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CIImageAccumulator mangling image data?
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CIImageAccumulator mangling image data?

Hi,

I have the following problem with CIImageAccumulator: when I set the image of such an accumulator to an image in any colour space different from CoreImage's internal colour space (say Generic RGB), and then extract it again, the pixel data are slightly different than those of the original image.

I've written a test program, reproduced below, to illustrate this problem. The program creates a CIImage containing 3 pixels, whose individual values are (in RGB, 8 bits, with alpha=255 in all cases): (1,2,3), (126,127,128) and (253,254,255). This image is then rendered into a bitmap context, and the pixel values are extracted from there. At this stage, they are equal to the input values. However, if I then store the CIImage inside an accumulator, extract the output image from that accumulator, and extract the pixel values from there, I now get (0,0,0), (125,126,128), (254,255,255). This close, but not equal, to the original data.

Here is what I think is happening: the accumulator stores its data internally in the linear variant of the Generic RGB colour space that is used by CoreImage. The problem I observe is due to rounding errors after the two-way conversion (non-linear -> linear -> non-linear).

Is that analysis right? If yes, does anybody know of a work-around? Ideally, I'd like to be able to specify the colour space to use for the accumulator, but there doesn't seem to be an API for that, unfortunately.

I'd be grateful for any help, thanks.
Michel.

Test program:

void showImagePixels(CIImage* image, CGColorSpaceRef colorSpace) {
unsigned char* data = malloc(3 * 8);
CGContextRef cgContext = CGBitmapContextCreate(data, 3,1,8,3*8,colorSpace,kCGImageAlphaPremultipliedFirst);
CIContext* context = [CIContext contextWithCGContext:cgContext options:[NSDictionary dictionary]];
[context drawImage:image atPoint:CGPointZero fromRect:[image extent]];
CGContextRelease(cgContext);

for (int i = 0; i < 3; ++i)
fprintf(stderr, "[= = =] ", data[i*4 + 1], data[i*4 + 2], data[i*4 + 3]);
fprintf(stderr, "\n");
free(data);
}

int main (int argc, const char * argv[]) {
  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

CGColorSpaceRef genColorSpace = CGColorSpaceCreateWithName (kCGColorSpaceGenericRGB);

unsigned char bitmapBytes[] = { 255,1,2,3, 255,126,127,128, 255,253,254,255 };
NSData* bitmapData = [NSData dataWithBytes:bitmapBytes length:sizeof(bitmapBytes)];
CIImage* image = [CIImage imageWithBitmapData:bitmapData bytesPerRow:3*8 size:CGSizeMake(3,1) format:kCIFormatARGB8 colorSpace:genColorSpace];

CIImageAccumulator* accu = [CIImageAccumulator imageAccumulatorWithExtent:[image extent] format:kCIFormatARGB8];
[accu setImage:image];

  showImagePixels(image, genColorSpace);
  showImagePixels([accu image], genColorSpace);

  [pool release];
  return 0;
}

Test program output:

[  1   2   3] [126 127 128] [253 254 255]
[  0   0   0] [125 126 128] [254 255 255]

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