Unity: How to get the visible bounds of an object, i.e. as it is seen from the camera?

Question

Imagine you'd like to highlight a 3D GameObject by drawing a (translucent) UI panel above it. The UI panel should cover the object exactly. Therefore the object's bounds, as it is seen from the camera, are needed. How do you get these bounds? (In word space, or screen, viewport or canvas space, any way's fine).

My problem is that [Collider|Renderer|Mesh].bounds does not take the camera's position into account. Hence, depending on the camera's location and the object's dimensions, the bounds are more or less off. Do you know how this can be fixed?

Any advise is very much appreciated. Thank you!

Answer 1

As DMGregory pointed out, the solution is quite simple: Just iterate through the vertices of a mesh, convert them from world to screen coordinates and store the min and max values of both x and y axis. In code it looks like this:

private List<Vector3> vertices = new List<Vector3>();

public Rect GetBoundingBoxOnScreen(Mesh mesh, Camera camera) {
    mesh.GetVertices(vertices);
    Rect retVal = Rect.MinMaxRect(float.MaxValue, float.MaxValue, float.MinValue, float.MinValue);

    for (int i = 0; i < vertices.Count; i++) {
        Vector3 v = camera.WorldToScreenPoint(vertices[i]);
        if (v.x < retVal.xMin) {
            retVal.xMin = v.x;
        }

        if (v.y < retVal.yMin) {
            retVal.yMin = v.y;
        }

        if (v.x > retVal.xMax) {
            retVal.xMax = v.x;
        }

        if (v.y > retVal.yMax) {
            retVal.yMax = v.y;
        }
    }

    return retVal;
}        
     

The min and max corners of the returned rectangle are them passed to RectTransformUtility.ScreenPointToLocalPointInRectangle() to convert them from screen to canvas coordinates. And that's it. @DMGregory, again, thank you for your advice.