# How to implement OpenCV fisheye camera model in OpenGL shader to undistort image

I’m trying to implement the OpenCV fisheye camera model in OpenGL shader to increase the render speed, I’m following OpenCV’s documentation. But the result I get still has some distortion.

Here is the result I get from my OpenGL shader(don’t worry about the scale issue):

And here is the result I get from OpenCV initUndistortRectifyMap and remap with exactly the same camera matrix.

It’s easy to find out that the shader result was still distorted a little bit, and the distortion would be more visible if I continue processing the perspective transformation, the following is the comparison between OpenGL shader perspective result and OpenCV warpPerspective result.

The perspective transformation in my shader has proved to be correct by passing the OpenCV undistorted image to the shader. So the problem should be with the undistortion part in my shader, here is the undistortion part of my shader:

``````void main() {
float i = textureCoords.x * windowWidth;
float j = textureCoords.y * windowHeight;
float xc = (i - cameraMatrix[0][2]) / cameraMatrix[0][0];
float yc = (j - cameraMatrix[1][2]) / cameraMatrix[1][1];
float r = sqrt(xc*xc + yc*yc);
float degree = atan(r);
float rd = degree + distortVector[0]*pow(degree,3) + distortVector[1]*pow(degree,5) + distortVector[2]*pow(degree,7) + distortVector[3]*pow(degree,9);
float scale = rd / r;
float x = xc * scale;
float y = yc * scale;
float u = cameraMatrix[0][0]*(x+0.1*y) + cameraMatrix[0][2];
float v = cameraMatrix[1][1]*y + cameraMatrix[1][2];
vec2 coords = vec2(u/textureWidth, v/textureHeight);
if (coords.x>=0.0 && coords.x<=1.0 && coords.y>=0.0 && coords.y<=1.0) {
gl_FragColor = texture(inputTexture, coords);
} else {
gl_FragColor = vec4(0.0,0.0,0.0,0.0);
}
}
``````

Any ideas? please let me know

From the OP:

@maGnet2C-cmd Could you perhaps explain how you solved the issue with your camera matrix? I’m facing the same issue. Specifically I’m not sure which camera matrix to use. The one obtained by calibrating the distorted image, or the one obtained in OpenCV after undistorting the image.

Hello！I am running into a tough problem that i cant deal with. I am trying to follow your method to realize opencv’s warpperspective function in opengl fragment shadet. But i cant get expected result. Could u plz offer me a demo code?Thanks a.lot!

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