Add support for textLength and lengthAdjust in SVG text elements

[oshmoun]

Add support for textLength and lengthAdjustby calculating a fitting letter spacing, and a fitting X scaling factor if lengthAdjust=spacingAndGlyphs, that make the text expand/shrink to fit the specified size.

There is one weird behaviour that browsers do, at least Chromium, that I noticed while implementing this:
If letter-spacing is specified alongside textLength, then it functions as a negative width offset, a TODO was added for that.

[liZe]

I’ve finished the small cleanup, here are some comments:

• the 3 first commits are just for styling purpose
• d001a23 changes the signature of draw_first_line to allow a matrix instead of separate properties
• cd5608c tries to simplify many small details, and lead to these questions:

1. Why did you introduce the - 0.5 value in spaces count? I didn’t find an important change with - 1 and it looks more "normal" 😄.
2. I’ve always overridden the letter-spacing value when "spacing" is set. The rendering is a bit different to what browsers do: they have some extra space on the right, but there’s nothing I can find in the specification about that ("Whether letter-spacing is applied at the end of a line is undefined in this level.") and what WeasyPrint does looks better in my opinion.
3. I prefer the rendering of Firefox for rotated + scaled letters, but WeasyPrint looks more like Chrome. What do you think? It should be possible to fix that by playing with transformation matrices.

Here’s a small sample I’ve used to test manually:

<svg width="150" height="1000" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" font-family="serif">
  <rect fill="pink" width="150" height="1000"/>
  <rect fill="white" width="100" height="1000" x="10"/>
  <text textLength="100" x="10" y="100" fill="black">abcd</text>
  <text textLength="100" x="10" y="200" fill="black" rotate="30">abcd</text>
  <text textLength="100" x="10" y="300" fill="black" lengthAdjust="spacingAndGlyphs">abcd</text>
  <text textLength="100" x="10" y="400" fill="black" rotate="30" lengthAdjust="spacingAndGlyphs">abcd</text>
  <text x="10" y="500" fill="black" letter-spacing="10">abcd</text>
  <text textLength="100" x="10" y="600" fill="black" letter-spacing="10">abcd</text>
  <text textLength="100" x="10" y="700" fill="black" letter-spacing="10" lengthAdjust="spacingAndGlyphs">abcd</text>
  <text textLength="100" x="10" y="800" fill="black" letter-spacing="10" lengthAdjust="spacingAndGlyphs" rotate="30">abcd</text>
</svg>

[oshmoun]

Many thanks for the adjustments!

1. Why did you introduce the `- 0.5` value in spaces count? I didn’t find an important change with `- 1` and it looks more "normal" 😄.

During my testing I noticed that without that, the letter spacing would be too much and move the last glyph outside of the specified text length. Hence I assumed there is an additional 0.5 gap that should be considered in the calculation. Now upon testing your code, I'm not so sure it wasn't a mea culpa during testing. Change to -1 appears to work as expected here 👍

2. I’ve always overridden the letter-spacing value when "spacing" is set. The rendering is a bit different to what browsers do: they have some extra space on the right, but there’s nothing I can find in the specification about that ("Whether letter-spacing is applied at the end of a line is undefined in this level.") and what WeasyPrint does looks better in my opinion.

Yes, I also can't find much in the specs, so that browser behaviour is kinda weird 😅
I see no issue with your suggestion, overwriting letter-spacing makes sense, though maybe a warning could be logged to let the user know, will keep a note of that.

3. I prefer the rendering of Firefox for rotated + scaled letters, but WeasyPrint looks more like Chrome. What do you think? It should be possible to fix that by playing with transformation matrices.

I don't have any personal preference in that regard 😅
But I think it'd be more time efficient to keep it like Chrome, as that's the one with least effort, YAGNI principle and all that 😄

[oshmoun]

Sorry about the delay in implementing the tests, will try to do that over the weekend 🙂

[liZe]

Sorry about the delay in implementing the tests, will try to do that over the weekend 🙂

Thanks. No need to hurry! 😄

[oshmoun]

three tests added, one for textLength with default lengthAdjust (spacing)
one for lengthAdjust=spacingAndGlyphs but glyphs only (no letter-spacing specified)
and one for both spacing and glyphs

[liZe]

Thanks a lot!