Recolor the page instead of inverting the colors

This patch makes it possible to define two colors
that are used for recoloring the document pages instead
of the classic color reverting.

For the old behaviour set recolor_darkcolor to "#FFFFFF"
and recolor_lightcolor to "#000000".

All credit of this commit belongs to fedcer (fedcer@gmail.com).

config.def.h

@@ -36,6 +36,8 @@ static const char notification_e_bgcolor[] = "#FF1212";
 static const char notification_e_fgcolor[] = "#FFFFFF";
 static const char notification_w_bgcolor[] = "#FFF712";
 static const char notification_w_fgcolor[] = "#000000";
+static const char recolor_darkcolor[]      = "#353535";
+static const char recolor_lightcolor[]     = "#DBDBDB";
 
 static const char search_highlight[]       = "#9FBC00";
 
@@ -57,7 +59,7 @@ Shortcut shortcuts[] = {
   {GDK_CONTROL_MASK,   GDK_m,             sc_toggle_inputbar,   NORMAL,   {0} },
   {GDK_CONTROL_MASK,   GDK_q,             sc_quit,              -1,       {0} },
   {GDK_CONTROL_MASK,   GDK_c,             sc_abort,             -1,       {0} },
-  {GDK_CONTROL_MASK,   GDK_i,             sc_revert_video,      NORMAL,   {0} },
+  {GDK_CONTROL_MASK,   GDK_i,             sc_recolor,           NORMAL,   {0} },
   {0,                  GDK_slash,         sc_focus_inputbar,    NORMAL,   { .data = "/" } },
   {GDK_SHIFT_MASK,     GDK_slash,         sc_focus_inputbar,    NORMAL,   { .data = "/" } },
   {GDK_SHIFT_MASK,     GDK_question,      sc_focus_inputbar,    NORMAL,   { .data = "?" } },
@@ -136,6 +138,6 @@ SpecialCommand special_commands[] = {
 /* settings */
 Setting settings[] = {
   /* name,         variable,                        type,  render,  description */
-  {"revertvideo",  &(Zathura.Global.reverse_video), 'b',   TRUE,    "Invert the image"},
+  {"recolor",      &(Zathura.Global.recolor),       'b',   TRUE,    "Invert the image"},
   {"password",     &(Zathura.PDF.password),         's',   FALSE,   "The password of the document"},
 };

zathura.c

@@ -170,6 +170,8 @@ struct
     GdkColor notification_e_bg;
     GdkColor notification_w_fg;
     GdkColor notification_w_bg;
+    GdkColor recolor_darkcolor;
+    GdkColor recolor_lightcolor;
     GdkColor search_highlight;
     PangoFontDescription *font;
   } Style;
@@ -180,7 +182,7 @@ struct
     GList   *history;
     int      mode;
     int      viewing_mode;
-    gboolean reverse_video;
+    gboolean recolor;
     GtkLabel *status_text;
     GtkLabel *status_buffer;
     GtkLabel *status_state;
@@ -272,7 +274,7 @@ void sc_change_buffer(Argument*);
 void sc_change_mode(Argument*);
 void sc_focus_inputbar(Argument*);
 void sc_navigate(Argument*);
-void sc_revert_video(Argument*);
+void sc_recolor(Argument*);
 void sc_rotate(Argument*);
 void sc_scroll(Argument*);
 void sc_search(Argument*);
@@ -384,13 +386,15 @@ init_zathura()
   gdk_color_parse(notification_e_bgcolor, &(Zathura.Style.notification_e_bg));
   gdk_color_parse(notification_w_fgcolor, &(Zathura.Style.notification_w_fg));
   gdk_color_parse(notification_w_bgcolor, &(Zathura.Style.notification_w_bg));
+  gdk_color_parse(recolor_darkcolor,      &(Zathura.Style.recolor_darkcolor));
+  gdk_color_parse(recolor_lightcolor,     &(Zathura.Style.recolor_lightcolor));
   gdk_color_parse(search_highlight,       &(Zathura.Style.search_highlight));
   Zathura.Style.font = pango_font_description_from_string(font);
 
   /* other */
   Zathura.Global.mode          = NORMAL;
   Zathura.Global.viewing_mode  = NORMAL;
-  Zathura.Global.reverse_video = FALSE;
+  Zathura.Global.recolor       = FALSE;
 
   Zathura.State.filename          = (char*) DEFAULT_TEXT;
   Zathura.State.pages             = "";
@@ -637,14 +641,46 @@ draw(int page_id)
   cairo_restore(cairo);
   cairo_destroy(cairo);
 
-  if(Zathura.Global.reverse_video)
+  if(Zathura.Global.recolor)
   {
     unsigned char* image = cairo_image_surface_get_data(Zathura.PDF.surface);
-    int x, y, z = 0;
+    int x, y;
 
-    for(x = 0; x < cairo_image_surface_get_width(Zathura.PDF.surface); x++)
+    int width     = cairo_image_surface_get_width(Zathura.PDF.surface);
-      for(y = 0; y < cairo_image_surface_get_height(Zathura.PDF.surface) * 4; y++)
+    int height    = cairo_image_surface_get_height(Zathura.PDF.surface);
-        image[z++] ^= 0x00FFFFFF;
+    int rowstride = cairo_image_surface_get_stride(Zathura.PDF.surface);
+
+    /* recolor code based on qimageblitz library flatten() function
+    (http://sourceforge.net/projects/qimageblitz/) */
+
+    int r1 = Zathura.Style.recolor_darkcolor.red    / 257;
+    int g1 = Zathura.Style.recolor_darkcolor.green  / 257;
+    int b1 = Zathura.Style.recolor_darkcolor.blue   / 257;
+    int r2 = Zathura.Style.recolor_lightcolor.red   / 257;
+    int g2 = Zathura.Style.recolor_lightcolor.green / 257;
+    int b2 = Zathura.Style.recolor_lightcolor.blue  / 257;
+
+    int min = 0x00;
+    int max = 0xFF;
+    int mean;
+
+    float sr = ((float) r2 - r1) / (max - min);
+    float sg = ((float) g2 - g1) / (max - min);
+    float sb = ((float) b2 - b1) / (max - min);
+
+    for (y = 0; y < height; y++) 
+    {
+      unsigned char* data = image + y * rowstride;
+
+      for (x = 0; x < width; x++) 
+      {
+        mean = (data[0] + data[1] + data[2]) / 3;
+        data[2] = sr * (mean - min) + r1 + 0.5;
+        data[1] = sg * (mean - min) + g1 + 0.5;
+        data[0] = sb * (mean - min) + b1 + 0.5;
+        data += 4;
+      }
+    }
   }
 
   gtk_widget_set_size_request(Zathura.UI.drawing_area, width, height);
@@ -1264,9 +1300,9 @@ sc_navigate(Argument* argument)
 }
 
 void
-sc_revert_video(Argument* argument)
+sc_recolor(Argument* argument)
 {
-  Zathura.Global.reverse_video = !Zathura.Global.reverse_video;
+  Zathura.Global.recolor = !Zathura.Global.recolor;
   draw(Zathura.PDF.page_number);
 }