Re: coverage to alpha

[Alexei Podtelezhnikov]

On Tue, Jun 8, 2021 at 11:14 AM Eric Muller <emuller@amazon.com> wrote:
>
> On the one hand, we have a coverage map, i.e. for each pixel how much of it 
> is covered by the outline.
>
> On the other hand, we are blending a foreground (text) color with a 
> background color. So we need some alpha, and use it for a linear blending, in 
> a linear colorspace.
>
> What is not clear to me is how to go from coverage to alpha. Identity is 
> plausible, but there are also reasons to believe it is not:


Coverage is a pixel proportion that draws the outline foreground
color: Coverage == opacity (aka alpha). It is more than plausible, it
is justifiable. It certainly helps to use gamma correction too so that
blending is linear.


> - things like White's illusion show that the perception works in strange ways


I do not know why you are concerned, or how, or why you want to
compensate for this using coverage or alpha.


> - "One usually begins by assuming that nothing is known about the object 
> world and then the diffraction limit outlines the range of object details 
> that an image transfer allows to be gained and, by exclusion, those that it 
> leaves undetermined. On the other hand, it might be known ahead of time that 
> the ensemble of possible objects is restricted. Then distinctions can be made 
> by concentrating on the expected differences and disregarding image aspects 
> that might have arisen from sources known beforehand to be absent." (Optical 
> superresolution and visual hyperacuity, Westheimer), which can explain how 
> readers can perceive gray pixels differently (i.e. expecting black and white, 
> and therefore perceiving gray as width)
>
> - may be the alpha should also depend on the foreground/background color


FreeType prefers to stay out of it. We sell *coverage*. Take it or
leave it. Once bought, you can do whatever you want with it in the
context of your perception experiments :)


> - may be the alpha should also depend on the ppem.


Again, we sell coverage. It looks great as alpha with gamma correction.