[elpa] externals/ement 298a902e07 4/7: Fix: Color-related compatibility with Emacs 27

[ELPA Syncer]

branch: externals/ement
commit 298a902e07443579db5aefc280d85361614cc832
Author: Adam Porter <adam@alphapapa.net>
Commit: Adam Porter <adam@alphapapa.net>

    Fix: Color-related compatibility with Emacs 27
    
    Copy (color-luminance-dark-limit) and (color-dark-p) from Emacs 28.
    
    Fixes #99.  Thanks to Phil Sainty (@phil-s).
    
    Reported-by: Phil Sainty <psainty@orcon.net.nz>
    Suggested-by: Phil Sainty <psainty@orcon.net.nz>
---
 README.org      |  1 +
 ement-lib.el    | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 ement-notify.el |  2 +-
 ement-room.el   |  2 +-
 4 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/README.org b/README.org
index 44f746a70f..a1d4674867 100644
--- a/README.org
+++ b/README.org
@@ -303,6 +303,7 @@ Note that, while ~matrix-client~ remains usable, and 
probably will for some time
 + Command ~ement-describe-room~ for rooms without topics.
 + Improve insertion of old messages around existing timestamp headers.
 + Reduce D-Bus notification system check timeout to 2 seconds (from the 
default of 25).
++ Compatibility with Emacs 27.
 
 ** 0.2.1
 
diff --git a/ement-lib.el b/ement-lib.el
index dfcb433114..31d5bb48e8 100644
--- a/ement-lib.el
+++ b/ement-lib.el
@@ -61,6 +61,49 @@
 (defvar ement-room-prism-color-adjustment)
 (defvar ement-room-prism-minimum-contrast)
 
+;;;; Compatibility
+
+;; These workarounds should be removed when they aren't needed.
+
+;;;;; Emacs 28 color features.
+
+;; Copied from Emacs 28.  See 
<https://github.com/alphapapa/ement.el/issues/99>.
+
+;; FIXME: Remove this workaround when possible.
+
+(eval-and-compile
+  (unless (boundp 'color-luminance-dark-limit)
+    (defconst ement--color-luminance-dark-limit 0.325
+      "The relative luminance below which a color is considered 'dark'.
+A 'dark' color in this sense provides better contrast with white
+than with black; see `color-dark-p'.
+This value was determined experimentally.")))
+
+(defalias 'ement--color-dark-p
+  (if (fboundp 'color-dark-p)
+      'color-dark-p
+    (lambda (rgb)
+      "Whether RGB is more readable against white than black.
+RGB is a 3-element list (R G B), each component in the range [0,1].
+This predicate can be used both for determining a suitable (black or white)
+contrast colour with RGB as background and as foreground."
+      (unless (<= 0 (apply #'min rgb) (apply #'max rgb) 1)
+        (error "RGB components %S not in [0,1]" rgb))
+      ;; Compute the relative luminance after gamma-correcting (assuming sRGB),
+      ;; and compare to a cut-off value determined experimentally.
+      ;; See https://en.wikipedia.org/wiki/Relative_luminance for details.
+      (let* ((sr (nth 0 rgb))
+             (sg (nth 1 rgb))
+             (sb (nth 2 rgb))
+             ;; Gamma-correct the RGB components to linear values.
+             ;; Use the power 2.2 as an approximation to sRGB gamma;
+             ;; it should be good enough for the purpose of this function.
+             (r (expt sr 2.2))
+             (g (expt sg 2.2))
+             (b (expt sb 2.2))
+             (y (+ (* r 0.2126) (* g 0.7152) (* b 0.0722))))
+        (< y ement--color-luminance-dark-limit)))))
+
 ;;;; Functions
 
 ;;;;; Commands
@@ -556,7 +599,7 @@ avatars, etc."
                                                  "white"))
                                               (_
                                                ;; The `contrast-with` color is 
usable: test it.
-                                               (if (color-dark-p 
(color-name-to-rgb contrast-with))
+                                               (if (ement--color-dark-p 
(color-name-to-rgb contrast-with))
                                                    "white" "black")))))))
       (apply #'color-rgb-to-hex (append color-rgb (list 2))))))
 
diff --git a/ement-notify.el b/ement-notify.el
index b73eb605b9..d929d0022f 100644
--- a/ement-notify.el
+++ b/ement-notify.el
@@ -325,7 +325,7 @@ If ROOM has no existing buffer, do nothing."
             (let ((color (color-desaturate-name
                           (ement--prism-color (ement-room-id room) 
:contrast-with (face-foreground 'default))
                           50)))
-              (if (color-dark-p (color-name-to-rgb (face-background 'default)))
+              (if (ement--color-dark-p (color-name-to-rgb (face-background 
'default)))
                   (color-darken-name color 25)
                 (color-lighten-name color 25))))))
 
diff --git a/ement-room.el b/ement-room.el
index 3a65731d0f..30f025d6f9 100644
--- a/ement-room.el
+++ b/ement-room.el
@@ -895,7 +895,7 @@ spec) without requiring all events to use the same margin 
width."
                                     (setf (ement-user-message-color sender)
                                           (let ((message-color 
(color-desaturate-name (ement--user-color sender)
                                                                                
       ement-room-prism-message-desaturation)))
-                                            (if (color-dark-p 
(color-name-to-rgb (face-background 'default)))
+                                            (if (ement--color-dark-p 
(color-name-to-rgb (face-background 'default)))
                                                 (color-lighten-name 
message-color ement-room-prism-message-lightening)
                                               (color-darken-name message-color 
ement-room-prism-message-lightening))))))))
                (redacted-face (when (or local-redacted-by unsigned-redacted-by)