Faux Vertex Lighting

This is a novel attempt at lighting that is identical to the real thing with a couple of added benefits for lazy people like me. I made it a couple of years ago for a project that I don’t have time for, so I’m publishing it as-is in case someone would find it useful.

Here are some features:

• Utilizes scene lights
• Simulates subdivided surfaces for more detailed lighting without the need for more vertices
  • Subdivisions are not constrained to triangle boundaries, so they aren’t strictly accurate
  • Removing this would result in an authentic implementation for better or for worse

This is not at all parameterized, readable, or well-optimized. I would need to rewrite it to alleviate those issues, but I feel it’s better to publish what I have for now in case I can’t.

Shader code

shader_type spatial;
render_mode skip_vertex_transform;

varying vec3 sfcPosVS;
varying vec3 sfcPosLS;
varying vec3 sfcNmlLS;
varying vec3 sfcRtLS;
varying vec3 sfcUpLS;

vec3 cvtSpcPos(vec3 v, mat4 m) {
	return (m * vec4(v, 1.)).xyz;
}

vec3 cvtSpcDir(vec3 v, mat4 m) {
	return normalize((m * vec4(v, 0.0)).xyz);
}

void vertex() {
	sfcPosLS = VERTEX;
	sfcNmlLS = NORMAL;
	sfcRtLS = TANGENT;
	sfcUpLS = -cross(NORMAL,TANGENT);
	
	VERTEX = cvtSpcPos(VERTEX,MODELVIEW_MATRIX);
	NORMAL = cvtSpcDir(NORMAL,MODELVIEW_MATRIX);
	sfcPosVS = VERTEX;
}

void fragment() {
	//
}

void light() {
	vec3 lgtPosVS = (LIGHT / ATTENUATION) + sfcPosVS;
	vec3 lgtPosWS = cvtSpcPos(lgtPosVS,INV_VIEW_MATRIX);
	
	vec3 uv = vec3(
		dot(sfcPosLS,sfcRtLS),
		dot(sfcPosLS,-sfcUpLS),
		dot(sfcPosLS,sfcNmlLS)
	);
	
	vec3 uvQtz = vec3(
		floor(dot(sfcPosLS,sfcRtLS)),
		floor(dot(sfcPosLS,-sfcUpLS)),
		dot(sfcPosLS,sfcNmlLS)
	);
	
	vec3 tstPosLS = cvtSpcPos(lgtPosWS,inverse(MODEL_MATRIX));
	
	vec3 tstPos2D = vec3(
		dot(tstPosLS,sfcRtLS),
		dot(tstPosLS,-sfcUpLS),
		dot(tstPosLS,sfcNmlLS)
	);
	
	vec3 prjPosUV = tstPos2D;
	
	float dst00 = clamp(
		distance(prjPosUV,uvQtz) * .33,
		0., 1.);
	float dst01 = clamp(
		distance(prjPosUV,uvQtz + vec3(0.,1.,0.)) * .33,
		0., 1.);
	float dst10 = clamp(
		distance(prjPosUV,uvQtz + vec3(1.,0.,0.)) * .33,
		0., 1.);
	float dst11 = clamp(
		distance(prjPosUV,uvQtz + vec3(1.,1.,0.)) * .33,
		0., 1.);
	
	vec3 uvFrc = fract(uv);
	float finalf;
	
	if (uvFrc.x > uvFrc.y) {
		finalf = 
			(1. - uvFrc.x) * dst00 + 
			(uvFrc.x - uvFrc.y) * dst10 + 
			uvFrc.y * dst11; }
	else {
		finalf = 
			(1. - uvFrc.y) * dst00 + 
			(uvFrc.y - uvFrc.x) * dst01 + 
			uvFrc.x * dst11;
	}
	
	float sfcAngMlt = clamp(dot(LIGHT,NORMAL),0.,1.);
	float atnMod = clamp(round(2. * ATTENUATION),0.,1.);
	
	DIFFUSE_LIGHT += atnMod * LIGHT_COLOR * clamp(1. - finalf,0.,1.) * sfcAngMlt;
}