You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and dots ('.'), can be up to 35 characters long. Letters must be lowercase.
44 lines
2.0 KiB
44 lines
2.0 KiB
# 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.
|
|
|