potato4.js

// Capture the first pointer down 
// And if we start on the potato we block camera control
var potatoHasControl;
scene.onPointerDown = (evt, pickResult) => {
    if (!pickResult.hit) {
        return;
    }

    if (pickResult.pickedMesh !== potato) {
        return;
    }

    potatoHasControl = true;
    scene.activeCamera.detachControl();
}

// Restore camera control
scene.onPointerUp = () => {
    if (!potatoHasControl) {
        return;
    }

    potatoHasControl = false;
    scene.activeCamera.attachControl();
}

// Get the pointer move and act accordingly
scene.onPointerMove = (evt, pickResult) => {
    if (!potatoHasControl) {
        return;
    }
    
    if (!pickResult || !pickResult.hit) {
       return;
    }

    if (pickResult.pickedMesh !== potato) {
        return;
    }

    // UV of the picked point
    var uv = pickResult.getTextureCoordinates();
    var size = heightmap.getSize();
    var centerX = Math.max(Math.round(uv.x * (size.width - 1)), 0);
    var centerY = Math.max(Math.round((1 - uv.y) * (size.height - 1)), 0); 
    var radius = 20;

    context.beginPath();
    context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
    context.fillStyle = evt.shiftKey ? 'black' : 'white';
    context.fill();
    heightmap.update(true);        
};