manim/manimlib/shaders/textured_surface_frag.glsl

#version 330

uniform sampler2D LightTexture;
uniform sampler2D DarkTexture;
uniform float num_textures;
uniform vec3 light_source_position;

in vec3 xyz_coords;
in vec3 v_normal;
in vec2 v_im_coords;
in float v_opacity;
in float v_gloss;
in float v_shadow;

out vec4 frag_color;

#INSERT add_light.glsl

const float dark_shift = 0.2;

void main() {
    vec4 color = texture(LightTexture, v_im_coords);
    if(num_textures == 2.0){
        vec4 dark_color = texture(DarkTexture, v_im_coords);
        float dp = dot(
            normalize(light_source_position - xyz_coords),
            normalize(v_normal)
        );
        float alpha = smoothstep(-dark_shift, dark_shift, dp);
        color = mix(dark_color, color, alpha);
    }

    frag_color = add_light(
        color,
        xyz_coords,
        normalize(v_normal),
        light_source_position,
        v_gloss,
        v_shadow
    );
    frag_color.a = v_opacity;
}