Getting exact pixel from texture

Question

I have a question about textures in OpenGL. I am trying to use them for GPGPU operations but I am stuck at beggining. I have created a texture like this (4x4 int matrix).

OGLTexImageFloat dataTexImage = new OGLTexImageFloat(4, 4, 4);
dataTexImage.setPixel(0, 0, 0, 0);
dataTexImage.setPixel(0, 1, 0, 10);
dataTexImage.setPixel(0, 2, 0, 5);
dataTexImage.setPixel(0, 3, 0, 15);
dataTexImage.setPixel(1, 0, 0, 10);
dataTexImage.setPixel(1, 1, 0, 0);
dataTexImage.setPixel(1, 2, 0, 2);
dataTexImage.setPixel(1, 3, 0, 1000);
dataTexImage.setPixel(2, 0, 0, 5);
dataTexImage.setPixel(2, 1, 0, 2);
dataTexImage.setPixel(2, 2, 0, 0);
dataTexImage.setPixel(2, 3, 0, 2);
dataTexImage.setPixel(3, 0, 0, 15);
dataTexImage.setPixel(3, 1, 0, 1000);
dataTexImage.setPixel(3, 2, 0, 2);
dataTexImage.setPixel(3, 3, 0, 0);
texture = new OGLTexture2D(gl, dataTexImage);

Now I would like to add value from [1,1] matrix position to value of each pixel (matrix entry). As I am speaking about every picture I should probably do it in fragment shader. But i dont know how can i get exact pixel form texture ([1,1] entry from matrix). Can someone explain me, how to do this?

Answer 1

If you are trying to add a single constant value (i.e. a value from [1,1]) to the entire image (every pixel of the rendered image), then you should pass that constant value as a separate uniform value into your shader program.

Then in the fragment shader, add this constant value to the current pixel color. The current pixel color comes as an input vec4 from your vertex shader.