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

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 ++;

    }

}