The Raster Tragedy in Skia
This is an extension of The Raster Tragedy at Low-Resolution Revisited as it applies to Skia. The Raster Tragedy describes a number of issues with typeface rasterization with a particular emphasis on proper hinting to overcome these issues. Since not all fonts are nicely hinted and sometimes hinting is not desired, there are additional hacks which may be applied. Generally, one wants to hint purely informational text laid out for a particular device, but not hint text which is art. Unless, of course, the hinting is part of the art like Shift_JIS art.
The Gamma Hack
First, one should be aware of transfer functions (of which 'gamma' is an example). A good introduction can be had at What Every Coder Should Know About Gamma.
In Skia, all color sources are converted into the destination color space and the blending is done in the destination color space by applying the linear blend function. Skia does not convert into a linear space, apply the linear blend, and convert back to the encoded space. If the destination color space does not have a linear encoding this will lead to 'incorrect' blending. The idea is that there are essentially two kinds of users of Skia. First there are existing systems which are already using a non-linear encoding with a linear blend function. While the blend isn't correct, these users generally don't want anything to change due to expectations. Second there are those who want everything done correctly and they are willing to pay for a linearly encoded destination in which the linear blend function is correct.
For bi-level glyph rendering a pixel is either covered or not, so there are no coverage blending issues.
For regular full pixel partial coverage (anti-aliased) glyph rendering the user may or may not want correct linear blending. In most non-linear encodings, using the linear blend function tends to make black on white look slightly heavier, using the pixel grid as a kind of contrast and optical sizing enhancement. It does the opposite for white on black, often making such glyphs a bit under-covered. However, this fights the common issue of blooming where light on dark on many displays tends to appear thicker than dark on light. (The black not being fully black also contributes.) If the pixels are small enough and there is proper optical sizing and perhaps anti-aliased drop out control (these latter two achieved either manually with proper font selection or 'opsz', automatically, or through hinting) then correct linear blending tends to look great. Otherwise black on white text tends to (correctly) get really anemic looking at small sizes. So correct blending of glyph masks here should be left up to the user of Skia. If they're really sophisticated and already tackled these issues then they may want linear blending of the glyphs for best effect. Otherwise the glyphs should just keep looking like they used to look due to expectations.
For subpixel partial coverage (subpixel anti-aliased) glyph masks linear blending in a linear encoding is more or less required to avoid color fringing effects. The intensity of the subpixels is being directly exploited so needs to be carefully controlled. The subpixels tend to alleviate the issues with no full coverage (though still problematic if blitting text in one of the display's primaries). One will still want optical sizing since the glyphs will still look somewhat too light when scaled down linearly.
So, if subpixel anti-aliased glyph masks (and sometimes full pixel anti-aliased glyph masks) need a correct blit how are they to be used with non-linearly encoded destinations?
One possible solution is to special case these blits. If blitting on the CPU it's often fast and close enough to take the square root of the source and destination values, do the linear blend function, then square the result (approximating the destination encoding as if its transfer function is square). Many GPUs have a mode where they can blend in sRGB, though unfortunately this generally applies to the whole framebuffer, not just individual draws. For various reasons, Skia has avoided special casing these blends.
What Skia currently does is the gamma hack. When creating the glyph mask one usually knows the approximate color which is going to be drawn, the transfer function, and that a linear source-over blend is going to be used. The destination color is then guessed to be a contrasting color (if there isn't any contrast the drawing won't be able to be seen anyway) so assume that the destination color will be the perceptually opposite color in the destination color space. One can now determine the desired value by converting the perceptual source and guessed destination into a linear encoding, do the linear source-over blend, and convert to the destination encoding. The coverage is then adjusted so that the result of a linear source-over blend on the non-linear encoded values will be as close as possible to this desired value.
This works, but makes many assumptions. The first is the guess at the destination. A perceptual middle gray could equally well (or poorly) contrast with black or white so the best guess is drawing it on top of itself. Subpixel anti-aliased glyph masks drawn with this guess will be drawn without any adjustment at all, leaving them color fringy. On macOS Skia tweaks the destination guess to reduce the correction when using bright sources. This helps reducing 'blooming' issues with light on dark (aka dark mode) and better matches how CoreText seems to do blending. The second is that a src-over blend is assumed, but users generally aren't as discriminating of the results of other blends.
The gamma hack works best with subpixel anti-aliasing since the adjustment can be made per-channel instead of full pixel. If this hack is applied to full pixel anti-aliased masks everything is essentially being done in the nearest gray instead of the nearest color (since there is only one channel through which to communicate), leading to poor results. Most users will not want the gamma hack applied to full pixel anti-aliased glyphs.
Since the gamma hack is logically part of the blend, it must always be the very last adjustment made to the coverage mask before being used by the blitter. The inputs are the destination transfer function and the current color (in the destination color space). In Skia these come from the color space on the SkSurface and the color on the SkPaint.
Optical Sizing Hack
In metal type a type designer will draw out on paper the design for the faces of the final sorts. Often there would be different drawings for different target sizes. Sometimes these different sizes look quite different (like Display and Text faces of the same family). Then a punch cutter takes these drawings and creates a piece of metal called a punch which can stamp out these faces. The punch is used to create a negative in a softer piece of metal forming a strike. This strike is made the correct width and called a matrix. This matrix is used as a mold to cast individual sorts. The sorts are collected into a box called a type case. A typesetter would take sorts from a type case and set them into a form. The printer would then ink the sorts in the form and press the paper. (Note that the terms 'typeface' and 'font' aren't used in this description, there is a lot of disagreement on what they apply to.)
Every step of this process is now automated in some way. Unfortunately, knowledge embedded in the manual process has not always been replicated in the automation. This can be for a wide variety of reasons, such as being overlooked or being difficult to emulate. One of these areas is the art of optical sizing and managing thin features.
In general smaller type should be relatively heavier in weight than would be expected if taking a larger size and linearly scaling it down. The type designer will draw out the faces with this in mind, potentially with an eye toward how it would vary with size and with potential need for ink traps. The punch cutter would then cut the punches at a given size with this in mind, adjusting until the soot proofs looked good. There may even be slight adjustments to the matrix itself if something seemed off. The typesetter would often know which specific cases contained slightly heavier or lighter sorts. The printer would then adjust the ink and pressure to get a good looking print.
Popular digital font formats didn't really support this until recently with the variable font 'opsz' axis. The presence of an 'opsz' axis is like the type designer giving really good instructions about how the faces should look at various sizes. However, not all fonts have or will have a 'opsz' axis. Since we don't always have an 'opsz' what can be done to prevent small glyphs from looking all washed out?
One way the type designer could influence the optical size is through hinting. Any manual hinting is done by someone looking at the result at small sizes. Tweaks are made until it 'looks good'. This often unconsciously bakes in optical sizing. Even autohinters eventually end up emulating this, generally making the text more contrasty and somewhat heavier at small pixel sizes (this depends on the target pixel size not the nominal requested size).
An alternate way the designer could provide optical sized fonts is using multiple font files for different optical sizes and expect the user to select the right one (like Display and Text variants). In theory the right one might also be selected automatically based on some criteria. Switching font files because of nominal size changes may seem drastic, but this is how the system font on macOS worked for a while.
One automatic way to do optical sizing is to artificially embolden the outline itself at small nominal sizes. This is the approach taken by CoreText. This is a lot like the dreaded fake-bold, but doesn't have the issue of the automatic fake bolding being heavier than ultra bold, since this emboldening is applied uniformally based on requested nominal size.
Another automatic way to do optical sizing is to over-cover all the edges when doing anti-aliasing. The pixels are a fixed physical size so affect small features more than large features. Skia currently has rudimentary support for this in its 'contrast hack' which is described more later.
As a note on optical sizing, this is one place where the nominal or optical size of the font is treated differently from the final pixel size of the font. A SkCanvas may have a scale transform active. Any hinting will be done at the final pixel size (the font size mapped through the current transformation matrix), but the optical size is not affected by the transform since it's actually a parameter for the SkTypeface (and maybe the SkFont).
The Contrast Hack
Consider the example of a pixel wide stroke at half-pixel boundaries. As stated in section 3.1.3 "if we were to choose the stroke positions to render the most faithful proportions and advance width, we may have to compromise the rendering contrast." If the stroke lands on a pixel boundary all is well. If the stoke lands at a half pixel boundary and correct linear blending is used then the same number of photons are reaching the eye, but the reduction in photons is spread out over twice the area. If the pixels were small enough then this wouldn't be an issue. Back of the envelope suggests an 8K 20inch desktop monitor or ~400ppi being the minimum. Note that RGB subpixel anti-aliasing brings a 100dpi display up to 300dpi, which is close, but still a bit short. Exploiting the RGB subpixels also doesn't really increase the resolution as much when getting close to the RGB primaries for the fill color.
One way to try to compensate for this is to cover some of these partially covered pixels more until it visually looks better. Note that this depends on a lot of factors like the pixel density of the display, the visual acuity of the user, the distance of the user from the display, and the user's sensitivity to the potential variations in stem darkness which may result. Automatically taking all these factors into account would be quite difficult. The correct function is generally determined by having the user look at the result of applying various amounts of extra coverage and having them to pick a setting that looks the least bad to them.
This specific form of over-covering is a form of drop out control for anti-aliasing and could be implemented in a similar way, detecting when a stem comes on in one pixel and goes out in the next and mark that for additional coverage. At raster time a cruder approximation could be made by doing a pass in each of the horizontal and vertical directions and finding runs of more than one non-fully-covered pixel and increasing their coverage.
If instead of doing these computationally expensive passes all coverage is boosted then in addition to the smeared stems the entire outside edge of the glyph will also be bolded. Making these outside edges heavier is a crude approximation of outsetting the initial path in a rather complicated way and amounts to an optical sizing tweak. Just as hinting can be used to approximate optical sizing if the user's perception of the pixel sizes is known in advance, this is a pixel level tweak tied to a specific user and display combination.
Much like the gamma hack can be modified to reduce the correction for light on dark to fight blooming, the contrast hack can be reduced when the color being drawn is known to be light. Generally the contrast correction goes to zero as one approaches white.