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