engine-v0.7.0

Added

• Caption layers. A segment or element override can carry stacking (CaptionStacking.UNDER_TOP_LAYER or OVER_TOP_LAYER) and blendMode (CaptionBlendMode, the blend modes CSS and the 2D canvas share under the same names). Whatever an element leaves out follows its segment, and whatever a segment leaves out is the base layer: under the top layer, plain. Each layer is rasterized apart and LayeredFrameCompositor draws it with its blend against everything already composited, the top layer included. A blend therefore holds whatever stacking contexts the stylesheet creates, which mix-blend-mode written on the element does not, and one word can sit behind a top layer such as an actor cutout while the rest of its caption stays in front. A word or glyph still in its line is painted by a copy of its segment's subtree in which only it is visible. A blended raster holds nothing of the video, so a blended element that does not move reuses one tile.
• RenderJob.background paints the captions on something other than the input's frames: { kind: 'color', color } for a solid colour, or { kind: 'transparent' } for an output with an alpha channel (WebM through VP9 or VP8; any other format is rejected). The input still sets the output's size, frame rate, duration and cuts. A top layer follows the background: it is cut out of the colour, or, on a transparent output, erases the captions under it instead of painting over them.
• RenderJob.audio: 'discard' leaves the output without audio.
• A render whose background is not the video no longer decodes it: each frame's time comes from the packets alone (PacketTimingVideoFrameDecoder). Measured on an 18 s 1080×1920 source in headless Chromium, a solid-colour render went from about 12 s to 9 s.
• SegmentSubtreeDecomposer and SubtreeIsolationMarks are exported, so a live preview can split a segment into the same subtrees and copies the export paints, and stamp the same classes on them.

Changed

• CaptionsOverlayFramePainterFactory takes a SubtitleLayerSourceFactory and a CaptionLayerDemand in place of one SubtitleLayerSource. It opens one source per layer the document paints in, each with only the styles that paint there, so a run with no stacking or blendMode opens the base layer alone, as before. BrowserSubtitleLayerSourceFactory builds the default source per layer, and BrowserSubtitleFrameRenderer.create takes the layer it paints through options.layer.
• CaptionsOverlayFramePainterFactory.create takes the FrameBackdrop to paint on (VideoFrameBackdrop for the previous behaviour), and its top layer is no longer optional. MediaBunnyTranscodeRequest gains alpha and audio, VideoTrackEncoderFactoryRequest gains alpha, AudioTrackBridgeRequest gains wanted, and VideoFrameDecoderRequest gains pixels. MediaBunnyTranscodeRequest gains sourcePixels, FrameBackdrop has showsFramePixels, and VideoFrameDecoderKind gains 'packet-timing'. SubtitleLayerSourceFactory.create takes framesHaveAlpha, FrameBackdrop has sampledByCaptions, and FrameCompositionRequest has blendCaptions. On a background that is not the video, captions blend with nothing: blended layers are composited plainly, and a caption that samples the video gets an empty frame, as PNG with its alpha instead of JPEG, so it stays transparent rather than black or tinted by the background.
• FrameCompositionRequest.captions is a list of CaptionLayerFrame in paint order instead of one SubtitleFrame.
• ComposedSubtitleLayerSource takes a SubtitleStyleFrameDemand as its third argument, the one place that decides whether a style reads the video frame.
• BaselineNeeds gains isolation, the rules a copy of a subtree needs, so composeOptional callers have to say whether they want them.

Fixed

• A caption that reads the video frame no longer erases the overlay beneath it. It was handed the bare frame and painted it back over the overlay wherever it drew, so a watermark disappeared inside a frosted caption's box. Caption sources are now handed the frame with the overlay drawn on it; the top layer still gets the bare frame.