Cropping a Pixel Buffer - ios

Question

I am using the following to turn image data from the camera into a UIImage. In order to save some time, and memory, I'd like to crop the image data before I turn it into an UIImage.
Ideally I pass in a cropRect, and get back a cropped UIImage. However, since the camera output could be sized differently based on whether I am using a photo or video preset, I may not know what dimensions to use for the cropRect. I could use a cropRect, similar to the focus or exposure points, that uses a CGPoint between (0,0) and (1,1) and do similarly for the CGSizeof the cropRect. Or I can get the dimensions of the sampleBuffer, before I call the following, and pass in an appropriate cropRect. I'd like some advice as to which I should use.
I also would like to know how best to crop in order not to have to create an entire UIImage and then crop it back down. Typically, I am only interested in keeping about 10-20% of the pixels. I assume I have to iterate through the pixels, and start copying the cropRect into a different pixel buffer, until I have all the pixels I want.
And keep in mind that there is possible rotation happening according to orientation.
+ (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer orientation:(UIImageOrientation) orientation
{
// Create a UIImage from sample buffer data
// Get a CMSampleBuffer's Core Video image buffer for the media data
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
// Lock the base address of the pixel buffer
CVPixelBufferLockBaseAddress(imageBuffer, 0);
// Get the number of bytes per row for the pixel buffer
void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
// Get the number of bytes per row for the pixel buffer
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
// Get the pixel buffer width and height
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
// Create a device-dependent RGB color space
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
// Create a bitmap graphics context with the sample buffer data
CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8,
bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
// Create a Quartz image from the pixel data in the bitmap graphics context
CGImageRef quartzImage = CGBitmapContextCreateImage(context);
// Unlock the pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
// Free up the context and color space
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
// Create an image object from the Quartz image
UIImage *image = [UIImage imageWithCGImage:quartzImage scale:(CGFloat)1.0 orientation:orientation];
// Release the Quartz image
CGImageRelease(quartzImage);
return (image);
}
In summary:
Should I pass in a cropRect which specifies a rect between (0,0,0,0) and (1,1,1,1) or do I pass in a cropRect that specifies exact pixel locations like (50,50,100,100)?
How best do I crop the pixel buffer?

Answer 1

I think you should use pixel as cropRect, as you have to convert the float-values to pixel-values at least at some point.
The following code is not tested, but should give you the idea.
CGRect cropRect = CGRectMake(50, 50, 100, 100); // cropRect
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVReturn lock = CVPixelBufferLockBaseAddress(pixelBuffer, 0);
if (lock == kCVReturnSuccess) {
int w = 0;
int h = 0;
int r = 0;
int bytesPerPixel = 0;
unsigned char *buffer;
w = CVPixelBufferGetWidth(pixelBuffer);
h = CVPixelBufferGetHeight(pixelBuffer);
r = CVPixelBufferGetBytesPerRow(pixelBuffer);
bytesPerPixel = r/w;
buffer = CVPixelBufferGetBaseAddress(pixelBuffer);
UIGraphicsBeginImageContext(cropRect.size); // create context for image storage, use cropRect as size
CGContextRef c = UIGraphicsGetCurrentContext();
unsigned char* data = CGBitmapContextGetData(c);
if (data != NULL) {
// iterate over the pixels in cropRect
for(int y = cropRect.origin.y, yDest = 0; y<CGRectGetMaxY(cropRect); y++, yDest++) {
for(int x = cropRect.origin.x, xDest = 0; x<CGRectGetMaxX(cropRect); x++, xDest++) {
int offset = bytesPerPixel*((w*y)+x); // offset calculation in cropRect
int offsetDest = bytesPerPixel*((cropRect.size.width*yDest)+xDest); // offset calculation for destination image
for (int i = 0; i<bytesPerPixel; i++) {
data[offsetDest+i] = buffer[offset+i];
}
}
}
}
UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}