Re: Text shadows

[Eli Zaretskii <eliz@gnu.org>]

> From: Kai Ma <justksqsf@gmail.com>
> Date: Sun, 12 Mar 2023 16:24:17 +0800
> Cc: emacs-devel@gnu.org
> 
> > 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.

Would you please investigate that aspect?  E.g., does Firefox support
this without Cairo, and if so, how?

At worst, perhaps it would be good enough not to support blurring on
some platforms, or support it by some minor modification of the color?
Or maybe we could implement the equivalent of gaussian_blur in our
code?

> >> 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.

The radius and angle are just polar specifications of the Cartesian
X-offset and Y-offset, that's all.  If the API needs Cartesian
specification, it should be trivial to convert polar to Cartesian
before calling the API.