Get 3D Position of shadow pixels in ThreeJS

Question

I have the following project below created using ThreeJS. You will notice the gold object creates a shadow behind it on a sphere where I'm only rendering the backside so we can see the objects inside. I'm using a point light in the very center of the eye model to create the shadow evenly in all directions. This is the reason the shadow is curved.

I need to know how to get the 3D coordinates (x,y,z) of each pixel of this shadow that was created. For reference here is the code that creates the shadow with a lot removed for simplicity.

renderer.shadowMap.enabled = true;
renderer.shadowMap.type = THREE.BasicShadowMap//THREE.PCFSoftShadowMap;

const light = new THREE.PointLight( 0xffffff, 20, 0 );
light.position.set( 0, 0, 0 );
light.castShadow = true;
light.shadow.mapSize.width = 512;
light.shadow.camera.near = 0.5;
light.shadow.camera.far = 500;
scene.add( light );

const sphereGeometry = new THREE.SphereGeometry( 25, 32, 32 );
const sphereMaterial = new THREE.MeshStandardMaterial( { color: 0xffffff } );
sphereMaterial.side=THREE.BackSide;

const sphere = new THREE.Mesh( sphereGeometry, sphereMaterial );
sphere.castShadow = false;
sphere.receiveShadow = true;
scene.add( sphere );

I have researched some into this and I think it may be storing the shadow information in the matrix property of the model but this is not clear for sure in any documentation, so I'm not sure where to look to get this information. Any help is appreciated!

--- Extra not important info ---

Also, in case you are curious, the reason I need the shadow coordinates is because I will use those to raycast back into the eye and create a different kind of shadow on an azimuthal equidistant project on the right (it's complicated...), but just know that if I have the 3D coordinates of the shadow pixels I can do this :). I'm already doing it for the muscles of the eye for example.

Answer 1

You can't extract the shadow into a new geometry because this is all calculated in the GPU shaders upon rendertime, so JavaScript doesn't really have access to the shadowMap positions. However, there is a solution.

Assuming your point light is at (0, 0, 0), and it's at the center of the sphere, you could iterate through the vertices of the gold object and project these positions onto the sphere:

// Sphere radius
const radius = 25;

const vec3 = new THREE.Vector3();

// Get the vertex position array
const vertices = goldObject.geometry.getAttribute("position").array;

// Loop that iterates through all vertex positions
for (let i3 = 0; i3 < vertices.length; i3 += 3) {
    // Set this vertex into our vec3
    vec3.set(
        vertices[i3 + 0], // x
        vertices[i3 + 1], // y
        vertices[i3 + 2]  // z
    );

    // Set vector magnitude to 1
    vec3.normalize();

    // Set vector magnitude to radius of sphere
    vec3.multiplyScalar(sphereRadius);

    // Now you have the spherical projection of this vertex!
    console.log(vec3);
}

Since the light source is the exact center of the sphere, you could take the position of each vertex of the gold object, normalize it, then multiply it by the radius of the sphere. Now that you have the vec3 on each iteration, you could add it to your own array to build your own THREE.BufferGeometry that's pushed against the sphere.

Of course, if you've translated or rotated the gold object, then that will affect the vertex positions, so you'd have to undo those translations, rotations, etc. when iterating through all the vertices.