uniform vec2 resolution;
void main(void)
{
//orthographic matrix
mat4 ortho_matrix = mat4(
2.0/resolution.x, 0, 0, 0,
0, 2.0/-resolution.y, 0, 0,
0, 0, -1, 0,
-1, 1, 0, 1
);
//surface normal of the 2D plane (looking straight down)
vec3 surface_normal = normalize(mix(vec3(-1), vec3(1), vec3(0.5, 0.5, 1.0)));
//screen position of the light
vec2 light_screen_pos = vec2(650, 150);
//translate light's position to normalized coordinates
//the z value makes sure it is slightly above the 2D plane
vec4 light_ortho_pos = ortho_matrix * vec4(light_screen_pos, -0.03, 1.0);
//calculate the light for this fragment
vec3 light_direction = light_ortho_pos.xyz - vec3(gl_FragCoord.x / resolution.x, gl_FragCoord.y / resolution.y, 0);
float dist = length(light_direction);
light_direction = normalize(light_direction);
vec3 light = clamp(dot(surface_normal, light_direction), 0.0, 1.0) * vec3(0.5, 0.5, 0.5);
vec3 cel_light = step(0.15, (light.r + light.g + light.b) / 3.0) * light;
gl_FragColor = vec4(pow(light + cel_light, vec3(0.4545)), 1.0);
}