Handling DrawGlyphRun missing characters in WPF

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?

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;
}