SKShaper: Support SKPaint.TextAlign property

[koolkdev]

Description of Change

Added handling of the SKPaint.TextAlign property in SKShaper.Shape.
The width is calculated by the sum of XAdvance

Befpre;
DrawText with TextAlign = Center:

DrawShapedText with TextAlign = Center

After this PR:
DrawShapedText with TextAlign = Center

Bugs Fixed

• Related to issue [BUG] SKShaper/DrawShapedText ignores SKPaint.TextAlign property #1909

Behavioral Changes

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Updated documentation

[Gillibald]

This might be a huge performance hit. I think text alignment should be evaluated when the text blob is being drawn onto a canvas. Better implement this in the DrawShapedText extension method. SKTextBlob.Bounds should give you the needed Size for your calculations.

[koolkdev]

Hi, thanks for your suggestion. I tried to implement it, but Bounds return much larger box than there is actually rendered.

For example:
SKTextBlob.Create("test", font).Bounds.Width
returns 157.3125
while
paint.MeasureText("test")
returns 67.125

According to the documentation of bounds:

    /** Returns conservative bounding box. Uses SkPaint associated with each glyph to
        determine glyph bounds, and unions all bounds. Returned bounds may be
        larger than the bounds of all glyphs in runs.

        @return  conservative bounding box
    */

When diving deeper into the code of SkTextBlobBuilder, there are two functions to calculate the bounds, one is called TightRunBounds and the other ConservativeRunBounds, where ConservativeRunBounds is used for sk_textblob_builder_alloc_run_pos and sk_textblob_builder_alloc_run_pos_h, and TightRunBounds is only used for sk_textblob_builder_alloc_run (and there is a FIXME comment to not use tight run in this case too).
So it seems that bounds is pretty useless for this use case.

Another option is to use paint.MeasureText but it would not be accurate with things like Arabic text (which is the point of SKShaper)

Another option to that I can think of is to return it as part of the Result of SKShaper.Shape (last xOffset minus the starting xOffset) and than use it in DrawShapedText. What do you think?
Thanks.

[koolkdev]

koolkdev@17da8ad

[Gillibald]

DrawPositionedText should use

SkiaSharp/binding/Binding/SKTextBlob.cs

Line 806 in fffae82

public SKPositionedRunBuffer AllocatePositionedRun (SKPaint font, int count, int textByteCount, SKRect? bounds)

if that doesn't work there is something wrong with the binding

[Gillibald]

Hmm, I think bounds is misused here and should be an out parameter. Only when a nullptr is used Skia calculates the bounds.

[koolkdev]

It seems that bounds is an input parameter, according to documentation:

        bounds defines an optional bounding box, used to suppress drawing when SkTextBlob
        bounds does not intersect SkSurface bounds. If bounds is nullptr, SkTextBlob bounds
        is computed from RunBuffer::pos, and RunBuffer::glyphs metrics.

Skia calculate the bounds anyway, but that parameter is just the initial value which it does SkRect.join to
https://github.com/mono/skia/blob/916a1e9721de74a05c75391f52393972ed7f736c/src/core/SkTextBlob.cpp#L514
https://github.com/mono/skia/blob/916a1e9721de74a05c75391f52393972ed7f736c/src/core/SkTextBlob.cpp#L383
At least if I am understanding the code correctly....

Anyway, the issue is the that Skia uses conservative way to calculate the bounds (probably for performance) which result in a much larger bounds than the actual bounds. (like twice as big or more in some cases), so it doesn't really help in this case.

What do you say about the current solution?

[mattleibow]

Thanks for this PR. It has been so long since I reviewed a PR on this repo 😢 Thanks for this, but I just got 1 suggestion about API/ABI compatibility.

[mattleibow]

/azp run SkiaSharp

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