How do I convert from a CVPixelBufferRef to an openCV cv::Mat
I found the answer in some excellent GitHub source code. I adapted it here for simplicity. It also does the greyscale conversion for me.
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
OSType format = CVPixelBufferGetPixelFormatType(pixelBuffer);
// Set the following dict on AVCaptureVideoDataOutput's videoSettings to get YUV output
// @{ kCVPixelBufferPixelFormatTypeKey : kCVPixelFormatType_420YpCbCr8BiPlanarFullRange }
NSAssert(format == kCVPixelFormatType_420YpCbCr8BiPlanarFullRange, @"Only YUV is supported");
// The first plane / channel (at index 0) is the grayscale plane
// See more infomation about the YUV format
// http://en.wikipedia.org/wiki/YUV
CVPixelBufferLockBaseAddress(pixelBuffer, 0);
void *baseaddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0);
CGFloat width = CVPixelBufferGetWidth(pixelBuffer);
CGFloat height = CVPixelBufferGetHeight(pixelBuffer);
cv::Mat mat(height, width, CV_8UC1, baseaddress, 0);
// Use the mat here
CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
I am thinking that the best order will be:
- Convert to grayscale (since it is done almost automatically)
- Crop (this should be a fast operation and will reduce the number of pixels to work with)
- Scale down
- Equalize the histogram
I'm using this. My cv:Mat
is configured BGR(8UC3) colorFormat.
CVImageBufferRef -> cv::Mat
- (cv::Mat) matFromImageBuffer: (CVImageBufferRef) buffer {
cv::Mat mat ;
CVPixelBufferLockBaseAddress(buffer, 0);
void *address = CVPixelBufferGetBaseAddress(buffer);
int width = (int) CVPixelBufferGetWidth(buffer);
int height = (int) CVPixelBufferGetHeight(buffer);
mat = cv::Mat(height, width, CV_8UC4, address, 0);
//cv::cvtColor(mat, _mat, CV_BGRA2BGR);
CVPixelBufferUnlockBaseAddress(buffer, 0);
return mat;
}
cv::Mat -> CVImageBufferRef (CVPixelBufferRef)
- (CVImageBufferRef) getImageBufferFromMat: (cv::Mat) mat {
cv::cvtColor(mat, mat, CV_BGR2BGRA);
int width = mat.cols;
int height = mat.rows;
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
// [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
// [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
[NSNumber numberWithInt:width], kCVPixelBufferWidthKey,
[NSNumber numberWithInt:height], kCVPixelBufferHeightKey,
nil];
CVPixelBufferRef imageBuffer;
CVReturn status = CVPixelBufferCreate(kCFAllocatorMalloc, width, height, kCVPixelFormatType_32BGRA, (CFDictionaryRef) CFBridgingRetain(options), &imageBuffer) ;
NSParameterAssert(status == kCVReturnSuccess && imageBuffer != NULL);
CVPixelBufferLockBaseAddress(imageBuffer, 0);
void *base = CVPixelBufferGetBaseAddress(imageBuffer) ;
memcpy(base, mat.data, _mat.total()*4);
CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
return imageBuffer;
}