Multiplying Mat matrices using reshape, Mat type issue in OpenCV

Question

I'm trying to implement color conversion from RGB-LMS and LMS-RGB back and using reshape for multiplication matrix, following answer from this question : Fastest way to apply color matrix to RGB image using OpenCV 3.0?

My ori Mat object is from an image with 3 channel (RGB), and I need to multiply them with matrix of 1 channel (lms), it seems like I have an issue with the matrix type. I've read reshape docs and questions related to this issue, like Issues multiplying Mat matrices, and I believe I have followed the instructions.

Here's my code : [UPDATED : Convert into flat image]

void test(const Mat &forreshape, Mat &output, Mat &pic, int rows, int cols)
{
    Mat lms(3, 3, CV_32FC3);
    Mat rgb(3, 3, CV_32FC3);
    Mat intolms(rows, cols, CV_32F);

    lms = (Mat_<float>(3, 3) << 1.4671, 0.1843, 0.0030,
                                3.8671, 27.1554, 3.4557,
                                4.1194, 45.5161 , 17.884 );
    /* switch the order of the matrix according to the BGR order of color on OpenCV */


    Mat transpose = (3, 3, CV_32F, lms).t();  // this will do transpose from matrix lms

    pic     = forreshape.reshape(1, rows*cols);
    Mat flatFloatImage;
    pic.convertTo(flatFloatImage, CV_32F);

    rgb         = flatFloatImag*transpose;
    output      = rgb.reshape(3, cols);
}

I define my Mat object, and I have converted it into float using convertTo

Mat ori = imread("ori.png", CV_LOAD_IMAGE_COLOR);
int rows = ori.rows;
int cols = ori.cols;

Mat forreshape;
ori.convertTo(forreshape, CV_32F);

Mat pic(rows, cols, CV_32FC3);
Mat output(rows, cols, CV_32FC3);

Error is :

OpenCV Error: Assertion failed (type == B.type() && (type == CV_32FC1 || type == CV_64FC1 || type == CV_32FC2 || type == CV_64FC2)) ,

so it's the type issue.

I tried to change all type into either 32FC3 of 32FC1, but doesn't seem to work. Any suggestion ?

Dmitrii Z. Dmitrii Z. · Accepted Answer · 2018-01-30T22:39:47

I believe what you need is to convert your input to a flat image and than multiply them

float lms [] = {1.4671, 0.1843, 0.0030,
                            3.8671, 27.1554, 3.4557,
                            4.1194, 45.5161 , 17.884};
Mat lmsMat(3, 3, CV_32F, lms );

Mat flatImage = ori.reshape(1, ori.rows * ori.cols);
Mat flatFloatImage;
flatImage.convertTo(flatFloatImage, CV_32F);
Mat mixedImage = flatFloatImage * lmsMat;
Mat output = mixedImage.reshape(3, imData.rows);

I might have messed up with lms matrix there, but I guess you will catch up from here.

Also see 3D matrix multiplication in opencv for RGB color mixing

EDIT: Problem with distortion is that you got overflow after float to 8U conversion. This would do the trick:

rgb         = flatFloatImage*transpose;
rgb.convertTo(pic, CV_32S);
output      = pic.reshape(3, rows)

Output: ;

Also I'm not sure but quick google search gives me different matrix for LMS see here. Also note that opencv stores colors in B-G-R format instead of RGB so change your mix mtraixes recordingly.

Multiplying Mat matrices using reshape, Mat type issue in OpenCV

1 Answers