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95 lines
4.5 KiB
Markdown
95 lines
4.5 KiB
Markdown
# 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. It will try to use arial.ttf from the
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Windows font directory unless you type the name of a .ttf file as
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a command-line argument.
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## Benefits of oversampling
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Oversampling is a mechanism for improving subpixel rendering of characters.
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Improving subpixel has a few benefits:
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* With horizontal-oversampling, text can remain sharper while still being sub-pixel positioned for better kerning
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* Horizontally-oversampled text significantly reduces aliasing when text animates horizontally
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* Vertically-oversampled text significantly reduces aliasing when text animates vertically
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* Text oversampled in both directions significantly reduces aliasing when text rotates
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## What text oversampling is
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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 de-sharpening
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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 now 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 independent
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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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An advantage of this formulation is that no changes are required to
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the rendering code; the exact same quad-rendering code works, it just
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uses different texture coordinates. (Note this does potentially increase
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texture bandwidth for text rendering since we end up minifying the texture
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without using mipmapping, but you probably are not going to be fill-bound
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by your text rendering.)
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## What about gamma?
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Gamma-correction for fonts just doesn't work. This doesn't seem to make
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much sense -- it's physically correct, it simulates what we'd see if you
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shrunk a font down really far, right?
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But you can play with it in the oversample.exe app. If you turn it on,
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white-on-black fonts become too thick (i.e. they become too bright), and
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black-on-white fonts become too thin (i.e. they are insufficiently dark). There is
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no way to adjust the font's inherent thickness (i.e. by switching to
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bold) to fix this for both; making the font thicker will make white
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text worse, and making the font thinner will make black text worse.
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Obviously you could use different fonts for light and dark cases, but
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this doesn't seem like a very good way for fonts to work.
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Multiple people who have experimented with this independently (me,
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Fabian Giesen,and Maxim Shemanarev of Anti-Grain Geometry) have all
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concluded that correct gamma-correction does not produce the best
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results for fonts. Font rendering just generally looks better without
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gamma correction (or possibly with some arbitrary power stuck in
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there, but it's not really correcting for gamma at that point). Maybe
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this is in part a product of how we're used to fonts being on screens
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which has changed how we expect them to look (e.g. perhaps hinting
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oversharpens them and prevents the real-world thinning you'd see in
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a black-on-white text).
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(AGG link on text rendering, including mention of gamma:
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http://www.antigrain.com/research/font_rasterization/ )
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Nevertheless, even if you turn on gamma-correction, you will find that
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oversampling still helps in many cases for small fonts.
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