How to train an SVM with opencv based on a set of images?

Question

I have a folder of positives and another of negatives images in JPG format, and I want to train an SVM based on that images, I\'ve done the following but I receive an error:

Answer 1

Assuming that you know what you are doing by reshaping an image and using it to train SVM, the most probable cause of this is that your

Mat img = Highgui.imread(file.getAbsolutePath());

fails to actually read an image, generating a matrix img with null data property, which will eventually trigger the following in the OpenCV code:

// check parameter types and sizes
if( !CV_IS_MAT(train_data) || CV_MAT_TYPE(train_data->type) != CV_32FC1 )
    CV_ERROR( CV_StsBadArg, "train data must be floating-point matrix" );

Basically train_data fails the first condition (being a valid matrix) rather than failing the second condition (being of type CV_32FC1).

In addition, even though reshape works on the *this object, it acts like a filter and its effect is not permanent. If it's used in a single statement without immediately being used or assigned to another variable it will be useless. Change the following lines in your code:

img.reshape(1, 1);
trainingImages.push_back(img);

to:

trainingImages.push_back(img.reshape(1, 1));

Answer 2

Just as the error says, You need to change type of Your matrix, from integer type, probably CV_8U, to floating point one, CV_32F or CV_64F. To do it You can use cv::Mat::convertTo(). Here is a bit about depths and types of matrices.