Re: Doc string and operation of color-distance

[Mark Oteiza <mvoteiza@udel.edu>]

Eli Zaretskii <eliz@gnu.org> writes:

> Mark, the new color-distance says something strange in its doc string:
>
>   (color-distance COLOR1 COLOR2 &optional FRAME METRIC)
>
>   Return an integer distance between COLOR1 and COLOR2 on FRAME.
>   COLOR1 and COLOR2 may be either strings containing the color name,
>   or lists of the form (RED GREEN BLUE), each in the range 0 to 65535 inclusive.
>   If FRAME is unspecified or nil, the current frame is used.
>   If METRIC is unspecified or nil, a modified L*u*v* metric is used.
>
> The last sentence should say "non-nil", I think, and it should
> document that METRIC is supposed to be a function of 2 colors.
>
> Also, this is unexpected:
>
>   (color-distance "red" "blue" nil 'lcms-cam02-ucs)
>   => (error "Invalid color" "red")
>
> I think the new lcms2 functions should support colors specified as
> strings, because all the other color-related functions do.

At the very least we should feed metrics the translated RGBs.

Beyond that, in order to use (for example) lcms2-cam02-ucs with
color-distance, one has to translate RGB to XYZ, so the metric would be

  (lambda (a b)
    (lcms2-cam02-ucs (color-srgb-to-xyz a) (color-srgb-to-xyz b))

diff --git a/src/xfaces.c b/src/xfaces.c
index 012de4e7af..9804c96c39 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -4093,7 +4093,9 @@ DEFUN ("color-distance", Fcolor_distance, Scolor_distance, 2, 4, 0,
 COLOR1 and COLOR2 may be either strings containing the color name,
 or lists of the form (RED GREEN BLUE), each in the range 0 to 65535 inclusive.
 If FRAME is unspecified or nil, the current frame is used.
-If METRIC is unspecified or nil, a modified L*u*v* metric is used.  */)
+If METRIC is specified, it should be a function that accepts two
+lists of the form (RED GREEN BLUE).  The default metric uses a
+modified L*u*v* space. */)
   (Lisp_Object color1, Lisp_Object color2, Lisp_Object frame,
    Lisp_Object metric)
 {
@@ -4112,7 +4114,15 @@ If METRIC is unspecified or nil, a modified L*u*v* metric is used.  */)
   if (NILP (metric))
     return make_number (color_distance (&cdef1, &cdef2));
   else
-    return call2 (metric, color1, color2);
+    {
+      Lisp_Object rgb1 = list3 (make_number (&cdef1.red),
+                                make_number (&cdef1.green),
+                                make_number (&cdef1.blue)); 
+      Lisp_Object rgb2 = list3 (make_number (&cdef2.red),
+                                make_number (&cdef2.green),
+                                make_number (&cdef2.blue)); 
+      return call2 (metric, rgb1, rgb2);
+    }
 }
 
 \f