Re: Can we use FRAME_RIF to return a Lisp_Object result?

[Eli Zaretskii]

> Date: Tue, 31 Oct 2017 11:44:02 -0700
> From: Keith David Bershatsky <address@hidden>
> 
> As part of the multiple fake cursors (22873) and crosshairs (17684) features 
> that I am working on, I am obtaining the red / green / blue values for the 
> buffer default background face by using platform specific functions within 
> nsterm.m, xterm.c, and w32term.c.
> 
> I would like `FRAME_RIF (f)->calc_lsl_default_bg (w)` to return a Lisp_Object 
> in the form of a vector consisting of 3 doubles.
> 
> All of the examples in redisplay_interface are void, which makes me think 
> that a Lisp_Object result may not be supported.
> 
> I have been using window pointers to store the values for red, green and 
> blue.  However, I would like to eliminate the need to use those window 
> pointers in favor of FRAME_RIF returning a Lisp_Object result.  Is this okay 
> to do, or is there a more appropriate way to get the result of those platform 
> specific functions?
> 
> 
> /* window.h */
> /* Suffixes for the `mc` window pointer prefix. */
> struct multiple_cursors
> {
>   * * *
> 
>   /* Values for the background color that are used to erase a glyphless 
> cursor. */
>   double bg_red, bg_green, bg_blue;
> 
>   * * *
> };
> 
> 
> /* nsterm.m */
> static void
> ns_calc_lsl_default_bg (struct window *w)
> {
>   struct frame *f = XFRAME (WINDOW_FRAME (w));
>   /* The default face, possibly remapped. */
>   struct face *default_face = FACE_FROM_ID (f, lookup_basic_face (f, 
> DEFAULT_FACE_ID));
>   NSColor *background_color = ns_lookup_indexed_color 
> (default_face->background, f);
>   EmacsCGFloat red, green, blue;
>   * * *
>   w->mc.bg_red = red;
>   w->mc.bg_green = green;
>   w->mc.bg_blue = blue;
>   * * *
> }

I don't understand why you need a platform-dependent implementation
for this.  Given the face (obtained using FACE_FROM_ID as above), you
have its Lisp attributes in face->lface, and the foreground and
background pixels are then at lface[LFACE_FOREGROUND_INDEX] and
lface[LFACE_BACKGROUND_INDEX] respectively.  These are XColor values,
so getting the RGB components from them is trivial.

For frame's default face, you can do this even simpler: by using
FRAME_FOREGROUND_PIXEL and FRAME_BACKGROUND_PIXEL.

Am I missing something?