ZDoom

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//SimSun Shader by Cherno

const float pi = 3.14159265359;
vec4 Process(vec4 color)
{
	vec3 lightDir = vec3(0.75,-1.0,-0.5);
	//Doom map axis: x,z,y
	//with 1.0-1.0,-1.0, the light comes from the north-west-west diagonally downwards, if north is towards the top of the (auto or editor)map

	vec2 texCoord = gl_TexCoord[0].st;
	vec3 l = lightDir;
	vec3 n = normalize(vWorldNormal.xyz);
	float angle = acos
	(
		(l.x*n.x + l.y*n.y + l.z * n.z) 
		/ 
		(
			(	
				sqrt
				(
					(l.x*l.x)+(l.y*l.y)+(l.z*l.z)
				) 
				* 
				sqrt
				(
					(n.x*n.x) + (n.y*n.y) + (n.z*n.z)
				)
			)
		)
	);
	float lightLevel = angle;
	lightLevel /= pi;
	//from here on out, you have a lightLevel between 0.0 and 1.0, depending on the angle of the surface relative to lightDir.
	//a lightLevel of 0.5 results in the pixel at it would appear with no shading from SimSun at all.
	//you can add calculations to the lightvalue if you wish to increase, decrease, or shoft the effect.
	lightLevel += 0.25;//shifts the lightLevel so it gets slightly lighter, so there are less extremely dark areas
	return getTexel(texCoord) * color * vec4(lightLevel,lightLevel,lightLevel,1.0);
}

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vec3 lightDir = vec3(0.75,-1.0,0.5);

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lightLevel

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vec4 Process(vec4 color)
{
    vec3 lightDir = vec3(0.75,-1.0,0.5);

    vec2 texCoord = gl_TexCoord[0].st;
    vec3 l = normalize(lightDir);
    vec3 n = normalize(vWorldNormal.xyz);

    float lightLevel = acos(dot(l,n)) / 3.14159265359;

    return getTexel(texCoord) * vec4(color.rgb * lightLevel, color.a);
}