Saving NSImage as PNG with no alpha and 5 bit colour

Question

I have an NSImage that I would like to save as a PNG, but remove the alpha channel and use 5 bit colour. I am currently doing this to create my PNG:

NSData *imageData = [image TIFFRepresentation];
NSBitmapImageRep *imageRep = [NSBitmapImageRep imageRepWithData:imageData];

NSDictionary *imageProps = nil;
imageData = [imageRep representationUsingType:NSPNGFileType properties:imageProps];

[imageData writeToFile:fileNameWithExtension atomically:YES];

I've read though lots of similar questions on SO but am confused as to the best/correct approach to use. Do I create a new CGGraphics context and draw into that? Can I create a new imageRep with these parameters directly? Any help, with a code snippet would be greatly appreciated.

Cheers

Dave

Answer 1

I did this in the end. Looks ugly and smells to me. Any better suggestions greatly appreciated.

// Create a graphics context (5 bits per colour, no-alpha) to render the tile
static int const kNumberOfBitsPerColour = 5;
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef tileGraphicsContext = CGBitmapContextCreate (NULL, rect.size.width, rect.size.height, kNumberOfBitsPerColour, 2 * rect.size.width, colorSpace, kCGBitmapByteOrder16Little | kCGImageAlphaNoneSkipFirst);

// Draw the clipped part of the image into the tile graphics context
NSData *imageData = [clippedNSImage TIFFRepresentation];
CGImageRef imageRef = [[NSBitmapImageRep imageRepWithData:imageData] CGImage];
CGContextDrawImage(tileGraphicsContext, rect, imageRef);

// Create an NSImage from the tile graphics context
CGImageRef newImage = CGBitmapContextCreateImage(tileGraphicsContext);
NSImage *newNSImage = [[NSImage alloc] initWithCGImage:newImage size:rect.size];

// Clean up
CGImageRelease(newImage);
CGContextRelease(tileGraphicsContext);
CGColorSpaceRelease(colorSpace);