[Tigervnc-devel] [RFC PATCH] unix vncviewer double buffering

[Pierre Ossman]

I thought I'd have a look at double buffering and it turned out to be a
lot simpler than I first thought.

Basically, the current code updates a local copy of the framebuffer,
and at the same time copies it to the X window. This is done on every
graphic operation.

The included patch changes the system to just update the backing store.
Once the entire framebuffer update is over, it copies the entire
modified area to the X window. This avoids tearing and gives a much
more pleasant experience.

The downsides with my patch is:

 - we'll be doing blits from the backing store even for CopyRect.

 - we'll be shuffling more data to the X server than before since we'll
   be copying the bounding box of the changes, and not just the changes.

 - we won't see the screen updating progressively on a slow connection

I plan to fix the last one by using a timer, and I think the other two
are acceptable losses to get rid of the tearing.

What do you think?

Rgds
-- 
Pierre Ossman            OpenSource-based Thin Client Technology
System Developer         Telephone: +46-13-21 46 00
Cendio AB                Web: http://www.cendio.com

Index: vncviewer/DesktopWindow.h
===================================================================
--- vncviewer/DesktopWindow.h	(revision 3714)
+++ vncviewer/DesktopWindow.h	(working copy)
@@ -24,6 +24,7 @@
 
 #include <rfb/Cursor.h>
 #include <rfb/Rect.h>
+#include <rfb/Region.h>
 #include <rfb/Timer.h>
 #include "TXWindow.h"
 #include "TXViewport.h"
@@ -61,13 +62,13 @@
   void fillRect(const rfb::Rect& r, rfb::Pixel pix) {
     if (r.overlaps(cursorBackingRect)) hideLocalCursor();
     im->fillRect(r, pix);
-    im->put(win(), gc, r);
+    damageRect(r);
     showLocalCursor();
   }
   void imageRect(const rfb::Rect& r, void* pixels) {
     if (r.overlaps(cursorBackingRect)) hideLocalCursor();
     im->imageRect(r, pixels);
-    im->put(win(), gc, r);
+    damageRect(r);
     showLocalCursor();
   }
   void copyRect(const rfb::Rect& r, int srcX, int srcY) {
@@ -75,11 +76,8 @@
         cursorBackingRect.overlaps(rfb::Rect(srcX, srcY,
                                              srcX+r.width(), srcY+r.height())))
       hideLocalCursor();
-    if (im->usingShm())
-      XSync(dpy, False);
     im->copyRect(r, rfb::Point(r.tl.x-srcX, r.tl.y-srcY));
-    XCopyArea(dpy, win(), win(), gc, srcX, srcY,
-              r.width(), r.height(), r.tl.x, r.tl.y);
+    damageRect(r);
     showLocalCursor();
   }
   void invertRect(const rfb::Rect& r);
@@ -97,12 +95,14 @@
   void createXCursors();
   void hideLocalCursor();
   void showLocalCursor();
+  void damageRect(const rfb::Rect& r) { damage.assign_union(rfb::Region(r)); };
   bool handleTimeout(rfb::Timer* timer);
   void handlePointerEvent(const rfb::Point& pos, int buttonMask);
 
   CConn* cc;
   TXImage* im;
   GC gc;
+  rfb::Region damage;
   ::Cursor dotCursor, noCursor, localXCursor;
 
   rfb::Cursor cursor;
Index: vncviewer/DesktopWindow.cxx
===================================================================
--- vncviewer/DesktopWindow.cxx	(revision 3714)
+++ vncviewer/DesktopWindow.cxx	(working copy)
@@ -216,7 +216,6 @@
   if (cursorVisible) {
     cursorVisible = false;
     im->imageRect(cursorBackingRect, cursorBacking.data);
-    im->put(win(), gc, cursorBackingRect);
   }
 }
 
@@ -238,7 +237,6 @@
     im->getImage(cursorBacking.data, cursorBackingRect);
 
     im->maskRect(cursorRect, cursor.data, cursor.mask.buf);
-    im->put(win(), gc, cursorBackingRect);
   }
 }
 
@@ -278,7 +276,13 @@
 
 void DesktopWindow::framebufferUpdateEnd()
 {
-  XSync(dpy, False);
+  Rect r;
+
+  r = damage.get_bounding_rect();
+  damage.clear();
+
+  fprintf(stderr, "Updating %dx%d+%d+%d\n", r.width(), r.height(), r.tl.x, r.tl.y);
+  im->put(win(), gc, r);
 }
 
 
@@ -297,7 +301,7 @@
       }
     }
   }
-  im->put(win(), gc, r);
+  damageRect(r);
 }
 
 

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