Failed Assertion Using HOGDescriptor - opencv

Question

Ok, so I've decided that using a histogram of oriented gradients is a better method for image fingerprinting vs. creating a histogram of sobel derivatives. I think I finally have it mostly figured out but when I test my code I get the following:
OpenCV Error: Assertion failed ((winSize.width - blockSize.width) % blockStride.width == 0 && (winSize.height - blockSize.height) % blockStride.height == 0).
As of now I'm just trying to figure out how to compute the HOG correctly and see the results; but not visually, I just want some very basic output to see if the HOG was created. Then I'll figure out how to use it in image comparison.
Here is my sample code:
using namespace cv;
using namespace std;
int main(int argc, const char * argv[])
{
// Initialize string variables.
string thePath, img, hogSaveFile;
thePath = "/Users/Mikie/Documents/Xcode/images/";
img = thePath + "HDimage.jpg";
hogSaveFile = thePath + "HDimage.yml";
// Create mats.
Mat src;
// Load image as grayscale.
src = imread(img, CV_LOAD_IMAGE_GRAYSCALE);
// Verify source loaded.
if(src.empty()){
cout << "No image data. \n ";
return -1;
}else{
cout << "Image loaded. \n" << "Size: " << src.cols << " X " << src.rows << "." << "\n";
}
// Initialize float variables.
float imgWidth, imgHeight, newWidth, newHeight;
imgWidth = src.cols;
imgHeight = src.rows;
newWidth = 320;
newHeight = (imgHeight/imgWidth)*newWidth;
Mat dst = Mat::zeros(newHeight, newWidth, CV_8UC3);
resize(src, dst, Size(newWidth, newHeight), CV_INTER_LINEAR);
// Was resize successful?
if (dst.rows < src.rows && dst.cols < src.cols) {
cout << "Resize successful. \n" << "New size: " << dst.cols << " X " << dst.rows << "." << "\n";
} else {
cout << "Resize failed. \n";
return -1;
}
vector<float>theHOG(Mat dst);{
if (dst.empty()) {
cout << "Image lost. \n";
} else {
cout << "Setting up HOG. \n";
}
imshow("Image", dst);
bool gammaC = true;
int nlevels = HOGDescriptor::DEFAULT_NLEVELS;
Size winS(newWidth, newHeight);
// int block_size = 16;
// int block_stride= 8;
// int cell_size = 8;
int gbins = 9;
vector<float> descriptorsValues;
vector<Point> locations;
HOGDescriptor hog(Size(320, 412), Size(16, 16), Size(8, 8), Size(8, 8), gbins, -1, HOGDescriptor::L2Hys, 0.2, gammaC, nlevels);
hog.compute(dst, descriptorsValues, Size(0,0), Size(0,0), locations);
printf("descriptorsValues.size() = %ld \n", descriptorsValues.size()); //prints 960
for (int i = 0; i <descriptorsValues.size(); i++) {
cout << descriptorsValues[i] << endl;
}
}
cvWaitKey(0);
return 0;
}
As you can see, I messed around with different variables to define the sizes but to no avail so, I commented them out and tried manually setting them. Still nothing. What am I doing wrong? Any help will be greatly appreciated.
Thank you!

Answer 1

You are initializing the HOGDescriptor incorrectly.
The assertion states that each of the first three input parameters must satisfy the constraint:
(winSize - blockSize) % blockStride == 0
in both height and width dimensions.
The problem is that winSize.height does not satisfy this constraint, considering the other parameters you initialize hog with:
(412 - 16) % 8 = 4 //Problem!!
Probably the simplest fix is to increase your window dimensions from cv::Size(320,412) to something divisible by 8, perhaps cv::Size(320,416), but the specific size will depend on your specific requirements. Just pay attention to what the assertion is saying!