OpenCV iOS - Unsupported Parameter Combination - c++

Question

I'm using OpenCV to test image operations. But using the following method results in an error, which i can't explain to myself.
- (cv::Mat)cvMatGrayFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols = image.size.width;
CGFloat rows = image.size.height;
cv::Mat cvMat = cv::Mat(rows, cols, CV_8UC1); // 8 bits per component, 1 channel
CGContextRef contextRef = CGBitmapContextCreate(cvMat.data, // Pointer to backing data
cols, // Width of bitmap
rows, // Height of bitmap
8, // Bits per component
cvMat.step[0], // Bytes per row
colorSpace, // Colorspace
kCGImageAlphaNone |
kCGBitmapByteOrderDefault); // Bitmap info flags
CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
CGContextRelease(contextRef);
CGColorSpaceRelease(colorSpace);
return cvMat;
}
When i run this method, i get the following output into the xcode console
Aug 17 11:14:24 OpenCVDemo[1250] <Error>: CGBitmapContextCreate: unsupported parameter combination: set CGBITMAP_CONTEXT_LOG_ERRORS environmental variable to see the details
Aug 17 11:14:24 OpenCVDemo[1250] <Error>: CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
For example here is an method, which is working fine.
- (cv::Mat)cvMat3ChannelFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols = image.size.width;
CGFloat rows = image.size.height;
cv::Mat rgba(rows, cols, CV_8UC4, cvScalar(1,2,3,4)); // 8 bits per component, 4 channels
CGContextRef contextRef = CGBitmapContextCreate(rgba.data, // Pointer to backing data
cols, // Width of bitmap
rows, // Height of bitmap
8, // Bits per component
rgba.step[0], // Bytes per row
colorSpace, // Colorspace
kCGImageAlphaNoneSkipLast |
kCGBitmapByteOrderDefault); // Bitmap info flags
CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
CGContextRelease(contextRef);
cv::Mat bgr( rgba.rows, rgba.cols, CV_8UC3 );
cv::Mat alpha( rgba.rows, rgba.cols, CV_8UC1 );
cv::Mat out[] = { bgr, alpha };
// rgba[0] -> bgr[2], rgba[1] -> bgr[1],
// rgba[2] -> bgr[0], rgba[3] -> alpha[0]
int from_to[] = { 0,2, 1,1, 2,0, 3,3 };
mixChannels( &rgba, 1, out, 2, from_to, 4 );
return bgr;
}
I hope here is somebody who can explain, why the gray-method is not working.

Answer 1

- (cv::Mat)cvMat3ChannelFromUIImage:(UIImage *)image
{
UIImage *theImage = [UIImage imageWithData:UIImagePNGRepresentation(image)];
CGColorSpaceRef colorSpace = CGImageGetColorSpace(theImage.CGImage);
CGFloat cols = theImage.size.width;
CGFloat rows = theImage.size.height;
cv::Mat rgba(rows, cols, CV_8UC4, cvScalar(1,2,3,4)); // 8 bits per component, 4 channels
CGContextRef contextRef = CGBitmapContextCreate(rgba.data, // Pointer to backing data
cols, // Width of bitmap
rows, // Height of bitmap
8, // Bits per component
rgba.step[0], // Bytes per row
colorSpace, // Colorspace
kCGImageAlphaNoneSkipLast |
kCGBitmapByteOrderDefault); // Bitmap info flags
CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), theImage.CGImage);
CGContextRelease(contextRef);
cv::Mat bgr( rgba.rows, rgba.cols, CV_8UC3 );
cv::Mat alpha( rgba.rows, rgba.cols, CV_8UC1 );
cv::Mat out[] = { bgr, alpha };
// rgba[0] -> bgr[2], rgba[1] -> bgr[1],
// rgba[2] -> bgr[0], rgba[3] -> alpha[0]
int from_to[] = { 0,2, 1,1, 2,0, 3,3 };
mixChannels( &rgba, 1, out, 2, from_to, 4 );
return bgr;
}

Answer 2

//Mat Conversion
+ (cv::Mat)cvMatFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols,rows;
if (image.imageOrientation == UIImageOrientationLeft
|| image.imageOrientation == UIImageOrientationRight) {
cols = image.size.height;
rows = image.size.width;
}
else{
cols = image.size.width;
rows = image.size.height;
}
cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels
CGContextRef contextRef = CGBitmapContextCreate(cvMat.data, // Pointer to backing data
cols, // Width of bitmap
rows, // Height of bitmap
8, // Bits per component
cvMat.step[0], // Bytes per row
colorSpace, // Colorspace
kCGImageAlphaNoneSkipLast |
kCGBitmapByteOrderDefault);
CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
CGContextRelease(contextRef);
cv::Mat cvMatTest;
cv::transpose(cvMat, cvMatTest);
if (image.imageOrientation == UIImageOrientationLeft
|| image.imageOrientation == UIImageOrientationRight) {
}
else{
return cvMat;
}
cvMat.release();
cv::flip(cvMatTest, cvMatTest, 1);
return cvMatTest;
}
//Gray Conversion
+ (cv::Mat)cvMatGrayFromUIImage:(UIImage *)image
{
cv::Mat cvMat = [self cvMatFromUIImage:image];
cv::Mat grayMat;
if ( cvMat.channels() == 1 ) {
grayMat = cvMat;
}
else {
grayMat = cv :: Mat( cvMat.rows,cvMat.cols, CV_8UC1 );
cv::cvtColor( cvMat, grayMat, CV_BGR2GRAY );
}
return grayMat;
}
//Call the above method
cv::Mat grayImage = [self cvMatGrayFromUIImage:"your image"];