Draw standard NSImage inverted (white instead of black)

Question

I'm trying to draw a standard NSImage in white instead of black. The following works fine for drawing the image in black in the current NSGraphicsContext:

NSImage* image = [NSImage imageNamed:NSImageNameEnterFullScreenTemplate];
[image drawInRect:r fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:1.0];

I expected NSCompositeXOR to do the trick, but no. Do I need to go down the complicated [CIFilter filterWithName:@"CIColorInvert"] path? I feel like I must be missing something simple.

Anders

Answer 1

The Core Image route would be the most reliable. It's actually not very complicated, I've posted a sample below. If you know none of your images will be flipped then you can remove the transform code. The main thing to be careful of is that the conversion from NSImage to CIImage can be expensive performance-wise, so you should ensure you cache the CIImage if possible and don't re-create it during each drawing operation.

CIImage* ciImage = [[CIImage alloc] initWithData:[yourImage TIFFRepresentation]];
if ([yourImage isFlipped])
{
    CGRect cgRect    = [ciImage extent];
    CGAffineTransform transform;
    transform = CGAffineTransformMakeTranslation(0.0,cgRect.size.height);
    transform = CGAffineTransformScale(transform, 1.0, -1.0);
    ciImage   = [ciImage imageByApplyingTransform:transform];
}
CIFilter* filter = [CIFilter filterWithName:@"CIColorInvert"];
[filter setDefaults];
[filter setValue:ciImage forKey:@"inputImage"];
CIImage* output = [filter valueForKey:@"outputImage"];
[output drawAtPoint:NSZeroPoint fromRect:NSRectFromCGRect([output extent]) operation:NSCompositeSourceOver fraction:1.0];

Note: release/retain memory management is left as an exercise, the code above assumes garbage collection.

If you want to render the image at an arbitrary size, you could do the following:

NSSize imageSize = NSMakeSize(1024,768); //or whatever size you want
[yourImage setSize:imageSize];
[yourImage lockFocus];
NSBitmapImageRep* bitmap = [[NSBitmapImageRep alloc] initWithFocusedViewRect:NSMakeRect(0, 0, imageSize.width, imageSize.height)];
[yourImage unlockFocus];
CIImage* image = [CIImage imageWithData:[bitmap TIFFRepresentation]];

Answer 2

Just one note: I found that CIColorInvert filter isn't always reliable. For example, if you want to invert back an image inverted in Photoshop, the CIFilter will produce a much lighter image. As far as I understood, it happens because of the differences in gamma value of CIFilter (gamma is 1) and images that came from other sources.

While I was looking for ways to change the gamma value for CIFilter, I found a note that there's a bug in CIContext: changing its gamma value from the default 1 will produce unpredictable results.

Regardless, there's another solution to invert NSImage, which always produces the correct results - by inverting pixels of NSBitmapImageRep.

I'm reposting the code from etutorials.org (http://bit.ly/Y6GpLn):

// srcImageRep is the NSBitmapImageRep of the source image
int n = [srcImageRep bitsPerPixel] / 8;           // Bytes per pixel
int w = [srcImageRep pixelsWide];
int h = [srcImageRep pixelsHigh];
int rowBytes = [srcImageRep bytesPerRow];
int i;

NSImage *destImage = [[NSImage alloc] initWithSize:NSMakeSize(w, h)];
NSBitmapImageRep *destImageRep = [[[NSBitmapImageRep alloc] 
      initWithBitmapDataPlanes:NULL
          pixelsWide:w
          pixelsHigh:h
          bitsPerSample:8
          samplesPerPixel:n
          hasAlpha:[srcImageRep hasAlpha] 
          isPlanar:NO
          colorSpaceName:[srcImageRep colorSpaceName]
          bytesPerRow:rowBytes 
          bitsPerPixel:NULL] autorelease];

unsigned char *srcData = [srcImageRep bitmapData];
unsigned char *destData = [destImageRep bitmapData];

for ( i = 0; i < rowBytes * h; i++ )
    *(destData + i) = 255 - *(srcData + i);

[destImage addRepresentation:destImageRep];