Create a sub-image from context - ios

Question

I was wondering if there was a way to create a CGImage corresponding to a rectangle inside the context?
What I am doing right now:
I am using CGBitmapContextCreateImage to create a CGImage from a context. Then, I use CGImageCreateWithImageInRect to extract that sub-image.
Anil

Answer 1

Try this:
static CGImageRef createImageWithSectionOfBitmapContext(CGContextRef bigContext,
size_t x, size_t y, size_t width, size_t height)
{
uint8_t *data = CGBitmapContextGetData(bigContext);
size_t bytesPerRow = CGBitmapContextGetBytesPerRow(bigContext);
size_t bytesPerPixel = CGBitmapContextGetBitsPerPixel(bigContext) / 8;
data += x * bytesPerPixel + y * bytesPerRow;
CGContextRef smallContext = CGBitmapContextCreate(data,
width, height,
CGBitmapContextGetBitsPerComponent(bigContext), bytesPerRow,
CGBitmapContextGetColorSpace(bigContext),
CGBitmapContextGetBitmapInfo(bigContext));
CGImageRef image = CGBitmapContextCreateImage(smallContext);
CGContextRelease(smallContext);
return image;
}
or this:
static CGImageRef createImageWithSectionOfBitmapContext(CGContextRef bigContext,
size_t x, size_t y, size_t width, size_t height)
{
uint8_t *data = CGBitmapContextGetData(bigContext);
size_t bytesPerRow = CGBitmapContextGetBytesPerRow(bigContext);
size_t bytesPerPixel = CGBitmapContextGetBitsPerPixel(bigContext) / 8;
data += x * bytesPerPixel + y * bytesPerRow;
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, data,
height * bytesPerRow, NULL);
CGImageRef image = CGImageCreate(width, height,
CGBitmapContextGetBitsPerComponent(bigContext),
CGBitmapContextGetBitsPerPixel(bigContext),
CGBitmapContextGetBytesPerRow(bigContext),
CGBitmapContextGetColorSpace(bigContext),
CGBitmapContextGetBitmapInfo(bigContext),
provider, NULL, NO, kCGRenderingIntentDefault);
CGDataProviderRelease(provider);
return image;
}

Answer 2

You can create a cropped image as follows as mentioned here,
For eg:-
UIImage *image = //original image
CGRect rect = //cropped rect
CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], rect);
UIImage *img = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
You need to get the CGImage from context to use the above code to crop it. You can use CGBitmapContextCreateImage as mentioned in question. Here is the documentation.

Answer 3

You could create your CGBitmapContext with a buffer that you allocated, and create a CGImage from scratch using the same buffer. With the context and the image sharing a buffer, you can draw into the context and then create a CGImage with that section of the master image.
Note that if you draw into the same context afterward, the cropped image may actually pick up the changes (depending on just how much shared-referencing-instead-of-copying is going on internally). Depending on what you're doing, you may or may not find this desirable.