objective-ccocoansimagecgimageref

Creating NSImage from CGImageRef causes image pixel size to double

capture is a CGImageRef returned from a call to CGWindowListCreateImage(). When I try to turn it into an NSImage directly via initWithCGImage:size: it mysteriously doubles in size. If I instead manually create an NSBitmapImageRep from capture and then add it to an empty NSImage everything works ok.

My hardware setup is a retina MBP + non-retina external display. The capture is taking place on the non-retina screen.

NSLog(@"capture image size: %d %d", CGImageGetWidth(capture), CGImageGetHeight(capture));
NSLog(@"logical image size: %f %f", viewRect.size.width, viewRect.size.height);

NSBitmapImageRep *debugRep;
NSImage *image;

//
// Create NSImage directly

image = [[NSImage alloc] initWithCGImage:capture size:NSSizeFromCGSize(viewRect.size)];

debugRep = [[image representations] objectAtIndex:0];
NSLog(@"pixel size, NSImage direct: %d %d", debugRep.pixelsWide, debugRep.pixelsHigh);

//
// Create representation manually

NSBitmapImageRep *imageRep = [[NSBitmapImageRep alloc] initWithCGImage:capture];
image = [[NSImage alloc] initWithSize:NSSizeFromCGSize(viewRect.size)];
[image addRepresentation:imageRep];
[imageRep release];

debugRep = [[image representations] objectAtIndex:0];
NSLog(@"pixel size, NSImage + manual representation: %d %d", debugRep.pixelsWide, debugRep.pixelsHigh);

Log output:

capture image size: 356 262
logical image size: 356.000000 262.000000
pixel size, NSImage direct: 712 524
pixel size, NSImage + manual representation: 356 262

Is this expected behaviour?

Solution

  • The documentation for initWithCGImage:size: states:

    You should not assume anything about the image, other than that drawing it is equivalent to drawing the CGImage.

    In the end I just continued on working with NSBitmapImageRep instances directly.