Style incrementality

[nicoburns]

Tracking issue for incremental application of styles in Parley.

Parley layout consists of 3 phases:

• Construction (includes Shaping and BiDi analysis)
• Layout (line breaking, alignment, etc)
• Paint (parley just passes through styles)

It should be possible to update styles and rerun later phases without having to redo earlier phases. This helps to avoid the expensive shaping phase when only layout/paint has updated, and can completely avoid doing work in Parley in the case of paint-only updates.

A table of which properties affect which phase can be found below:

Property	Affects
FontStack(FontStack<'a>)	Shaping
FontSize(f32)	Shaping
FontWidth(FontWidth)	Shaping
FontStyle(FontStyle)	Shaping
FontWeight(FontWeight)	Shaping
FontVariations(FontSettings<'a, FontVariation>)	Shaping
FontFeatures(FontSettings<'a, FontFeature>)	Shaping
Locale(Option<&'a str>)	Shaping
LineHeight(LineHeight)	Layout (possibly actually Shaping)
WordSpacing(f32)	Layout
LetterSpacing(f32)	Layout
WordBreak(WordBreakStrength)	Layout
OverflowWrap(OverflowWrap)	Layout
TextWrapMode(TextWrapMode)	Layout
VerticalAlign(VerticalAlign)	Layout
Brush(B)	Paint
Underline(bool)	Paint
UnderlineOffset(Option)	Paint
UnderlineSize(Option)	Paint
UnderlineBrush(Option)	Paint
Strikethrough(bool)	Paint
StrikethroughOffset(Option)	Paint
StrikethroughSize(Option)	Paint
StrikethroughBrush(Option)	Paint

[taj-p]

I'm interested in whether we see this as an evolution of the parley crate or whether this requirement should form part of the design of the parley-core crate?

In other words, if we spent the time adopting this into parley, would it be soon superseded by parley-core or are the learnings/implementation sufficiently transferable?

edit: Separately, should the Paint properties be owned by a separate crate (e.g. Parley Draw - in relation to #parley > Ink Skipping Underline @ 💬)?

[nicoburns]

The way to think about this in the context of parley-core is probably that the "Shaping" styles would be the realm of parley-core, the "Layout" styles would only exist in the higher-level parley crate, and the "Paint" styles probably shouldn't be in parley at all.

I'm not 100% about ink-skipping underline. But if it requires rasterizing the glyph then it's probably still not in the realm of parley?

[DJMcNab]

edit: Separately, should the Paint properties be owned by a separate crate (e.g. Parley Draw - in relation to #parley > Ink Skipping Underline @ 💬)?

My understanding of things is that is that paint styles probably just an opaque id (see #230, especially #230 (comment) this would probably be a u32, see #107), which is passed through, so that you can do your own mapping of them to actual painting when needed.

[nicoburns]

would probably be a u32, see #107), which is passed through, so that you can do your own mapping of them to actual painting when needed.

Mostly agree, although I think it would probably be best if it was a u64 in this case (note that #230 specifies that internal IDs should be u32, but this would be an external ID). Certainly it would be a problem for Blitz if it was a u32 as our node IDs are 64bit (32bit index, 32bit version from the slotmap crate).