gistfile1.js

        var getDirectionSpriteIndex = function (entity, world) {
            var entitiesWithCamera = world.getEntities('camera');

            var index = 0;

            if (entitiesWithCamera.length) {
                var activeCamera = entitiesWithCamera[0].getComponent('camera').camera;

                var camWorldPos = new THREE.Vector3();
                camWorldPos.setFromMatrixPosition(activeCamera.matrixWorld);
                var directionVec = camWorldPos;
                directionVec.sub(entity.position);

                var rotVec = new THREE.Vector3(0, 0, 1);
                rotVec.applyEuler(entity.rotation);

                var rotY = (Math.atan2(rotVec.z, rotVec.x));
                if ( rotY < 0 ) {
                    rotY += (Math.PI*2);
                }
                rotY = (Math.PI*2) - rotY;

                // Rotate vector with our own rotation
                var tx = ((directionVec.x * Math.cos(rotY)) - (directionVec.z * Math.sin(rotY)));
                var tz = ((directionVec.x * Math.sin(rotY)) + (directionVec.z * Math.cos(rotY)));

                directionVec.x = tx;
                directionVec.z = tz;

                var result = Math.atan2(directionVec.z, directionVec.x);

                result += Math.PI;

                while (result < 0) {
                    result += (Math.PI * 2);
                }
                while (result > (Math.PI * 2)) {
                    result -= (Math.PI * 2);
                }

                if (result >= 0.39 && result <= 1.17) {
                    index = 3;
                } else if (result > 1.17 && result <= 1.96) {
                    index = 2;
                } else if (result > 1.96 && result <= 2.74) {
                    index = 1;
                } else if (result > 2.74 && result <= 3.53) {
                    index = 0;
                } else if (result > 3.53 && result <= 4.31) {
                    index = 7;
                } else if (result > 4.31 && result <= 5.10) {
                    index = 6;
                } else if (result > 5.10 && result <= 5.89) {
                    index = 5;
                } else {
                    index = 4;
                }
            }

            return index;
        };