纹理中的OpenGL片段着色器

Question

I have a simple RGBA texture that I will project on a rectangle/quad in OpenGL.

However, I want to do some operations in the rgb pixels of that texture, eg, I want the displayed color to be some function of the RGB pixels of the original image.

My questions are: can I apply a fragment shader to a 2D texture? And if I can, How do I access the rgb value of the original texture image?

Any help would be appreciated.

Answer 1

You can certainly do this. Render the quad as usual and send the texture to the fragment shader as a sampler2D uniform.

Inside the fragment shader, you can then make calls to either texture or texelFetch . texture samples your texture, and texelFetch looks up an RGB texel from the texture without performing interpolation, etc.

Here is an example of what your fragment shader might look like:

Fragment shader

#version 330 core

uniform vec2 resolution;
uniform sampler2D myTexture;

out vec3 color;

void main()
{
    vec2 pos = gl_FragCoord.xy / resolution.xy;
    pos.y = resolution.y - pos.y;

    color = texture(myTexture, pos).xyz;
}

And then on the application side:

Initialization

glGenTextures(1, &m_textureID);
glBindTexture(GL_TEXTURE_2D, m_textureID);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, m_imageWidth, m_imageHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, pImage);

m_textureUniformID = glGetUniformLocation(m_programID, "myTexture");

Render loop

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, m_textureID);
glUniform1i(m_textureUniformID, 0);