I have code that produces a PNG file from a cropped version of a bitmap that
can be 8-bit grayscale, 16-bit grayscale, 24-bit color, or 32-bit color+alpha.
It basically looks like this:
- (NSData *) getPNGCropped: (NSRect)cropRect
{
unsigned char* planes[5];
int offset = (int)cropRect.origin.y * _bytesPerRow +
(int)cropRect.origin.x * (_bitsPerPixel >> 3);
for (unsigned int i = 0; i < 5; ++i)
planes[i] = _imagePlanes[i] ? (_imagePlanes[i] + offset) : NULL;
int width = (int)cropRect.size.width;
int height = (int)cropRect.size.height;
NSBitmapImageRep* bits =
[[[NSBitmapImageRep alloc]
initWithBitmapDataPlanes: planes
pixelsWide: width
pixelsHigh: height
bitsPerSample: _bitsPerSample
samplesPerPixel: _samplesPerPixel
hasAlpha: _hasAlpha
isPlanar: _isPlanar
colorSpaceName: _colorSpace
bytesPerRow: _bytesPerRow
bitsPerPixel: _bitsPerPixel
] autorelease];
return [bits representationUsingType: NSPNGFileType properties: nil];
}
_planes[0] points to the mutableBytes of an NSMutableData and _planes[1 .. 4]
point to nil
The NSMutableData contains a bitmap that my code is continuously modifying.
Periodically the code makes an NSBitmapImageRep from this bitmap and displays
it or creates a PNG file. There is a companion method that creates an uncropped
NSBitmapImageRep, this method never crashes.
This cropping method crashes with SIGABRT in the representationUsingType method
call if the source bitmap is 8-bit grayscale and planes[0] is not aligned on an
8-byte boundary. For 16-bit grayscale or 24- or 32- bit color there does not
seem to be any alignment requirement. If the bitmap is 8-bit grayscale and
planes[0] is on an 8-byte boundary then it works fine.
This seems to be a specific problem with creating a PNG file. If I take the
same NSBitmapImageRep and drop it into a NSImage I can draw with it just fine.
If I change the file type to NSJPEGFileType it does not crash.
-- john
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