Performance|Widgets|libappfw: Update background blur only when needed

GuiWidget was redrawing the blur FBO even when no one needed it.
Now it gets updated just-in-time when someone wants it.

doomsday/sdk/libappfw/src/guiwidget.cpp

@@ -73,6 +73,7 @@ DENG2_PIMPL(GuiWidget)
     // Background blurring.
     struct BlurState
     {
+        Time updatedAt;
         Vector2ui size;
         QScopedPointer<GLTextureFramebuffer> fb[2];
         Drawable drawable;
@@ -244,45 +245,31 @@ DENG2_PIMPL(GuiWidget)
         }
     }
 
-    void drawBlurredBackground()
+    void updateBlurredBackground()
     {
-        if (background.type == Background::SharedBlur ||
-            background.type == Background::SharedBlurWithBorderGlow)
-        {
-            // Use another widget's blur.
-            DENG2_ASSERT(background.blur != 0);
-            if (background.blur)
-            {
-                self().root().painter().flush();
-                background.blur->drawBlurredRect(self().rule().recti(), background.solidFill);
-            }
-            return;
-        }
+        // Make sure blurring is initialized.
+        initBlur();
 
-        if (background.type != Background::Blurred &&
-            background.type != Background::BlurredWithBorderGlow &&
-            background.type != Background::BlurredWithSolidFill)
+        auto const now = Time::currentHighPerformanceTime();
+        if (blur->updatedAt == now)
         {
-            deinitBlur();
             return;
         }
-
+        blur->updatedAt = now;
+
         // Ensure normal drawing is complete.
         auto &painter = self().root().painter();
         painter.flush();
 
         auto const oldClip = painter.normalizedScissor();
 
-        // Make sure blurring is initialized.
-        initBlur();
-
         DENG2_ASSERT(blur->fb[0]->isReady());
 
         // Pass 1: render all the widgets behind this one onto the first blur
         // texture, downsampled.
         GLState::push()
-                .setTarget(*blur->fb[0])
-                .setViewport(Rectangleui::fromSize(blur->size));
+            .setTarget(*blur->fb[0])
+            .setViewport(Rectangleui::fromSize(blur->size));
         blur->fb[0]->clear(GLFramebuffer::Depth);
         self().root().drawUntil(self());
         GLState::pop();
@@ -292,8 +279,8 @@ DENG2_PIMPL(GuiWidget)
         // Pass 2: apply the horizontal blur filter to draw the background
         // contents onto the second blur texture.
         GLState::push()
-                .setTarget(*blur->fb[1])
-                .setViewport(Rectangleui::fromSize(blur->size));
+            .setTarget(*blur->fb[1])
+            .setViewport(Rectangleui::fromSize(blur->size));
         blur->uTex = blur->fb[0]->colorTexture();
         blur->uMvpMatrix = Matrix4f::ortho(0, 1, 0, 1);
         blur->uWindow = Vector4f(0, 0, 1, 1);
@@ -303,6 +290,33 @@ DENG2_PIMPL(GuiWidget)
 
         blur->fb[1]->resolveSamples();
 
+        painter.setNormalizedScissor(oldClip);
+    }
+
+    void drawBlurredBackground()
+    {
+        if (background.type == Background::SharedBlur ||
+            background.type == Background::SharedBlurWithBorderGlow)
+        {
+            // Use another widget's blur.
+            DENG2_ASSERT(background.blur != 0);
+            if (background.blur)
+            {
+                self().root().painter().flush();
+                background.blur->d->updateBlurredBackground();
+                background.blur->drawBlurredRect(self().rule().recti(), background.solidFill);
+            }
+            return;
+        }
+
+        if (background.type != Background::Blurred &&
+            background.type != Background::BlurredWithBorderGlow &&
+            background.type != Background::BlurredWithSolidFill)
+        {
+            deinitBlur();
+            return;
+        }
+
         // Pass 3: apply the vertical blur filter, drawing the final result
         // into the original target.
         Vector4f blurColor = background.solidFill;
@@ -311,12 +325,11 @@ DENG2_PIMPL(GuiWidget)
         {
             blurColor.w = 1;
         }
-        if (!attribs.testFlag(DontDrawContent) && blurColor.w > 0 && blurOpacity > 0)
+        if (blurColor.w > 0 && blurOpacity > 0)
         {
+            updateBlurredBackground();
             self().drawBlurredRect(self().rule().recti(), blurColor, blurOpacity);
         }
-
-        painter.setNormalizedScissor(oldClip);
     }
 
     inline float currentOpacity() const
@@ -908,10 +921,10 @@ void GuiWidget::draw()
         // Detect mistakes in GLState stack usage.
         dsize const depthBeforeDrawingWidget = GLState::stackDepth();
 #endif
-        d->drawBlurredBackground();
-
         if (!d->attribs.testFlag(DontDrawContent))
         {
+            d->drawBlurredBackground();
+
             auto &painter = root().painter();
             painter.setSaturation(d->saturation);