Create README.md

tests/oversample/README.md

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