Handling DrawGlyphRun missing characters in WPF

Question

I am using drawingContext.DrawGlyphRun in my scenario, as I need the best performance I can get as I am rendering a large amount of text at lots of different positions on screen.

However, I have come into a slight snag. The font I am using is missing some unicode characters for codes such as ✓, ∞

It looks like drawingContext.DrawText has a very handy feature where a fallback font is used where characters are missing, such that you still at least get something rendered.

So now I am wondering how I could achieve something similar without losing all the performance gains I got from switching to DrawGlyphRun

Seems like I would need to break up the call in multiple parts

"My string with ∞ unsupported character"
[My string with ][∞][ unsupported character]

So 3 calls to drawingContext.DrawGlyphRun, with the 2nd one using a different font :(

And then I would also need to compute the position offsets so it all aligns correctly.

Other than changing the font (something I can't really do) am I missing an obvious easy way of doing this?

• Gerry Schmitz: How about a TextBlock with multiple Inline (text) Runs, that specify the font attributes. I (can) use a single TextBlock to display Segoe (colored) MDL2, Emoji and (colored) Symbol characters at the same time; including unicode. As for performance, I animate a few hundred user controls (at the same time) using this pattern to show "status" on the element.
• jessejbweld: TextBlock uses drawingContext.DrawText. I'm rendering about 1500+ of these strings on a large display (a bit like a spreadsheet, and all are in view at the same time, so virtualisation is no help) and it gets very slow with DrawText. One option I am thinking of is to just fall back to DrawText if the string contains an unknown character. But knowing my luck, all 1500 strings will have the unknown char :P
• Gerry Schmitz: I'm not doing any "virtualization"; these are individual user controls that host a TextBlock that displays different strings based on a toggle. The "canvas" is 6500 by 5800. A ScrollViewer zooms the whole thing in and out. I used a "few hundred" as a number. I can host thousands. All animating.
• jessejbweld: @GerrySchmitz interesting. Do you think you could share the code on github or something please? I am curious to see what you're doing
• Gerry Schmitz: The essence of my response is TextBlock.Inlines and Run; which this link touches on. learn.microsoft.com/en-us/uwp/api/…

Answer 1

I managed to get this working by using the Global User Interface fontfamily. Which a collection of typefaces that are used as fallbacks when a character code is not found in the target font.

I decided to cache the typeface for these missing character codes.

private readonly Dictionary<int, GlyphTypeface?> _fallbackGlyphMapping = [];
private bool TryGetFallbackGlyphTypeface(int code, [NotNullWhen(true)] out GlyphTypeface? glyphTypeface)
{
 if (!_fallbackGlyphMapping.TryGetValue(code, out glyphTypeface))
 {
 _globalFontFamily ??= new FontFamily("Global User Interface");
 foreach (var map in _globalFontFamily.FamilyMaps)
 {
 var typeface = new Typeface(new FontFamily(map.Target), FontStyles.Normal, FontWeights.Normal, FontStretches.Normal);
 if (typeface.TryGetGlyphTypeface(out GlyphTypeface? tempGlyphTypeface))
 {
 if (tempGlyphTypeface.CharacterToGlyphMap.TryGetValue(code, out var index))
 {
 _fallbackGlyphMapping[code] = tempGlyphTypeface;
 glyphTypeface = tempGlyphTypeface;
 return true;
 }
 }
 }
 // couldn't locate
 _fallbackGlyphMapping[code] = null;
 return false;
 }
 return glyphTypeface != null;
}