added easing functions + demo

examples/basic/easing_functions.py

@@ -0,0 +1,52 @@
+from pgzero.builtins import *
+from pgzero import animation
+import itertools
+
+WIDTH = 800
+HEIGHT = 800
+#print(animation.TWEEN_FUNCTIONS
+MARGIN = 20
+
+def position_for_block(i):
+    BLOCK_POSITIONS = [[(150, (i * 50 + i * MARGIN)), (WIDTH - 150, (i * 50 + i * MARGIN))] for i in range(len(animation.TWEEN_FUNCTIONS))]
+    return BLOCK_POSITIONS[i]
+
+
+
+positions = [itertools.cycle(position_for_block(i)) for i in range(len(animation.TWEEN_FUNCTIONS))]
+blocks = [Actor('block', center=next(positions[i])) for i in range(len(animation.TWEEN_FUNCTIONS))]
+
+line_centers = [(WIDTH/2, i * (50 + MARGIN)) for i in range(len(animation.TWEEN_FUNCTIONS))]
+
+def draw():
+    screen.clear()
+    #[block.draw() for block in blocks]
+    for i, easing_f_name in enumerate(animation.TWEEN_FUNCTIONS):
+        block = blocks[i]
+        screen.draw.text(easing_f_name, line_centers[i])
+        block.draw()
+
+
+def move_blocks():
+    for i, easing_f_name in enumerate(animation.TWEEN_FUNCTIONS):
+        animate(
+            blocks[i],
+            easing_f_name,
+            duration=5,
+            pos = next(positions[i])
+
+        )
+
+"""
+def move_block():
+    animate(
+        block,
+        'bounce_end',
+        duration=1,
+        pos=next(block_positions)
+    )
+""";
+
+move_blocks()  # start one move now
+clock.schedule_interval(move_blocks, 6)
+

src/pgzero/animation.py

@@ -2,7 +2,7 @@
 #  http://www.clutter-project.org/docs/clutter/stable/ClutterAlpha.html
 
 
-from math import sin, pow, pi
+from math import sin, cos, pow, pi
 
 from .clock import each_tick, unschedule
 from .spellcheck import suggest
@@ -75,6 +75,26 @@ def in_out_elastic(n):
         return pow(2, -10 * q) * sin((q - s) * (2.0 * pi) / p) * .5 + 1.0
 
 
+@tweener
+def out_sine(n):
+    return sin(n * pi/2)
+
+
+@tweener
+def in_sine(n):
+    return 1 - cos(n * pi/2)
+
+
+@tweener
+def in_out_sine(n):
+    return -(cos(pi * n) - 1)/2
+
+
+@tweener
+def out_expo(n):
+    return 1 if n == 1 else 1 - pow(2, -10 * n)
+
+
 def _out_bounce_internal(t, d):
     p = t / d
     if p < (1.0 / 2.75):