Re: Incorrect description of FT_GASP_SYMMETRIC_SMOOTHING

[Alexei Podtelezhnikov]

Hi Piotr,

On Thu, Sep 9, 2021 at 2:34 PM piotrunio-2004@wp.pl <piotrunio-2004@wp.pl> wrote:

In https://www.freetype.org/freetype2/docs/reference/ft2-gasp_table.html#ft_gasp_xxx ,

For better (but still insufficient) explanations,

https://docs.microsoft.com/en-us/typography/opentype/spec/gasp

but, to be honest, I do not know what symmetric smoothing even means.

FT_GASP_SYMMETRIC_SMOOTHING

If set, smoothing along multiple axes must be used with ClearType.

This is entirely incorrect.

From the observed behaviour of Microsoft ClearType renderers, the behaviour is as following:

If set, 6×5 or 6×1 oversampling may be used. If unset, 6×1 oversampling is always used.

They do say that the flag is used in combination with FT_GASP_SYMMETRIC_GRIDFIT and might interact. Perhaps you're onto something and

smoothing really means LCD filtering, which is not necessary if GRIDFITing. Unfortunately, OpenType specifications do not explain it.

 

This in turn roughly translates to FreeType language to the following:

If set, smoothing along multiple axes may or may not be used with ClearType.

If unset, smoothing along multiple axes should not be used with ClearType.

The sentence "If set, smoothing along multiple axes must be used with ClearType." is therefore incorrect and should be changed.

We do not use or apply gasp directly but we might have that functionality at user's discretion. We need more clarity on what they actually mean by CleatType smoothing.

Alexei