[Solved] Applying Color Palette to Texture

You can do this with a texture lookup in a shader. Create a bunch of 1d texture containing your palettes. When you're drawing something bind the appropriate palette texture to another texture unit. Do a texture lookup on the greyscale texture to get a 0...1 result, then do a lookup on the palette texture using that result to get the final colour.

You can fine-tune the behaviour by e.g. selecting a GL_NEAREST sampling mode.

That's one way of doing it; there are others, such as loading your palette into an array of uniforms (or using a UBO) and using the greyscale lookup * 255 as an index into that.

This is called gradient mapping, L4D2 used it to create a great variation of zombies. Take a look at their publication Shading a Bigger, Better Sequel: Techniques in Left 4 Dead 2 from valve, which you can download here.

Thanks guys! I've made it work and thought I'd post the relevant code here to help anyone else trying to do this.

So I did basically exactly what mhagain suggested. I load both my sprite and palette as textures (both 2D textures for now) and then I have a fragment shader which gets the brightness value from my sprite and returns a color from the palette based on that value.

Fragment Shader:

uniform sampler2D texture;
uniform sampler2D palette;
void main()
{
vec4 textureColor = texture2D(texture, gl_TexCoord[0].xy).rgba;
vec3 paletteColor = texture2D(palette, vec2(textureColor.r, 0)).rgb;
gl_FragColor = vec4(paletteColor.r, paletteColor.g, paletteColor.b, textureColor.a);
}


In the above code only the red value of the texture color is used to look up the color in the palette. The reason for this is that the sprite is greyscaled, meaning that the R,G and B values are equal, and I felt it would be a waste to calculate the average (brightness) since it would give the same value. The gl_TexCoord[0] is set in the vertex shader which is quite simple as well.

Vertex Shader:

void main() {
gl_Position = ftransform();
gl_TexCoord[0] = gl_MultiTexCoord0;
}


Lastly I'll just include the Java code for actually using these shaders (excluding the loading/compiling part):


int paletteLocation = GL20.glGetUniformLocation(shaderProgram, "palette");
GL20.glUniform1i(paletteLocation, 1);
GL13.glActiveTexture(GL13.GL_TEXTURE1);
GL11.glBindTexture(GL11.GL_TEXTURE_2D, palette.getTextureID());


int textureLocation = GL20.glGetUniformLocation(shaderProgram, "texture");
GL20.glUniform1i(textureLocation, 0);
GL13.glActiveTexture(GL13.GL_TEXTURE0);
GL11.glBindTexture(GL11.GL_TEXTURE_2D, texture.getTextureID());

GL11.glBegin(GL11.GL_QUADS);
GL11.glTexCoord2f(0, 0);
GL11.glVertex2f(0, 0);
GL11.glTexCoord2f(1, 0);
GL11.glVertex2f(64, 0);
GL11.glTexCoord2f(1, 1);
GL11.glVertex2f(64, 128);
GL11.glTexCoord2f(0, 1);
GL11.glVertex2f(0, 128);
GL11.glEnd();


Just to summarize the above code: I first bind the palette texture to GL_TEXTURE1 and enable the shader to reference it using the variable name "palette". Then I do the same thing for the sprite texture except I use GL_TEXTURE0 and the variable name "texture". I then simply draw a quad with the sprite texture in immediate mode.

So thanks again for the help and I hope that this helps anyone else with a similar issue.