Description

Shader code

shader_type canvas_item;

uniform sampler2D screen_texture: hint_screen_texture, filter_nearest;
// Use a Gradient1D
uniform sampler2D colours : source_color, filter_nearest;

float brightness(vec4 color) {
    return (color.r + color.g + color.b)/3.0;
}

void fragment() {
    vec4 color = texture(screen_texture, SCREEN_UV);
    COLOR = texture(colours, vec2(brightness(color)));
}