Template matching with Orb [closed]
[I also posted this on StackOverflow]
Hi,
I'm trying to detect whether a template image (logo) is present in a pdf document. The document can be either a scan encapsulated in a pdf or a "pure" pdf document, but this is completely random.
First, I convert the pdf document to a png image using ImageMagick's convert tool, then I cut the output images in half because they're so big, and after that I try to match a logo from a database with any of the shapes present in the half-cut image.
To do so, I use an Orb Feature Detector with an Orb Descriptor, and a RobustMatcher (sort of improved BruteForce matcher, source code available here). Here is a snippet of code from my adaptation of it :
// Read input images
Mat image1 = imread(argv[1], CV_LOAD_IMAGE_GRAYSCALE);
Mat image2 = imread(argv[2], CV_LOAD_IMAGE_GRAYSCALE);
if (!image1.data || !image2.data) {
std::cout << " --(!) Error reading images " << std::endl;
exit(1);
}
// Setting up values for ORB Detector
int nfeatures = 800;
//float scaleFactor = 1.10;
int nlevels = 8;
int edgeThreshold = 12;
int firstLevel = 0;
int WTA_K = 2;
int scoreType = 0;
int patchSize = 31;
// Prepare the matcher
RobustMatcher rmatcher;
rmatcher.setConfidenceLevel(0.98);
rmatcher.setMinDistanceToEpipolar(1.0);
rmatcher.setRatio(0.80f);
cv::Ptr<cv::FeatureDetector> pfd = new cv::OrbFeatureDetector(nfeatures, scaleFactor, nlevels, edgeThreshold, firstLevel, WTA_K, scoreType, patchSize);
rmatcher.setFeatureDetector(pfd);
cv::Ptr<cv::DescriptorExtractor> pde = new cv::OrbDescriptorExtractor();
rmatcher.setDescriptorExtractor(pde);
// Match the two images
std::vector<cv::DMatch> matches;
std::vector<cv::KeyPoint> keypoints1, keypoints2;
cv::Mat fundemental = rmatcher.match(image1, image2, matches, keypoints1, keypoints2);
// If nothing could be matched, stop here
if(matches.size() < 4){
exit(2);
}
The code works great on some examples that I chose carefully, with a highly-recognizable logo and a clean image, with certain proportions... etc. But when I try to apply the process to random pdf files, I start to get this error from OpenCV :
OpenCV Error: Assertion failed (type == src2.type() && src1.cols == src2.cols && (type == CV_32F || type == CV_8U)) in batchDistance, file /home/das/Downloads/opencv-2.4.5/modules/core/src/stat.cpp, line 1797 terminate called after throwing an instance of 'cv::Exception' what(): /home/das/Downloads/opencv-2.4.5/modules/core/src/stat.cpp:1797: error: (-215) type == src2.type() && src1.cols == src2.cols && (type == CV_32F || type == CV_8U) in function batchDistance
Aborted (core dumped)
I checked for this error, and it appeared that src1.cols != src2.cols, and a quick fix for it would be to test the condition before trying to match the images. The problem is that I miss a lot of images doing so, and this would be OK only if I were working on a video stream... but I'm not, and the next image has nothing in common with the previous one, and I can't determine whether my logo was present or not in the document.
Here is the code from stat.cpp, lines 1789 to 1826 : (assertion is at the beginning on line 1797)
void cv::batchDistance ...