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webp-container-spec: clarify background clear on loop

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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.

(cherry picked from commit 560394798ffd932de1ec51a9f5c8d580cec38d3b)

Change-Id: Ia3a52cf4482c6176334a5c9c99a0ddd07d1776e7
This commit is contained in:
James Zern 2015-02-04 18:07:00 -08:00
parent 4ba83c1759
commit f97b3f86bf
1 changed files with 29 additions and 22 deletions
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51
doc/webp-container-spec.txt
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@ -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
frame_params.frameX = Frame X
frame_params.frameY = Frame Y
frame_params.frameWidth = Frame Width Minus One + 1
frame_params.frameHeight = Frame Height Minus One + 1
frame_params.frameDuration = Frame Duration
assert VP8X.canvasWidth >= frame_params.frameX + frame_params.frameWidth
assert VP8X.canvasHeight >= frame_params.frameY + frame_params.frameHeight
for subchunk in 'Frame Data':
if subchunk.tag == "ALPH":
assert alpha subchunks not found in 'Frame Data' earlier
frame_params.alpha = alpha_data
else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
assert bitstream subchunks not found in 'Frame Data' earlier
frame_params.bitstream = bitstream_data
render frame with frame_params.alpha and frame_params.bitstream on canvas
with top-left corner in (frame_params.frameX, frame_params.frameY), using
dispose method dispose_method.
Show the contents of the image for frame_params.frameDuration * 1ms.
canvas contains the decoded canvas.
assert next chunk in image_data is ANMF
for loop = 0..loop_count - 1
clear canvas to ANIM.background_color or application defined color
until eof or non-ANMF chunk
frame_params.frameX = Frame X
frame_params.frameY = Frame Y
frame_params.frameWidth = Frame Width Minus One + 1
frame_params.frameHeight = Frame Height Minus One + 1
frame_params.frameDuration = Frame Duration
frame_right = frame_params.frameX + frame_params.frameWidth
frame_bottom = frame_params.frameY + frame_params.frameHeight
assert VP8X.canvasWidth >= frame_right
assert VP8X.canvasHeight >= frame_bottom
for subchunk in 'Frame Data':
if subchunk.tag == "ALPH":
assert alpha subchunks not found in 'Frame Data' earlier
frame_params.alpha = alpha_data
else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
assert bitstream subchunks not found in 'Frame Data' earlier
frame_params.bitstream = bitstream_data
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.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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