OpenCV displaying a 2-channel image (optical flow)

Question

I have optical flow stored in a 2-channel 32F matrix. I want to visualize the contents, what\'s the easiest way to do this?

How do I convert a CV_32FC2 to R

Answer 1

The MPI Sintel Dataset provides C and MatLab code for visualizing computed flow. Download the ground truth optical flow of the training set from here. The archive contains a folder flow_code containing the mentioned source code.

You can port the code to OpenCV, however, I wrote a simple OpenCV wrapper to easily use the provided code. Note that the method MotionToColor is taken from the color_flow.cpp file. Note the comments in the listing below.

// Important to include this before flowIO.h!
#include "imageLib.h"
#include "flowIO.h"
#include "colorcode.h"
// I moved the MotionToColor method in a separate header file.
#include "motiontocolor.h"

cv::Mat flow;
// Compute optical flow (e.g. using OpenCV); result should be
// 2-channel float matrix.

assert(flow.channels() == 2);
// assert(flow.type() == CV_32F);

int rows = flow.rows;
int cols = flow.cols;

CFloatImage cFlow(cols, rows, 2);

// Convert flow to CFLoatImage:
for (int i = 0; i < rows; i++) {
    for (int j = 0; j < cols; j++) {
        cFlow.Pixel(j, i, 0) = flow.at(i, j)[0];
        cFlow.Pixel(j, i, 1) = flow.at(i, j)[1];
    }
}

CByteImage cImage;
MotionToColor(cFlow, cImage, max);

cv::Mat image(rows, cols, CV_8UC3, cv::Scalar(0, 0, 0));

// Compute back to cv::Mat with 3 channels in BGR:
for (int i = 0; i < rows; i++) {
    for (int j = 0; j < cols; j++) {
        image.at(i, j)[0] = cImage.Pixel(j, i, 0);
        image.at(i, j)[1] = cImage.Pixel(j, i, 1);
        image.at(i, j)[2] = cImage.Pixel(j, i, 2);
    }
}

// Display or output the image ...

Below is the result when using the Optical Flow code and example images provided by Ce Liu.