Implement shadow sampling in fragment shader

default-frag.glsl

@@ -1,6 +1,6 @@
 #version 450
 
-float ambient = 0.05;
+float ambient = 0.1;
 vec3 light_dir = normalize(vec3(2, 2, -1));
 vec3 base_colour = vec3(1.0, 0.94, 0.8);
 
@@ -13,7 +13,16 @@ layout(binding = 2) uniform sampler2DShadow shadow_map;
 
 void main()
 {
-	float lambertian = max(dot(frag_normal, light_dir), 0.0);
-	float intensity = min(lambertian + ambient, 1.0);
+	const float lambertian = max(dot(frag_normal, light_dir), 0.0);
+
+	const float shadow_bias = 0.005;
+	const vec3 proj_coords = light_pos.xyz / light_pos.w;
+	const vec3 shadow_map_pos
+		= vec3(0.5 * proj_coords.xy + 0.5, proj_coords.z);
+	float shadow = texture(
+		shadow_map,
+		vec3(shadow_map_pos.xy, shadow_map_pos.z - shadow_bias));
+
+	const float intensity = min(shadow * lambertian + ambient, 1.0);
 	out_colour = vec4(intensity * base_colour, 1.0);
 }