OpenGL，调色板纹理不起作用。 我在这里错过了什么？

Question

I am trying to create a texture which uses a predefined palette (think of the NES). However no matter what I try it does not seem to go the way I want it to.

The sources I have used are these:

Paletted textures

Answer to a related question

This is the (WIP) code I am using at the moment:

//UV and Vertex are already set-up and can confirm they work   

//A Sprite of 16 x 16 
for (int i = 0; i < 16 * 16; i++) {
        PixelMap[i] = 1; //indexed colour for green, as seen below
    }
    glGenTextures(1, &pixelmap_texture_buffer);
    glBindTexture(GL_TEXTURE_2D, pixelmap_texture_buffer);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, 16, 16, 0, GL_R8, GL_UNSIGNED_BYTE, PixelMap);

//Now to set up the palette
//Blue colour
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(255);
   pixelcanvas.push_back(255);

//Green colour
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(255);
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(255);

//Red colour
   pixelcanvas.push_back(255);
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(0);
   pixelcanvas.push_back(255);

//Palette with a width of 3 for the three colours
   glGenBuffers(1, &palette);
   glBindTexture(GL_TEXTURE_2D, palette);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
   glEnable(GL_BLEND);
   glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 3, 1, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixelcanvas.data());

Here is the GLSL code for the fragment shader:

#version 330 core
in vec2 UV;
uniform sampler2D myTextureSampler;
uniform sampler2D myColorTable;
void main()
{
    vec4 index = texture2D(myTextureSampler, UV);
    vec4 texel = texture2D(myColorTable, index.xy);
    gl_FragColor = texel;
};

The result: A blue texture. (ClearColor is set to purple)

It keeps using the first color of the palette. No matter which index color I set it to.

Moreover, the line:

glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, 16, 16, 0, GL_R8, L_UNSIGNED_BYTE, PixelMap);

Does not seem to be doing anything. I can leave it out and the result remains the same. As if GL_R8 is not supported by OpenGL 3.3. Going by my logic, it is supposed to be completely green. What am I missing here? Please let me know your thoughts.

I have tried changing OpenGL versions.

I have tried setting the Texture2D to Texture1D

and I have tried setting the uniform locations manually through:

GLuint unit = 0;

glActiveTexture(GL_TEXTURE0 + unit);
glBindTexture(GL_TEXTURE_2D, pixelmap_texture_buffer);
glUniform1i(glGetUniformLocation(shaderRef, "myTextureSampler"), unit);

unit = 1;

glActiveTexture(GL_TEXTURE0 + unit);
glBindTexture(GL_TEXTURE_2D, palette);
glUniform1i(glGetUniformLocation(shaderRef, "myColorTable"), unit);

Answer 1

When you use unsigned bytes to make a texture, 0 is treated as 0 and 255 is treated as 1. When you read the texture in the shader and the original byte value at that pixel was 255, the value you get is 1.0, not 255.0. Usually you just set the fragment colour to this value and it does what you expect, since fragment colour 1.0 becomes 255 when it gets written into the framebuffer.

In your pixelmap texture you use the byte value 1, that means that index.r is 1/255.0, about 0.0039. Then you use 0.0039 as the U coordinate in the colourmap texture, so it reads 1/255.0 of the way along the texture, that is 1.17% of the way into the first pixel, and the first pixel is red.

To get unchanged integer values you could make the pixelmap texture in GL_R8UI format, and sample it with a usampler2D which returns a uvec4. See here for an example using this texture format. This example does not implement a paletted texture.

Or - the lazy way - multiply the sampler output by 255.

To access the palette texture using integer coordinates, instead of floating-point coordinates 0-1, you can useimage load functions . The palette texture must be an image and not a sampler.

Or - the lazy way - you can take the palette index, add 0.5 (so you get the middle of the texel and no boundary artifacts) and divide by 3 (the width of the palette texture).

Also, your palette texture doesn't need to be 2D. It could be a 1D texture.