Add support for midnight mode with color

This inversion method attempts to maintain the color hue and saturation but
inverts the lightness using the OKLab color space[^1].

[^1]: https://bottosson.github.io/posts/oklab/

* server/epdfinfo.c (image-recolor): Add feature to support the OKLab
inversion method functionality

* lisp/pdf-view.el (pdf-view-midnight-invert): Add new variable to
invert the image color lightness while maintaining hue.

(pdf-view-midnight-minor-mode): Account for above.

* lisp/pdf-info.el (pdf-info-query--parse-response): Handle changes to
:render/usecolors command

Closes: #69
Closes: #169
Closes: politza/pdf-tools#698
Closes: politza/pdf-tools#608

2 files changed, 47 insertions, 5 deletions

diff --git a/lisp/pdf-info.el b/lisp/pdf-info.el
index d3e17bf..7cc4388 100644
--- a/lisp/pdf-info.el
+++ b/lisp/pdf-info.el
@@ -570,8 +570,14 @@ interrupted."
          (let ((key (intern (car key-value)))
                (value (cadr key-value)))
            (cl-case key
-             ((:render/printed :render/usecolors)
-              (setq value (equal value "1"))))
+             ((:render/printed)
+              (setq value (equal value "1")))
+             ((:render/usecolors)
+              (setq value (ignore-errors
+                            (let ((int-val (cl-parse-integer value)))
+                              (if (> int-val 0)
+                                  int-val
+                                nil))))))
            (push value options)
            (push key options)))
        options))
@@ -1726,8 +1732,19 @@ Returns a list \(LEFT TOP RIGHT BOT\)."
             ((:render/foreground :render/background)
              (push (pdf-util-hexcolor value)
                    soptions))
-            ((:render/usecolors :render/printed)
+            ((:render/printed)
              (push (if value 1 0) soptions))
+            ((:render/usecolors)
+             ;; 0 -> original color
+             ;; 1 -> recolor document to grayscale mapping black to
+             ;;      :render/foreground and white to :render/background
+             ;; 2 -> recolor document by inverting the perceived lightness
+             ;;      preserving hue
+             (push (cond ((integerp value) value)
+                         ;; Map nil -> 0 and t -> 1
+                         (value 1)
+                         (t 0))
+                   soptions))
             (t (push value soptions)))
           (push key soptions)))
       soptions)))
diff --git a/lisp/pdf-view.el b/lisp/pdf-view.el
index afca46f..6197029 100644
--- a/lisp/pdf-view.el
+++ b/lisp/pdf-view.el
@@ -118,6 +118,20 @@ This should be a cons \(FOREGROUND . BACKGROUND\) of colors."
   :type '(cons (color :tag "Foreground")
                (color :tag "Background")))
 
+(defcustom pdf-view-midnight-invert nil
+  "In midnight mode invert the image color lightness maintaining hue.
+
+This is particularly useful if you are viewing documents with
+color coded data in plots.  This will maintain the colors such
+that 'blue' and 'red' will remain these colors in the inverted
+rendering.  This inversion is non-trivial.  This makes use of the
+OKLab color space which is well calibrated to have equal
+perceptual brightness across hue, but not all colors are within
+the RGB gamut after inversion, causing some colors to saturate.
+Nevertheless, this seems to work well in most cases."
+  :group 'pdf-view
+  :type 'boolean)
+
 (defcustom pdf-view-change-page-hook nil
   "Hook run after changing to another page, but before displaying it.
 
@@ -1243,7 +1257,16 @@ The colors are determined by the variable
                   (pdf-info-setoptions
                    :render/foreground (or (car pdf-view-midnight-colors) "black")
                    :render/background (or (cdr pdf-view-midnight-colors) "white")
-                   :render/usecolors t))))
+                   :render/usecolors
+                   (if pdf-view-midnight-invert
+                       ;; If midnight invert is enabled, pass "2" indicating
+                       ;; that :render/foreground and :render/background should
+                       ;; be ignored and to instead invert the PDF (preserving
+                       ;; hue)
+                       2
+                     ;; If invert is not enabled, pass "1" indictating that
+                     ;; :render/foreground and :render/background should be used
+                     1)))))
     (cond
      (pdf-view-midnight-minor-mode
       (add-hook 'after-save-hook enable nil t)
@@ -1252,7 +1275,9 @@ The colors are determined by the variable
      (t
       (remove-hook 'after-save-hook enable t)
       (remove-hook 'after-revert-hook enable t)
-      (pdf-info-setoptions :render/usecolors nil))))
+      (pdf-info-setoptions
+       ;; Value "0" indicates that colors should remain unchanged
+       :render/usecolors 0))))
   (pdf-cache-clear-images)
   (pdf-view-redisplay t))