[pnelsonAG]

When using cameras from Ab3d.PowerToys library, you can easily get the 2D point on the screen from the 3D position with using the Point3DTo2D method on a Ab3d.PowerToys camera.

Then should I switch to using the Ab3dPowerToys camera instead of the Viewport3D camera? How easy is it to replace the Viewport3D camera, what does the XAML and c# look like and what are the consequences?

If you are not using Ab3d.PowerToys cameras, you can get the 2D screen position with first calculating the view-projection matrix (see viewProj in my previous post).

I didn't understand that matrix, which depends on viewProj because I didn't understand width and height as mentioned in that post . . .

Matrix.LookAtRH(ref cameraPosition, ref target, ref up, out viewMatrix);

var orthoMatrix = Matrix.OrthoRH(width, height, zNear, zFar);

var viewProj = viewMatrix * orthoMatrix ;

... but I haven't explicitly declared a view volume anywhere and I don't see how a camera with near and far plane distances and a look direction can implicitly specify a volume (because there are no explicit right/left sides or top/bottom, so what goes in width and height for the orthoRH? The MainViewport has a Height and ActualHeight and Width and ActualWith in pixels space - 564 and 936 - but that's clearly not the coordinate system the lines are in. Orthographic cameras have a Width property (but no Height) too but I'm not specifying it in my example.

Then you need to need to multiply the 3D position (converted into Point4D) with viewProjection and divide by w:

Code:

Point4D point4d = ((Point4D)point3D) * viewProj;

if (point4d.W != 0)
     screenPosition = new Point(point4d.X / point4d.W, point4d.Y / point4d.W);
else
     screenPosition = new Point(double.NaN, double.NaN);

To calculate the ray from 2D position to 3D space, you can use the Ab3d.Cameras.BaseCamera.CreateMouseRay3D static method - the following is the method signature:

public static bool CreateMouseRay3D(Point mousePosition, Size viewportSize, ref Matrix3D viewMatrix, ref Matrix3D projectionMatrix, out Point3D rayOrigin, out Vector3D rayDirection)

Summary:
1. I need to decide if it would be easier to use a power toys camera instead of the one I have now
2. If I stick with the camera I have now I need to understand how to build the matrices