Re: Overlap oversampling

[Alexei Podtelezhnikov]

David,

On Mon, Jun 29, 2020 at 6:58 PM David Turner <david@freetype.org> wrote:
> So, could have a deep look at the patches here. They're pretty neat. I'll 
> just recommend documenting the subtle computations in ft_smooth_slow_spans() 
> a little better, and avoid branches altogether, by using bit twiddling to 
> perform saturated addition instead (removing branches from loops is always 
> best for performance). I.e. something like the following:

I completely agree with sum | -(sum>>8). Another idea is sum - (sum>>8).

> Can you tell me how to actually test that the code works as expected though?

The proof-of-concept patch is set up to replace normal rendering with
oversampling when SCALE is 2 or 4. I was using this font:
https://github.com/adobe-fonts/source-serif-pro/tree/release/VAR

I suggested FT_RENDER_MODE_SLOW to explicitly discourage its use for
good fonts. We can do FT_RENDER_MODE_OVERSAMPLE instead. Werner also
suggested using OVERLAP_SIMPLE and OVERLAP_COMPOUND flags to trigger
this mode but they may be unset or unavailable. I think this mode
should be explicit.

Regards,
Alexei