From d21b67eaad50c9389e1ad83cc595d2e77783942c Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Mon, 11 Jan 2016 08:37:42 -0500
Subject: render: Add alpha value to render context.

Necessary for interpolating moving objects to avoid temporal aliasing.

* sly/render.scm (<graphics>)[alpha]: New field.
  (graphics-alpha, set-graphics-alpha!): New procedure.
* sly/game.scm (run-game-loop): Set the frame's alpha value before each
  render call.
---
 sly/game.scm   | 4 +++-
 sly/render.scm | 5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/sly/game.scm b/sly/game.scm
index 5f1a1ec..722bfc0 100644
--- a/sly/game.scm
+++ b/sly/game.scm
@@ -84,7 +84,9 @@ instead of becoming completely unresponsive and possibly crashing."
         (gl-viewport 0 0 (vx size) (vy size)))
       (gl-clear (clear-buffer-mask color-buffer depth-buffer))
       (run-hook draw-hook dt alpha)
-      (with-graphics gfx ((signal-ref scene) gfx))
+      (with-graphics gfx
+        (set-graphics-alpha! gfx alpha)
+        ((signal-ref scene) gfx))
       (swap-window))
 
     (define (update lag)
diff --git a/sly/render.scm b/sly/render.scm
index 6bc917b..deac23a 100644
--- a/sly/render.scm
+++ b/sly/render.scm
@@ -57,6 +57,8 @@
             set-graphics-framebuffer!
             graphics-viewport
             set-graphics-viewport!
+            graphics-alpha
+            set-graphics-alpha!
             graphics-model-view-transform
             graphics-model-view-mul!
             graphics-model-view-identity!
@@ -177,7 +179,8 @@
   (viewport graphics-viewport %set-graphics-viewport!)
   (projection graphics-projection)
   (model-view graphics-model-view)
-  (uniforms graphics-uniforms set-graphics-uniforms!))
+  (uniforms graphics-uniforms set-graphics-uniforms!)
+  (alpha graphics-alpha set-graphics-alpha!))
 
 (define (graphics-uniform-ref gfx uniform)
   (hashq-ref (graphics-uniforms gfx) uniform))
-- 
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