Re: Text shadows

[Kai Ma]

> On Mar 12, 2023, at 15:12, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Kai Ma <justksqsf@gmail.com>
>> Date: Sun, 12 Mar 2023 12:06:28 +0800
>> 
>> Recently, I implemented the “text shadows” feature for Cairo-powered 
>> windowing systems, including X11 and Wayland.  See [1] if you wonder what 
>> that looks like.
> 
> Which parts of the feature implementation really require Cairo?  IOW,
> why cannot this be available to non-Cairo builds as well?

Theoretically, nothing.  I targeted Cairo because (I think) it is the most 
popular option on GNU/Linux.

Speaking of this specific implementation, gaussian_blur() relies on a specific 
Cairo image format ARGB32, (this assumption could be easily removed), and then 
it is used by ftcrfont_draw() to generate shadows.

I don’t know whether non-Cairo builds (non-cairo X, win32) can/should use 
gaussian_blur() though, since I’m not familiar with them.

>> This feature adds a new face attribute :shadow, and accepts values like:
>> 
>> - FLOAT                 how much blurring?
>> - (FLOAT . COLOR)       additionally, specifies a color for the shadow
>> - (FLOAT COLOR OFFSET)  additionally, specifies an offset (x . y)
> 
> I think at least in some GUI systems the offset is specified as radius
> and angle.

The shadow is generated by this process:

- draw glyphs (in the shadow color) onto a blank canvas;
- apply the blurring function;
- copy the “shadow” back to the original canvas at the specified x-y coordinate.

So I don’t think radius and angle is relevant here.

>> I’m writing to ask if the community is interested in accepting this feature 
>> into mainline Emacs?  If so, I will improve things and make a formal patch.
> 
> Yes, I think we'd like to support this feature.  But it looks to me
> like the current WIP is incomplete, as it doesn't handle the change in
> glyph metrics due to the shadowing, see below.

Thanks!

> It is best to submit a feature-request bug report using
> report-emacs-bug, and then continue the discussion on our issue
> tracker.

Will do after I fix the glyph metrics.

>> 1. The shadows sometimes should be drawn outside the glyph extents, but they 
>> aren’t drawn.
>> 2. Previously drawn shadows are not cleared, and they even accumulate.  This 
>> can be observed with blink-cursor-mode enabled.
> 
> This probably means you need further changes in the layout part of the
> display code, in xdisp.c, to account for the changes in the glyph
> metrics due to the shadow attribute.  See, for example, how the 'box'
> face attribute is handled there.

Thanks for this pointer.  I will work this out first, and then let’s continue 
the discussion on the bug tracker.

— Kai