st-scroll-view-fade: Pass a precomputed fade area to the shader

Instead of doing complex computations in the shader just pass in the correct
fade area (taking padding, scrollbars and rtl into account) and just work
with that in the shader.

That fixes a bug where we would fade the scrollbar when padding is present.

https://bugzilla.gnome.org/show_bug.cgi?id=659159

src/st/st-scroll-view-fade.c

@@ -25,6 +25,8 @@
 
 #include "st-scroll-view-fade.h"
 #include "st-scroll-view.h"
+#include "st-widget.h"
+#include "st-theme-node.h"
 #include "st-scroll-bar.h"
 #include "st-scrollable.h"
 
@@ -39,11 +41,18 @@ static const gchar *fade_glsl_shader =
 "uniform sampler2D tex;\n"
 "uniform float height;\n"
 "uniform float width;\n"
-"uniform float scrollbar_width;\n"
-"uniform float scrollbar_height;\n"
 "uniform float offset_bottom;\n"
 "uniform float offset_top;\n"
-"uniform bool rtl;\n"
+/*
+ * Used to pass the fade area to the shader
+ *
+ * [0][0] = x1
+ * [0][1] = y1
+ * [1][0] = x2
+ * [1][1] = y2
+ *
+ */
+"uniform mat2 fade_area;\n"
 "\n"
 "void main ()\n"
 "{\n"
@@ -51,18 +60,19 @@ static const gchar *fade_glsl_shader =
 " float y = height * cogl_tex_coord_in[0].y;\n"
 " float x = width * cogl_tex_coord_in[0].x;\n"
 " float ratio = 1.0;\n"
-" float fade_bottom_start = height - offset_bottom;\n"
+" float fade_bottom_start = fade_area[1][1] - offset_bottom;\n"
 " float ratio_top = y / offset_top;\n"
-" float ratio_bottom = (height - scrollbar_height - y)/(height - scrollbar_height - fade_bottom_start);\n"
+" float ratio_bottom = (fade_area[1][1] - y)/(fade_area[1][1] - fade_bottom_start);\n"
-" bool in_scroll_area = ((rtl && x > scrollbar_width) || (!rtl && x < (width - scrollbar_width)));\n"
+" bool in_scroll_area = fade_area[0][0] <= x && fade_area[1][0] >= x;\n"
-" bool fade_top = y < offset_top && in_scroll_area;\n"
+" bool fade_top = y < offset_top && in_scroll_area && (y >= fade_area[0][1]);\n"
-" bool fade_bottom = y > fade_bottom_start && in_scroll_area && (y < (height - scrollbar_height));\n"
+" bool fade_bottom = y > fade_bottom_start && in_scroll_area && (y <= fade_area[1][1]);\n"
 "\n"
 " if (fade_top) {\n"
-"  ratio = ratio_top;\n"
+"  ratio *= ratio_top;\n"
 " }\n"
-" else if (fade_bottom) {\n"
+"\n"
-"  ratio = ratio_bottom;\n"
+" if (fade_bottom) {\n"
+"  ratio *= ratio_bottom;\n"
 " }\n"
 "\n"
 "  cogl_color_out = color * ratio;\n"
@@ -81,11 +91,9 @@ struct _StScrollViewFade
   gint tex_uniform;
   gint height_uniform;
   gint width_uniform;
-  gint scrollbar_width_uniform;
+  gint fade_area_uniform;
-  gint scrollbar_height_uniform;
   gint offset_top_uniform;
   gint offset_bottom_uniform;
-  gint rtl_uniform;
 
   StAdjustment *vadjustment;
 
@@ -145,12 +153,8 @@ st_scroll_view_fade_pre_paint (ClutterEffect *effect)
         cogl_program_get_uniform_location (self->program, "height");
       self->width_uniform =
         cogl_program_get_uniform_location (self->program, "width");
-      self->scrollbar_width_uniform =
+      self->fade_area_uniform =
-        cogl_program_get_uniform_location (self->program, "scrollbar_width");
+        cogl_program_get_uniform_location (self->program, "fade_area");
-      self->scrollbar_height_uniform =
-        cogl_program_get_uniform_location (self->program, "scrollbar_height");
-      self->rtl_uniform =
-        cogl_program_get_uniform_location (self->program, "rtl");
       self->offset_top_uniform =
         cogl_program_get_uniform_location (self->program, "offset_top");
       self->offset_bottom_uniform =
@@ -184,13 +188,54 @@ st_scroll_view_fade_paint_target (ClutterOffscreenEffect *effect)
   ClutterActor *hscroll = st_scroll_view_get_hscroll_bar (ST_SCROLL_VIEW (self->actor));
   gboolean h_scroll_visible, v_scroll_visible;
 
+  ClutterActorBox allocation, content_box, paint_box;
+
+  /*
+   * Used to pass the fade area to the shader
+   *
+   * [0][0] = x1
+   * [0][1] = y1
+   * [1][0] = x2
+   * [1][1] = y2
+   *
+   */
+  float fade_area[2][2];
+  ClutterVertex verts[4];
+
+  if (self->program == COGL_INVALID_HANDLE)
+    goto out;
+
+  clutter_actor_get_paint_box (self->actor, &paint_box);
+  clutter_actor_get_abs_allocation_vertices (self->actor, verts);
+
+  clutter_actor_get_allocation_box (self->actor, &allocation);
+  st_theme_node_get_content_box (st_widget_get_theme_node (ST_WIDGET (self->actor)),
+                                (const ClutterActorBox *)&allocation, &content_box);
+
+  /*
+   * The FBO is based on the paint_volume's size which can be larger then the actual
+   * allocation, so we have to account for that when passing the positions
+   */
+  fade_area[0][0] = content_box.x1 + (verts[0].x - paint_box.x1);
+  fade_area[0][1] = content_box.y1 + (verts[0].y - paint_box.y1);
+  fade_area[1][0] = content_box.x2 + (verts[3].x - paint_box.x2);
+  fade_area[1][1] = content_box.y2 + (verts[3].y - paint_box.y2);
+
   g_object_get (ST_SCROLL_VIEW (self->actor),
                 "hscrollbar-visible", &h_scroll_visible,
                 "vscrollbar-visible", &v_scroll_visible,
                 NULL);
 
-  if (self->program == COGL_INVALID_HANDLE)
+  if (v_scroll_visible)
-    goto out;
+    {
+      if (st_widget_get_direction (ST_WIDGET (self->actor)) == ST_TEXT_DIRECTION_RTL)
+          fade_area[0][0] += clutter_actor_get_width (vscroll);
+
+      fade_area[1][0] -= clutter_actor_get_width (vscroll);
+    }
+
+  if (h_scroll_visible)
+      fade_area[1][1] -= clutter_actor_get_height (hscroll);
 
   st_adjustment_get_values (self->vadjustment, &value, &lower, &upper, NULL, NULL, &page_size);
 
@@ -214,12 +259,8 @@ st_scroll_view_fade_paint_target (ClutterOffscreenEffect *effect)
     cogl_program_set_uniform_1f (self->program, self->height_uniform, clutter_actor_get_height (self->actor));
   if (self->width_uniform > -1)
     cogl_program_set_uniform_1f (self->program, self->width_uniform, clutter_actor_get_width (self->actor));
-  if (self->scrollbar_width_uniform > -1)
+  if (self->fade_area_uniform > -1)
-    cogl_program_set_uniform_1f (self->program, self->scrollbar_width_uniform, v_scroll_visible ? clutter_actor_get_width (vscroll) : 0);
+    cogl_program_set_uniform_matrix (self->program, self->fade_area_uniform, 2, 1, FALSE, (const float *)fade_area);
-  if (self->scrollbar_height_uniform > -1)
-    cogl_program_set_uniform_1f (self->program, self->scrollbar_height_uniform, h_scroll_visible ? clutter_actor_get_height (hscroll) : 0);
-  if (self->rtl_uniform > -1)
-    cogl_program_set_uniform_1i (self->program, self->rtl_uniform, (st_widget_get_direction (ST_WIDGET (self->actor)) == ST_TEXT_DIRECTION_RTL));
 
   material = clutter_offscreen_effect_get_target (effect);
   cogl_material_set_user_program (material, self->program);
@@ -428,9 +469,7 @@ st_scroll_view_fade_init (StScrollViewFade *self)
   self->tex_uniform = -1;
   self->height_uniform = -1;
   self->width_uniform = -1;
-  self->scrollbar_width_uniform = -1;
+  self->fade_area_uniform = -1;
-  self->scrollbar_height_uniform = -1;
-  self->rtl_uniform = -1;
   self->offset_top_uniform = -1;
   self->offset_bottom_uniform = -1;
   self->fade_offset = DEFAULT_FADE_OFFSET;