Question

I am creating a monochrome image with the following code:

  CGColorSpaceRef cgColorSpace = CGColorSpaceCreateDeviceGray();
  CGImageRef cgImage = CGImageCreate (width, height, 1, 1, rowBytes, colorSpace, 0, dataProvider, decodeValues, NO, kCGRenderingIntentDefault);

where decodeValues is an array of 2 CGFloat‘s, equal to {0,1}. This gives me a fine image, but apparently my data (which comes from a PDF image mask) is black-on-white instead of white-on-black. To invert the image, I tried to set the values of decodeValues to {1,0}, but this did not change anything at all. Actually, whatever nonsensical values I put into decodeValues, I get the same image.

Why is decodeValues ignored here? How do I invert black and white?

Answer 1

here’s some code for creating and drawing a mono image. It’s the same as yours but with more context (and without necessary cleanup):

size_t width = 200;
size_t height = 200;
size_t bitsPerComponent = 1;
size_t componentsPerPixel = 1;
size_t bitsPerPixel = bitsPerComponent * componentsPerPixel;
size_t bytesPerRow = (width * bitsPerPixel + 7)/8;
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceGray();
CGBitmapInfo bitmapInfo = kCGImageAlphaNone;
CGFloat decode[] = {0.0, 1.0};
size_t dataLength = bytesPerRow * height;
UInt32 *bitmap = malloc( dataLength );
memset( bitmap, 255, dataLength );

CGDataProviderRef dataProvider = CGDataProviderCreateWithData( NULL, bitmap, dataLength, NULL);

CGImageRef cgImage = CGImageCreate (
                          width,
                          height,
                          bitsPerComponent,
                          bitsPerPixel,
                          bytesPerRow,
                          colorspace,
                          bitmapInfo,
                          dataProvider,
                          decode,
                          false,
                          kCGRenderingIntentDefault
                          );  

CGRect destRect = CGRectMake(0, 0, width, height);
CGContextDrawImage( context, destRect, cgImage );

If i change the decode array to CGFloat decode[] = {0.0, 0.0}; i always get a black image.
If you have tried that and it didn’t have any effect (you say you get the same image whatever values you use), either: you aren’t actually passing in those values but you think you are, or: somehow you aren’t actually examining the output of CGImageCreate.