Game Maker - How to change color palette using shader and texture?

Question

How can I change the color of the application surface using a texture?

https://en.wikipedia.org/wiki/List_of_8-bit_computer_hardware_palettes

https://en.wikipedia.org/wiki/List_of_8-bit_computer_hardware_palettes#/media/File:CGA_palette_color_test_chart.png

My fragment shader looks like this:

varying vec2 v_vTexcoord;
varying vec4 v_vColour;

void main()
{
    gl_FragColor = v_vColour * texture2D( gm_BaseTexture, v_vTexcoord );
}

Answer 1

One way would be to have another 1D texture which holds your texture palette. You could then have a uniform which is an offset for the start of the palette. If you turn on texture wrapping, you can simply add the offset to the x coordinate when you sample the texture and you'll be able to rotate the palette.