node-2d: Improve teleporting a node that a camera is tracking.

1 files changed, 17 insertions, 2 deletions

diff --git a/starling/node-2d.scm b/starling/node-2d.scm
index 095c5ca..9a18284 100644
--- a/starling/node-2d.scm
+++ b/starling/node-2d.scm
@@ -284,9 +284,24 @@
   (move-by node dx dy duration smoothstep))
 
 (define-method (teleport (node <node-2d>) x y)
-  (move-to node x y)
+  ;; When teleporting, we want to avoid position interpolation and odd
+  ;; looking camera jumps.  Interpolation is avoided by setting all 3
+  ;; position vectors to the same values.  This prevents a visual
+  ;; artifact where the player sees 1 frame where the node is
+  ;; somewhere in between its former position and the new position.
+  ;; The camera jump problem occurs when a camera has a node as its
+  ;; tracking target and that node teleports. Normally, the camera's
+  ;; view matrix is updated before any nodes are rendered, and thus
+  ;; *before* the node can recompute its world matrix based on the new
+  ;; position.  This creates 1 frame where the camera is improperly
+  ;; positioned at the target's old location.  This 1 frame lag is not
+  ;; an issue during normal movement, but when teleporting it causes a
+  ;; noticably unsmooth blip.  Forcing the matrices to be recomputed
+  ;; immediately solves this issue.
+  (set-vec2! (position node) x y)
   (set-vec2! (last-position node) x y)
-  (set-vec2! (render-position node) x y))
+  (set-vec2! (render-position node) x y)
+  (compute-matrices! node))
 
 (define-method (rotate-to (node <node-2d>) theta)
   (set! (rotation node) theta)