Why not alway create a CGBitmapContext of the desired size and in a
supported pixel format (see <http://developer.apple.com/mac/library/qa/qa2001/qa1037.html
>), then call CGContextDrawImage to draw the CGImage into the
context? Then you're always controlling the parameters.
CGRect bounds = CGRectMake(0, 0, scaleFactorX * image.size.width,
scaleFactorY * image.size.height);
context = CGBitmapContextCreate(NULL,
bounds.size.width,
bounds.size.height,
8,
4 * bounds.size.width,
CGColorSpaceCreateDeviceRGB(),
kCGImageAlphaPremultipliedFirst);
CGContextDrawImage(context, bounds, image.CGImage);
On Nov 1, 2009, at 8:45 AM, thatsanicehatyouh...@mac.com wrote:
I have some code where I'm downloading images from the internet.
They're probably all going to be jpegs, but I'm not making that
assumption in my code. I have no control over the images. I'm
creating a CGBitmapContext to manipulate them. Further, I'm scaling
the image (could be up or down depending on the original size) by
creating the context with the size I want and drawing the original
image into that.
I'm trying to write code to avoid messages such as this:
<Error>: CGBitmapContextCreate: unsupported parameter combination: 8
integer bits/component; 16 bits/pixel; 1-component colorspace;
kCGImageAlphaPremultipliedFirst; 352 bytes/row.
I only rarely get these now, but I want to make that "never". I'm
pasting my code below that I have so far - can anyone suggest
something better? (I'm working on an iPhone if it makes any
difference.)
Cheers,
Demitri
size_t bitsPerComponent = CGImageGetBitsPerComponent(image.CGImage);
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
// The value returned by CGImageGetBytesPerRow might be smaller than
the final
// image if we are scaling to a larger size.
float contextWidth = (float)ceil((double)scaleX*image.size.width) *
4.0;
int bytesPerRow = (CGImageGetBytesPerRow(image.CGImage) >
contextWidth) ? CGImageGetBytesPerRow(image.CGImage)
: contextWidth;
context = CGBitmapContextCreate(NULL, // let the OS manage the
memory for me
// image
dimensions (desired width, desired height)
scaleFactorX*image.size.width, scaleFactorY*image.size.height,
bitsPerComponent,
bytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedFirst);
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