Screen: add get_pixels_at_depth.

src/screen.nim

@@ -116,6 +116,15 @@ proc get_ray_direction_local*(viewport: Viewport, position: Vec2): Vec3 =
     let y = (position.y - viewport.rect_pix[1].float32) / viewport.rect_pix[3].float32
     return viewport.camera.get_ray_direction_local(x,y)
 
+proc get_pixels_at_depth*(viewport: Viewport, depth: float32): float32 =
+    ## Returns the length of one world unit (usually one meter) in screen pixels,
+    ## at a given depth.
+    
+    # TODO: shift_x
+    let p = vec4(1.0, 0.0, -depth, 1.0)
+    let s = viewport.camera.projection_matrix * p
+    return viewport.rect_pix[2].float32 * 0.5 * s.x/s.w
+
 proc clear_all_callbacks*(self: Screen) =
     self.key_callbacks.setLen 0
     self.mouse_button_callbacks.setLen 0