scripting: Add a tweening procedure.

* chickadee/scripting.scm (tween): Reimplement procedure.
* chickadee/math/easings.scm: New file.
* Makefile.am (SOURCES): Add it.
* doc/api.text (Easings, Tweening): New subsections.

1 files changed, 20 insertions, 7 deletions

diff --git a/chickadee/scripting.scm b/chickadee/scripting.scm
index 7a661bb..5d24ce9 100644
--- a/chickadee/scripting.scm
+++ b/chickadee/scripting.scm
@@ -16,6 +16,8 @@
 ;;; <http://www.gnu.org/licenses/>.
 
 (define-module (chickadee scripting)
+  #:use-module (chickadee math)
+  #:use-module (chickadee math easings)
   #:use-module (chickadee scripting agenda)
   #:use-module (chickadee scripting channel)
   #:use-module (chickadee scripting coroutine)
@@ -42,12 +44,23 @@
   "Wait DURATION before resuming the current coroutine."
   (yield (lambda (cont) (schedule-after duration cont))))
 
-(define (tween duration start end ease proc)
+(define* (tween duration start end proc #:key
+                (step 1)
+                (ease smoothstep)
+                (interpolate lerp))
+  "Transition a value from START to END over DURATION, sending each
+succesive value to PROC.  STEP controls the amount of time between
+each update of the animation.
+
+The EASE procedure controls the rate at which the animation advances.
+The smoothstep easing function is used by default.
+
+The INTERPOLATE procedure computes the values in between START and
+END.  By default, linear interpolation is used."
   (let loop ((t 0))
-    (if (= t duration)
+    (if (>= t duration)
         (proc end)
-        (let ((alpha (/ t duration)))
-          (proc (+ (* start (- 1 alpha))
-                   (* end alpha)))
-          (wait 1)
-          (loop (1+ t))))))
+        (let ((alpha (ease (/ t duration))))
+          (proc (interpolate start end alpha))
+          (wait step)
+          (loop (+ t step))))))