Parameter to control text line height/leading

[antonfirsov]

Follow-up on SixLabors/ImageSharp.Drawing#112:

We need to introduce a parameter in title in RenderOptions.

That parameter should then somehow alter the following offset value:

Fonts/src/SixLabors.Fonts/TextLayout.cs

Line 279 in 4beaff1

var offset = new Vector2(0, lineHeightOfFirstLine - top);

/cc @tocsoft @JimBobSquarePants

[JimBobSquarePants]

So I've been doing some reading and according to these Flutter docs LineHeight appears to be a combination of the Leading value (additional spacing split evenly above and below the line) and the font Height value (height (ascent plus descent)).

However, we calculate the Font.LineHeight property as TypoAscender - TypoDescender + TypoLineGap based on the Microsoft docs.

This, as you can imagine, leads me to some confusion.

@antonfirsov @tocsoft I'll definitely need your guidance here. Should we be simply adding a float? LineHeight property and calculating everything from that?

On a side note we should refactor RenderOptions to remove all the constructors and use property initializers.

[tocsoft]

I wouldn't add a LineHeight property instead we should be provide a LineSpacing property (which is how such a feature is presented in Word) and if we are using Word as a strategy then it should be a multiply we apply the calculated LineHeight.

So if you want to increase the spacing between the lines you would provide a number > 1 and to get lines closer a value < 1 with one being our current size and default value.... I also would not stop negative values as I can see it producing some interesting results and can't see why it would break anything.

[JimBobSquarePants]

Is Word something we want to base our approach on?

Perhaps we should follow the WC3 CSS Fonts Module Level 4 spec and provide overrides for ascent, descent, and line-gap?

https://www.w3.org/TR/css-fonts-4/#font-metrics-override-desc

I'm trying to figure out what Skia and System.Drawing do but it's bizarrely difficult to discover an straight answer.

[JimBobSquarePants]

Ok @tocsoft I did some reading and, as ever, your plan is definitely the most sane!

I'm assuming applying the value would be as simple as this then?

var offset = new Vector2(0, (lineHeightOfFirstLine * options.LineSpacing) - top);

Edit. Nope. Adjusting that value appears to offset the first line and doesn't affect wrapped lines.