vImage not putting channels correctly back together - objective-c

Question

I tried to extract all 3 channels from an image with vImageConvert_RGB888toPlanar8 and then put them back together with vImageConvert_Planar8toRGB888 but the image gets totally messed up. Why is that?
vImage_Buffer blueBuffer;
blueBuffer.data = (void*)blueImageData.bytes;
blueBuffer.width = size.width;
blueBuffer.height = size.height;
blueBuffer.rowBytes = [blueImageData length]/size.height;
vImage_Buffer rBuffer;
rBuffer.width = size.width;
rBuffer.height = size.height;
rBuffer.rowBytes = size.width;
void *rPixelBuffer = malloc(size.width * size.height);
if(rPixelBuffer == NULL)
{
NSLog(#"No pixelbuffer");
}
rBuffer.data = rPixelBuffer;
vImage_Buffer gBuffer;
gBuffer.width = size.width;
gBuffer.height = size.height;
gBuffer.rowBytes = size.width;
void *gPixelBuffer = malloc(size.width * size.height);
if(gPixelBuffer == NULL)
{
NSLog(#"No pixelbuffer");
}
gBuffer.data = gPixelBuffer;
vImage_Buffer bBuffer;
bBuffer.width = size.width;
bBuffer.height = size.height;
bBuffer.rowBytes = size.width;
void *bPixelBuffer = malloc(size.width * size.height);
if(bPixelBuffer == NULL)
{
NSLog(#"No pixelbuffer");
}
bBuffer.data = bPixelBuffer;
vImageConvert_RGB888toPlanar8(&blueBuffer, &rBuffer, &gBuffer, &bBuffer, kvImageNoFlags);
size_t destinationImageBytesLength = size.width*size.height*3;
const void* destinationImageBytes = valloc(destinationImageBytesLength);
NSData* destinationImageData = [[NSData alloc] initWithBytes:destinationImageBytes length:destinationImageBytesLength];
vImage_Buffer destinationBuffer;
destinationBuffer.data = (void*)destinationImageData.bytes;
destinationBuffer.width = size.width;
destinationBuffer.height = size.height;
destinationBuffer.rowBytes = [destinationImageData length]/size.height;
vImage_Error result = vImageConvert_Planar8toRGB888(&rBuffer, &gBuffer, &bBuffer, &destinationBuffer, 0);
NSImage* image = nil;
if(result == kvImageNoError)
{
//TODO: If you need color matching, use an appropriate colorspace here
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((__bridge CFDataRef)(destinationImageData));
CGImageRef finalImageRef = CGImageCreate(size.width, size.height, 8, 24, destinationBuffer.rowBytes, colorSpace, kCGBitmapByteOrder32Big|kCGImageAlphaNone, dataProvider, NULL, NO, kCGRenderingIntentDefault);
CGColorSpaceRelease(colorSpace);
CGDataProviderRelease(dataProvider);
image = [[NSImage alloc] initWithCGImage:finalImageRef size:NSMakeSize(size.width, size.height)];
CGImageRelease(finalImageRef);
}
free((void*)destinationImageBytes);
return image;

Answer 1

Working with vImage means to work with pixels only. So you must never use the size of an image (or imageRep), you only use pixelsWide and pixelsHigh. Replace all size.width with pixelsWide and all size.height with pixelsHigh. Apple has example code for vImage and they use size values! Don't believe them! Not all Apple example codes are correct.
The size of an image or imageRep determines how big an image shall be drawn on the screen (or a printer). Size values have the dimension of a length and the units are meter, cm, inch or (as in Cocoa) 1/72 inch aka point. They are represented as float values.
PixelsWide and pixelsHigh have no dimension and no unit (they are simply numbers) and are represented as int values.
There may be more bugs in your code, but the first step should be to replace all size values.

Answer 2

Strictly speaking, you want kCGBitmapByteOrderDefault instead of kCGBitmapByteOrder32Big. 32Big doesn't make much sense for a 24 bit pixel format.
This seems like a weak link:
destinationBuffer.rowBytes = [destinationImageData length]/size.height;
Check to see it is the right number.
A picture of the output would help diagnose the problem. Also check the console to see if CG is giving you any spew. Did you try vImageCreateCGImageFromBuffer() with kvImagePrintDiagnosticsToConsole to see if it has anything to say?

Answer 3

Working with vImage means to work with pixels only. So you must never use the size of an image (or imageRep), you only use pixelsWide and pixelsHigh. Replace all size.width with pixelsWide and all size.height with pixelsHigh. Apple has example code for vImage and they use size values! Don't believe them! Not all Apple example codes are correct.
The size of an image or imageRep determines how big an image shall be drawn on the screen (or a printer). Size values have the dimension of a length and the units are meter, cm, inch or (as in Cocoa) 1/72 inch aka point. They are represented as float values.
PixelsWide and pixelsHigh have no dimension and no unit (they are simply numbers) and are represented as int values.
There may be more bugs in your code, but the first step should be to replace all size values.