How can I detect the number of lit faces on a mesh? - webgl

Question

I have a number of objects arranged in a THREE.scene, and I want to calculate or retrieve a relative value indicating how much light each object is receiving from a single PointLight source. Simplified example:
With the light positioned at the camera, Block 1's value might be 0.50 since 3 of 6 faces are completely exposed, while 2 is ~0.33 and 3 is ~1.67.
I could probably do this the hard way by drawing a ray from the light toward the center of each face and looking at the intersects, but I'm assuming it's possible to directly retrieve the light level of each face.

Answer 1

This code takes the object's global matrix in consideration.
var amount = 0;
var rotationMatrix = new THREE.Matrix4();
var vector = new THREE.Vector3();
var centroid = new THREE.Vector3();
var normal = new THREE.Vector3();
for ( var i = 0; i < objects.length; i ++ ) {
var object = objects[ i ];
rotationMatrix.extractRotation( object.matrixWorld );
for ( var j = 0; j < object.geometry.faces.length; j ++ ) {
var face = object.geometry.faces[ j ];
centroid.copy( face.centroid );
object.matrixWorld.multiplyVector3( centroid );
normal.copy( face.normal );
rotationMatrix.multiplyVector3( normal );
vector.sub( light.position, centroid ).normalize();
if ( normal.dot( vector ) > 0 ) amount ++;
}
}

Answer 2

I think something like this should do the trick.
var amount = 0;
var faces = mesh.geometry.faces;
for ( var i = 0; i < geometry.faces.length; i ++ ) {
if ( geometry.faces[ i ].normal.dot( light.position ) > 0 ) amount ++;
}

Answer 3

(Warning: Brute force method!)
I'm including this for reference since it's what I'm currently using to meet all of the requirements described in the question. This function considers a face unlit if its center is not directly visible from the light's position.
I have no rotation matrix to consider for my application.
function getLightLevel(obj) {
/* Return percentage of obj.geometry faces exposed to light */
var litCount = 0;
var faces = obj.geometry.faces;
var faceCount = faces.length;
var direction = new THREE.Vector3();
var centroid = new THREE.Vector3();
for (var i=0; i < faceCount; i++) {
// Test only light-facing faces (from mrdoob's first answer).
if (faces[i].normal.dot(light.position) > 0) {
centroid.add(obj.position, faces[i].centroid);
direction.sub(centroid, light.position).normalize();
// Exclude face if centroid is obscured by another object.
var ray = new THREE.Ray(light.position, direction);
var intersects = ray.intersectObjects(objects);
if (intersects.length > 0 && intersects[0].face === faces[i]) {
litCount ++;
}
}
}
return litCount / faceCount;
}