View Full Version : Noise while mapping input RGB values using CPU and GPU

05-08-2017, 02:19 AM
I was trying to map the input image RGB value with gray-scale value using a mapping texture which looks like this:


So basically the idea is, we take `r` and `g` components of input pixel, then access the mapping using `r` component as column and `g` component as row. Using this convention would yield us a value in range `0-255` or `0.0 - 1.0` in context of OpenGL.

I tried to perform the same operation using GPU and CPU, but both outputs differ by a significant amount.

I am trying to Map `RGB` input pixel to some pre-defined palette, using the closest neighbour technique. The following problem arose performing an intermediate step in the solution. I would also like to know if there is a better way to solve the problem ?

GPU fragment shader:

#version 100
precision mediump float;
varying vec2 texture_mapping;
uniform sampler2D texture;
uniform sampler2D mapping;
void main() {
vec4 texel = texture2D(texture, texture_mapping);
gl_FragColor = texture2D(mapping, vec2(texel.r, texel.g));

CPU Code:

for (int row=0; row<originalImage.rows; row++) {
for (int col=0; col<originalImage.cols; col++) {
cv::Vec4b pix = originalImage.at<cv::Vec4b>(row, col);
cv::Point mappingPoint = cv::Point(pix.val[0], pix.val[1]);

cv::Vec4b mappedPixel = mapping.at<cv::Vec4b>(mappingPoint);
canvas.at<cv::Vec4b>(row, col) = mappedPixel;

I got Following results:

Input image:


GPU Output:


CPU Output: