LDA loaded does not work

asked 2015-07-06 01:50:41 -0500

jackbrucesimspon gravatar image

updated 2015-07-06 19:52:35 -0500

I've trained my LDA on several thousand images and was keen to save it so that I could load it in the main program that would be doing predictions (rather than running it fresh each time). I knew that with an SVM I can use a command like this to save it:


So I tried a similar thing for LDA, and that also seemed to work:

cv::LDA lda(2);
lda.compute(trainData, trainLabels);
cv::Mat projected = lda.project(trainData);

Then, in my prediction program, I load the LDA and try to project the new data:

cv::LDA lda(2);
cv::Mat projected = lda.project(testData);

The loading part worked fine, however when I try to project the new data, I get this error:

OpenCV Error: Bad argument (Wrong shapes for given matrices. Was size(src) = (256,30), size(W) = (256,2).) in subspaceProject error: (-5) Wrong shapes for given matrices. Was size(src) = (256,30), size(W) = (256,2). in function subspaceProject

But that doesn't make sense to me: the data I trained the LDA on had 256 columns, it was grayscale pixel values. I did exactly the same processing for my test images as I did for my training images. If I don't bother about loading the LDA and just have the prediction stage in the same program it works perfectly (same code for processing the test images too!), so I can't understand why this error keeps occurring.

To clarify, I've been running std::cout on trainData and to confirm it has 256 cols and 30 rows but still get the error. This is driving me crazy because if I deliberately make testData an incorrectly sized matrix (not the 256 cols it's expecting) than I get the same answer!

image description

The main section of code

cv::Mat testData;

for (int i=0; i < 30; i++)
    std::string filename = "/Users/u5305887/Desktop/tags/test/";
    filename = filename + std::to_string(i);
    filename = filename + ".jpg";
    cv::Mat image = cv::imread(filename, 0);
    cv::Mat flat_image = image.clone().reshape(0, 1);

cv::LDA lda;


std::cout << "testData columns: " << testData.cols << std::endl;
std::cout << "testData rows: " << testData.rows << std::endl;

cv::Mat projected = lda.project(testData);

cv::Mat proj_float;
projected.convertTo(proj_float, CV_32FC1);
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please check your testData. it should have 256 cols (and any number of rows)

berak gravatar imageberak ( 2015-07-06 05:31:18 -0500 )edit

I've been doing std::cout for the number of cols and rows for trainData, I keep getting 256 for trainData.cols and 30 for trainData.rows. That's what I don't understand about the error.

jackbrucesimspon gravatar imagejackbrucesimspon ( 2015-07-06 06:49:45 -0500 )edit

But thank-you for the suggestion, it's something I should have been more explicit about and would have made a lot of sense given the error messages, which is why I'm finding this so confusing!

jackbrucesimspon gravatar imagejackbrucesimspon ( 2015-07-06 06:56:20 -0500 )edit

Do you happen to know what the error "Was size(src) = (256,30), size(W) = (256,2). in function subspaceProject" means?

jackbrucesimspon gravatar imagejackbrucesimspon ( 2015-07-06 07:03:06 -0500 )edit

berak said testData (not trainData), that is the data you are projecting in LDA (after loading the LDA)

thdrksdfthmn gravatar imagethdrksdfthmn ( 2015-07-06 09:06:45 -0500 )edit

your testdata seems to have 256 rows, and 30 cols. here's the code (lda.cpp, 178):

int d = src.cols;
// make sure the data has the correct shape
if(W.rows != d) {
    String error_message = format("Wrong shapes for given matrices. Was size(src) = (%d,%d), size(W) = (%d,%d).", src.rows, src.cols, W.rows, W.cols);
    CV_Error(Error::StsBadArg, error_message);
berak gravatar imageberak ( 2015-07-06 09:14:24 -0500 )edit

Apologies, I mixed myself up with variable names when I wrote the example and the variables in my code. I've pasted the main section of code into the question now, where I'm still getting that error even though my matrix is the right size.

jackbrucesimspon gravatar imagejackbrucesimspon ( 2015-07-06 19:57:45 -0500 )edit