mirror of https://github.com/nothings/stb.git
45 lines
2.0 KiB
Markdown
45 lines
2.0 KiB
Markdown
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# Font character oversampling for rendering from atlas textures
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TL,DR: Run oversample.exe on a windows machine to see the
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benefits of oversampling. Type the name of a .ttf file on
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the command-line, or it will look for Arial in the Windows
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font directory.
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## About oversampling
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A common strategy for rendering text is to cache character bitmaps
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and reuse them. For hinted characters, every instance of a given
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character is always identical, so this works fine. However, stb_truetype
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doesn't do hinting.
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For anti-aliased characters, you can actually position the characters
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with subpixel precision, and get different bitmaps based on that positioning
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if you re-render the vector data.
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However, if you simply cache a single version of the bitmap and
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draw it at different subpixel positions with a GPU, you will get
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either the exact same result (if you use point-sampling on the
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texture) or linear filtering. Linear filtering will cause a sub-pixel
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positioned bitmap to blur further, causing a visible desharpening
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of the character. (And, since the character wasn't hinted, it was
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already blurrier than a hinted one would be, and not it gets even
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more blurry.)
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You can avoid this by caching multiple variants of a character which
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were rendered independently from the vector data. For example, you
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might cache 3 versions of a char, at 0, 1/3, and 2/3rds of a pixel
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horizontal offset, and always require characters to fall on integer
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positions vertically.
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When creating a texture atlas for use on GPUs, which support bilinear
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filtering, there is a better approach than caching several indepdent
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positions, which is to allow lerping between the versions to allow
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finer subpixel positioning. You can achieve these by interleaving
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each of the cached bitmaps, but this turns out to be mathematically
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equivalent to a simpler operation: oversampling and prefiltering the
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characters.
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So, setting oversampling of 2x2 in stb_truetype is equivalent to caching
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each character in 4 different variations, 1 for each subpixel position
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in a 2x2 set.
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