vwebp: Fix bug when Dispose then NoBlend frames

The graphical bug happens when there is a frame disposed to background
color, followed by another frame that does not blend, and their areas
don't fully overlap. Only the previous frame clears its part of the
viewport. The fix consists in clearing the screen for the previous and
the current frame if needed.

Change-Id: I3425cf7297f0c7b2cf13a3a61b517cc0b1c031d8

examples/vwebp.c

@@ -334,6 +334,24 @@ static void DrawBackground(void) {
   }
 }
 
+// Draw background in a scissored rectangle.
+static void DrawBackgroundScissored(int window_x, int window_y, int frame_w,
+                                    int frame_h) {
+  // Only update the requested area, not the whole canvas.
+  window_x = window_x * kParams.viewport_width / kParams.canvas_width;
+  window_y = window_y * kParams.viewport_height / kParams.canvas_height;
+  frame_w = frame_w * kParams.viewport_width / kParams.canvas_width;
+  frame_h = frame_h * kParams.viewport_height / kParams.canvas_height;
+
+  // glScissor() takes window coordinates (0,0 at bottom left).
+  window_y = kParams.viewport_height - window_y - frame_h;
+
+  glEnable(GL_SCISSOR_TEST);
+  glScissor(window_x, window_y, frame_w, frame_h);
+  DrawBackground();
+  glDisable(GL_SCISSOR_TEST);
+}
+
 static void HandleDisplay(void) {
   const WebPDecBuffer* const pic = kParams.pic;
   const WebPIterator* const curr = &kParams.curr_frame;
@@ -351,36 +369,20 @@ static void HandleDisplay(void) {
 
   if (kParams.only_deltas) {
     DrawBackground();
-  } else if (prev->dispose_method == WEBP_MUX_DISPOSE_BACKGROUND ||
-      curr->blend_method == WEBP_MUX_NO_BLEND) {
-    // glScissor() takes window coordinates (0,0 at bottom left).
-    int window_x, window_y;
-    int frame_w, frame_h;
+  } else {
+    // The rectangle of the previous frame might be different than the current
+    // frame, so we may need to DrawBackgroundScissored for both.
     if (prev->dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
       // Clear the previous frame rectangle.
-      window_x = prev->x_offset;
-      window_y = kParams.canvas_height - prev->y_offset - prev->height;
-      frame_w = prev->width;
-      frame_h = prev->height;
-    } else {  // curr->blend_method == WEBP_MUX_NO_BLEND.
+      DrawBackgroundScissored(prev->x_offset, prev->y_offset, prev->width,
+                              prev->height);
+    }
+    if (curr->blend_method == WEBP_MUX_NO_BLEND) {
       // We simulate no-blending behavior by first clearing the current frame
-      // rectangle (to a checker-board) and then alpha-blending against it.
-      window_x = curr->x_offset;
-      window_y = kParams.canvas_height - curr->y_offset - curr->height;
-      frame_w = curr->width;
-      frame_h = curr->height;
+      // rectangle and then alpha-blending against it.
+      DrawBackgroundScissored(curr->x_offset, curr->y_offset, curr->width,
+                              curr->height);
     }
-    glEnable(GL_SCISSOR_TEST);
-    // Only update the requested area, not the whole canvas.
-    window_x = window_x * kParams.viewport_width / kParams.canvas_width;
-    window_y = window_y * kParams.viewport_height / kParams.canvas_height;
-    frame_w = frame_w * kParams.viewport_width / kParams.canvas_width;
-    frame_h = frame_h * kParams.viewport_height / kParams.canvas_height;
-    glScissor(window_x, window_y, frame_w, frame_h);
-
-    DrawBackground();
-
-    glDisable(GL_SCISSOR_TEST);
   }
 
   *prev = *curr;