augmented reality - Mapping image onto 3D face mesh

Question

I am using the iPhone X and ARFaceKit to capture the user's face. The goal is to texture the face mesh with the user's image.

I'm only looking at a single frame (an ARFrame) from the AR session. From ARFaceGeometry, I have a set of vertices that describe the face. I make a jpeg representation of the current frame's capturedImage.

I then want to find the texture coordinates that map the created jpeg onto the mesh vertices. I want to:

1. map the vertices from model space to world space;

2. map the vertices from world space to camera space;

3. divide by image dimensions to get pixel coordinates for the texture.

   let geometry: ARFaceGeometry = contentUpdater.faceGeometry! let theCamera = session.currentFrame?.camera

   let theFaceAnchor: SCNNode = contentUpdater.faceNode let anchorTransform = float4x4((theFaceAnchor?.transform)!)

   for index in 0..<totalVertices { let vertex = geometry.vertices[index]

    // Step 1: Model space to world space, using the anchor's transform
    let vertex4 = float4(vertex.x, vertex.y, vertex.z, 1.0)
    let worldSpace = anchorTransform * vertex4
   
    // Step 2: World space to camera space
    let world3 = float3(worldSpace.x, worldSpace.y, worldSpace.z)
    let projectedPt = theCamera?.projectPoint(world3, orientation: .landscapeRight, viewportSize: (theCamera?.imageResolution)!)
   
    // Step 3: Divide by image width/height to get pixel coordinates
    if (projectedPt != nil) {
        let vtx = projectedPt!.x / (theCamera?.imageResolution.width)!
        let vty = projectedPt!.y / (theCamera?.imageResolution.height)!
        textureVs += "vt (vtx) (vty)
   "
    }
   

   }

This is not working, but instead gets me a very funky looking face! Where am I going wrong?

Answer 1

Texturing the face mesh with the user's image is now available in the Face-Based sample code published by Apple (section Map Camera Video onto 3D Face Geometry).

One can map camera video onto 3D Face Geometry using this following shader modifier.

// Transform the vertex to the camera coordinate system.
float4 vertexCamera = scn_node.modelViewTransform * _geometry.position;

// Camera projection and perspective divide to get normalized viewport coordinates (clip space).
float4 vertexClipSpace = scn_frame.projectionTransform * vertexCamera;
vertexClipSpace /= vertexClipSpace.w;

// XY in clip space is [-1,1]x[-1,1], so adjust to UV texture coordinates: [0,1]x[0,1].
// Image coordinates are Y-flipped (upper-left origin).
float4 vertexImageSpace = float4(vertexClipSpace.xy * 0.5 + 0.5, 0.0, 1.0);
vertexImageSpace.y = 1.0 - vertexImageSpace.y;

// Apply ARKit's display transform (device orientation * front-facing camera flip).
float4 transformedVertex = displayTransform * vertexImageSpace;

// Output as texture coordinates for use in later rendering stages.
_geometry.texcoords[0] = transformedVertex.xy;