data. This is the destination in memory // where any drawing to the bitmap context will be rendered. bitmapData = malloc( bitmapByteCount ); if (bitmapData == NULL) { fprintf (stderr, "Memory not allocated!"); CGColorSpaceRelease( colorSpace ); return NULL; }

  // Create the bitmap context. We want pre-multiplied ARGB, 8-bits // per component. Regardless of what the source image format is // (CMYK, Grayscale, and so on) it will be converted over to the format // specified here by CGBitmapContextCreate. context = CGBitmapContextCreate (bitmapData, pixelsWide, pixelsHigh, 8, // bits per component bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst); if (context == NULL) { free (bitmapData); fprintf (stderr, "Context not created!"); }

  // Make sure and release colorspace before returning CGColorSpaceRelease( colorSpace );

  return context; }