webp-container-spec: clarify background clear on loop

at the beginning of the loop there's an implicit clear of the entire
canvas to the background (or application defined) color. this avoids
adding the final composited frame to the first.

Change-Id: Ia3a52cf4482c6176334a5c9c99a0ddd07d1776e7

doc/webp-container-spec.txt

@@ -378,6 +378,9 @@ color is also used when disposal method is `1`.
   * Viewer applications SHOULD treat the background color value as a hint, and
     are not required to use it.
 
+  * The canvas is cleared at the start of each loop. The background color MAY be
+    used to achieve this.
+
 Loop Count: 16 bits (_uint16_)
 
 : The number of times to loop the animation. `0` means infinitely.
@@ -719,33 +722,37 @@ pseudocode:
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 assert VP8X.flags.hasAnimation
 canvas ← new image of size VP8X.canvasWidth x VP8X.canvasHeight with
-background color ANIM.background_color.
+         background color ANIM.background_color.
 loop_count ← ANIM.loopCount
 dispose_method ← ANIM.disposeMethod
 if loop_count == 0:
     loop_count = ∞
 frame_params ← nil
-for loop = 0, ..., loop_count - 1
+assert next chunk in image_data is ANMF
-    assert next chunk in image_data is ANMF
+for loop = 0..loop_count - 1
-    frame_params.frameX = Frame X
+    clear canvas to ANIM.background_color or application defined color
-    frame_params.frameY = Frame Y
+    until eof or non-ANMF chunk
-    frame_params.frameWidth = Frame Width Minus One + 1
+        frame_params.frameX = Frame X
-    frame_params.frameHeight = Frame Height Minus One + 1
+        frame_params.frameY = Frame Y
-    frame_params.frameDuration = Frame Duration
+        frame_params.frameWidth = Frame Width Minus One + 1
-    assert VP8X.canvasWidth >= frame_params.frameX + frame_params.frameWidth
+        frame_params.frameHeight = Frame Height Minus One + 1
-    assert VP8X.canvasHeight >= frame_params.frameY + frame_params.frameHeight
+        frame_params.frameDuration = Frame Duration
-    for subchunk in 'Frame Data':
+        frame_right = frame_params.frameX + frame_params.frameWidth
-        if subchunk.tag == "ALPH":
+        frame_bottom = frame_params.frameY + frame_params.frameHeight
-            assert alpha subchunks not found in 'Frame Data' earlier
+        assert VP8X.canvasWidth >= frame_right
-            frame_params.alpha = alpha_data
+        assert VP8X.canvasHeight >= frame_bottom
-        else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
+        for subchunk in 'Frame Data':
-            assert bitstream subchunks not found in 'Frame Data' earlier
+            if subchunk.tag == "ALPH":
-            frame_params.bitstream = bitstream_data
+                assert alpha subchunks not found in 'Frame Data' earlier
-    render frame with frame_params.alpha and frame_params.bitstream on canvas
+                frame_params.alpha = alpha_data
-    with top-left corner in (frame_params.frameX, frame_params.frameY), using
+            else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
-    dispose method dispose_method.
+                assert bitstream subchunks not found in 'Frame Data' earlier
-    Show the contents of the image for frame_params.frameDuration * 1ms.
+                frame_params.bitstream = bitstream_data
-canvas contains the decoded canvas.
+        render frame with frame_params.alpha and frame_params.bitstream on
+            canvas with top-left corner at (frame_params.frameX,
+            frame_params.frameY), using dispose method dispose_method.
+        canvas contains the decoded image.
+        Show the contents of the canvas for frame_params.frameDuration * 1ms.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Example file layouts