[evince] libview: use annotation color for the close button

[Germán Poo-Caamaño <gpoo src gnome org>]

commit da99e881a4e68b9f849c99cd4fccd18b56ca5492
Author: vanadiae <vanadiae35 gmail com>
Date:   Sat Jun 6 22:30:53 2020 +0200

    libview: use annotation color for the close button
    
    Currently, the close button of annotations takes the color from
    the theme on hover, which means it appears white/black on the
    colorful headerbar.
    
    This commit stop using the internal close button icon and now
    uses GTK window-close-symbolic instead, which means it can now
    be colored as well. The deprecated GTK object GtkStyleProperties
    has been replaced by GtkCssProvider. Finally, the color of the
    button is now the same as annotation's one and the close icon
    takes the most readable color between white and black.

 libview/ev-annotation-window.c | 108 ++++++++++++++++++++++++++++++-----------
 1 file changed, 81 insertions(+), 27 deletions(-)
---
diff --git a/libview/ev-annotation-window.c b/libview/ev-annotation-window.c
index f247ca5e..4b5011ca 100644
--- a/libview/ev-annotation-window.c
+++ b/libview/ev-annotation-window.c
@@ -22,6 +22,7 @@
 #include "config.h"
 
 #include <string.h>
+#include <math.h>
 
 #include "ev-annotation-window.h"
 #include "ev-stock-icons.h"
@@ -118,28 +119,89 @@ ev_annotation_window_sync_contents (EvAnnotationWindow *window)
        g_free (contents);
 }
 
+static double
+get_srgb (const double color_component)
+{
+       /* calculation of sRGB color is based on note 1 of 
https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef */
+       if (color_component <= 0.03928)
+               return color_component / 12.92;
+       else
+               return powf (((color_component + 0.055) / 1.055), 2.4);
+}
+
+static double
+get_relative_luminance (const GdkRGBA *color)
+{
+       /* calculation of relative luminance is based on note 1 of 
https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef */
+       return get_srgb (color->red) * 0.2126 + get_srgb (color->blue) * 0.0722 + get_srgb (color->green) * 
0.7152;
+}
+
+static double
+get_contrast_level (const GdkRGBA *bg_color,
+                   const GdkRGBA *fg_color)
+{
+       /* the contrast level calculus is based on WCAG 2.0 guideline 1.4  */
+       /* https://www.w3.org/WAI/GL/UNDERSTANDING-WCAG20/visual-audio-contrast7.html#key-terms */
+       const double bg_luminance = get_relative_luminance (bg_color);
+       const double fg_luminance = get_relative_luminance (fg_color);
+       return (fmax (bg_luminance, fg_luminance) + 0.05) / (fmin (bg_luminance, fg_luminance) + 0.05);
+}
+
+/**
+ * get_most_readable_color:
+ *
+ * Returns: (transfer none): the most readable color on bg_color between first_color and second_color
+ */
+static GdkRGBA *
+get_most_readable_color (const GdkRGBA *bg_color,
+                        GdkRGBA *first_color,
+                        GdkRGBA *second_color)
+{
+       const double first_contrast = get_contrast_level (bg_color, first_color);
+       const double second_contrast = get_contrast_level (bg_color, second_color);
+       /* higher is more readable (more contrast) */
+       return first_contrast > second_contrast ? first_color : second_color;
+}
+
+/**
+ * get_best_foreground_color:
+ *
+ * Returns: (transfer full): the most readable foreground color on bg_color between black #000000 and white 
#FFFFFF
+ */
+static GdkRGBA *
+get_best_foreground_color (const GdkRGBA *bg_color)
+{
+       GdkRGBA black, white;
+       gdk_rgba_parse (&black, "#000000");
+       gdk_rgba_parse (&white, "#FFFFFF");
+       return gdk_rgba_copy (get_most_readable_color (bg_color, &black, &white));
+}
+
 static void
 ev_annotation_window_set_color (EvAnnotationWindow *window,
                                GdkRGBA            *color)
 {
-        GtkStyleProperties *properties;
-        GtkStyleProvider   *provider;
-
-        properties = gtk_style_properties_new ();
-        gtk_style_properties_set (properties, 0,
-                                  "background-color", color,
-                                  NULL);
-
-        provider = GTK_STYLE_PROVIDER (properties);
-        gtk_style_context_add_provider (gtk_widget_get_style_context (GTK_WIDGET (window)),
-                                        provider, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
-        gtk_style_context_add_provider (gtk_widget_get_style_context (window->close_button),
-                                        provider, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
-        gtk_style_context_add_provider (gtk_widget_get_style_context (window->resize_se),
-                                        provider, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
-        gtk_style_context_add_provider (gtk_widget_get_style_context (window->resize_sw),
-                                        provider, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
-        g_object_unref (properties);
+       GtkCssProvider     *css_provider = gtk_css_provider_new ();
+       g_autofree char    *rgba_str = gdk_rgba_to_string (color);
+       g_autofree char    *css_data = NULL;
+       g_autoptr (GError)  error = NULL;
+       g_autoptr (GdkRGBA) icon_color = get_best_foreground_color (color);
+       g_autofree char    *icon_color_str = gdk_rgba_to_string (icon_color);
+       css_data = g_strdup_printf ("button {border-color: %1$s; color: %2$s; -gtk-icon-shadow:0 0; 
box-shadow:0 0;}\n\
+                                    button:hover {background: lighter(%1$s); border-color: darker(%1$s);}\n\
+                                    button:active {background: darker(%1$s);}\n\
+                                    evannotationwindow.background, button {background: %1$s}",
+                                   rgba_str, icon_color_str);
+
+       gtk_css_provider_load_from_data (css_provider, css_data, strlen (css_data), &error);
+       if (error != NULL)
+               g_error ("%s", error->message);
+
+       gtk_style_context_add_provider (gtk_widget_get_style_context (GTK_WIDGET (window)),
+                                       GTK_STYLE_PROVIDER (css_provider), 
GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+       gtk_style_context_add_provider (gtk_widget_get_style_context (window->close_button),
+                                       GTK_STYLE_PROVIDER (css_provider), 
GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+       gtk_style_context_add_class (gtk_widget_get_style_context (window->close_button), "circular");
 }
 
 static void
@@ -333,18 +395,10 @@ ev_annotation_window_init (EvAnnotationWindow *window)
        gtk_box_pack_start (GTK_BOX (hbox), header, TRUE, TRUE, 0);
        gtk_widget_show (header);
 
-       window->close_button = gtk_button_new ();
-       gtk_button_set_relief (GTK_BUTTON (window->close_button), GTK_RELIEF_NONE);
-       gtk_container_set_border_width (GTK_CONTAINER (window->close_button), 0);
+       window->close_button = gtk_button_new_from_icon_name ("window-close-symbolic", GTK_ICON_SIZE_BUTTON);
        g_signal_connect_swapped (window->close_button, "clicked",
                                  G_CALLBACK (ev_annotation_window_close),
                                  window);
-       pixbuf = gtk_icon_theme_load_icon (icon_theme, EV_STOCK_CLOSE, 8,
-                                          GTK_ICON_LOOKUP_FORCE_SIZE, NULL);
-       icon = gtk_image_new_from_pixbuf (pixbuf);
-       g_object_unref (pixbuf);
-       gtk_container_add (GTK_CONTAINER (window->close_button), icon);
-       gtk_widget_show (icon);
 
        gtk_box_pack_start (GTK_BOX (hbox), window->close_button, FALSE, FALSE, 0);
        gtk_widget_show (window->close_button);