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webp-container-spec: add prose for rendering process

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this is given in addition to the pseudocode, which has been corrected in
the process

based on comments in:
https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_lars-eggert

Bug: webp:448
Change-Id: I96bc063c2a71572ff61609a731a9c4e8edc2b971
This commit is contained in:
James Zern 2022-10-10 17:23:58 -07:00
parent 73b19b64fd
commit 83270c7f89
1 changed files with 30 additions and 7 deletions
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37
doc/webp-container-spec.txt
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@ -746,19 +746,40 @@ original order (unless they specifically intend to modify these chunks).
### Assembling the Canvas from frames
Here we provide an overview of how a reader should assemble a canvas in the
case of an animated image. The notation _VP8X.field_ means the field in the
'VP8X' chunk with the same description.
Here we provide an overview of how a reader MUST assemble a canvas in the case
of an animated image.
Displaying an _animated image_ canvas MUST be equivalent to the following
pseudocode:
The process begins with creating a canvas using the dimensions given in the
'VP8X' chunk, `Canvas Width Minus One + 1` pixels wide by `Canvas Height Minus
One + 1` pixels high. The `Loop Count` field from the 'ANIM' chunk controls how
many times the animation process is repeated. This is `Loop Count - 1` for
non-zero `Loop Count` values or infinitely if `Loop Count` is zero.
At the beginning of each loop iteration the canvas is filled using the
background color from the 'ANIM' chunk or an application defined color.
'ANMF' chunks contain individual frames given in display order. Before rendering
each frame, the previous frame's `Disposal method` is applied.
The rendering of the decoded frame begins at the Cartesian coordinates (`2 *
Frame X`, `2 * Frame Y`) using the top-left corner of the canvas as the origin.
`Frame Width Minus One + 1` pixels wide by `Frame Height Minus One + 1` pixels
high are rendered onto the canvas using the `Blending method`.
The canvas is displayed for `Frame Duration` milliseconds. This continues until
all frames given by 'ANMF' chunks have been displayed. A new loop iteration is
then begun or the canvas is left in its final state if all iterations have been
completed.
The following pseudocode illustrates the rendering process. The notation
_VP8X.field_ means the field in the 'VP8X' chunk with the same description.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assert VP8X.flags.hasAnimation
canvas ← new image of size VP8X.canvasWidth x VP8X.canvasHeight with
background color ANIM.background_color.
loop_count ← ANIM.loopCount
dispose_method ← ANIM.disposeMethod
dispose_method ← Dispose to background color
if loop_count == 0:
loop_count = ∞
frame_params ← nil
@ -784,10 +805,12 @@ for loop = 0..loop_count - 1
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.
frame_params.frameY), using blending method
frame_params.blendingMethod.
canvas contains the decoded image.
Show the contents of the canvas for
frame_params.frameDuration * 1ms.
dispose_method = frame_params.disposeMethod
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~