Implement OpenVG buffer scrolling and enable it on Symbian.

Add support for accelerated scrolling in the "direct" window surface
implementation. Using vgCopyPixels(), the already rasterized content on
the surface can be shifted to a new location such that only a portion
of the suqsequent frame needs to be repainted instead of the entire
frame.

This only works when the "preserved" EGL swap behavior is enabled and
the impact on performance is highly dependant on the specific hardware
platform in use.

Task-number: QT-2972
Reviewed-by: Rhys Weatherley

6 files changed, 52 insertions, 2 deletions

diff --git a/doc/src/howtos/openvg.qdoc b/doc/src/howtos/openvg.qdoc
index 9c805bb..f70ed54 100644
--- a/doc/src/howtos/openvg.qdoc
+++ b/doc/src/howtos/openvg.qdoc
@@ -295,6 +295,16 @@
     Convolution, colorize, drop shadow, and blur filters are accelerated
     using OpenVG operations.
 
+    \section2 Scrolling
+
+    By default, accelerated scrolling is not enabled because the impact on
+    performance is very much tied to the hardware platform. To enable
+    accelerated scrolling, you should ensure that QVG_BUFFER_SCROLLING is
+    defined when compiling the QtOpenVG module.
+
+    You should only enable this feature if vgCopyPixels() is known to be
+    efficient on your hardware platform.
+
     \section1 Known issues
 
     Performance of copying the contents of an OpenVG-rendered window to the
diff --git a/doc/src/platforms/emb-openvg.qdocinc b/doc/src/platforms/emb-openvg.qdocinc
index 37ccb9c..2f9cc21 100644
--- a/doc/src/platforms/emb-openvg.qdocinc
+++ b/doc/src/platforms/emb-openvg.qdocinc
@@ -259,6 +259,12 @@ at present.
         \o Convolution, colorize, drop shadow, and blur filters are
 accelerated using OpenVG operations.
 
+    \row
+        \o Scrolling
+        \o Accelerated scrolling is implemented but disabled by default
+unless QVG_BUFFER_SCROLLING is defined. This should only be enabled on
+OpenVG engines where vgCopyPixels() is known to be efficient.
+
 \endtable
 
 \section2 Known issues
diff --git a/src/openvg/openvg.pro b/src/openvg/openvg.pro
index 3790492..883f0f3 100644
--- a/src/openvg/openvg.pro
+++ b/src/openvg/openvg.pro
@@ -33,7 +33,7 @@ contains(QT_CONFIG, egl) {
         qwindowsurface_vgegl.cpp
 }
 
-symbian: DEFINES += QVG_RECREATE_ON_SIZE_CHANGE
+symbian: DEFINES += QVG_RECREATE_ON_SIZE_CHANGE QVG_BUFFER_SCROLLING
 
 include(../qbase.pri)
 
diff --git a/src/openvg/qwindowsurface_vg.cpp b/src/openvg/qwindowsurface_vg.cpp
index 83b0764..c19d5d1 100644
--- a/src/openvg/qwindowsurface_vg.cpp
+++ b/src/openvg/qwindowsurface_vg.cpp
@@ -57,6 +57,7 @@ QVGWindowSurface::QVGWindowSurface(QWidget *window)
 {
     // Create the default type of EGL window surface for windows.
     d_ptr = new QVGEGLWindowSurfaceDirect(this);
+    setStaticContentsSupport(d_ptr->supportsStaticContents());
 }
 
 QVGWindowSurface::QVGWindowSurface
@@ -89,7 +90,9 @@ void QVGWindowSurface::setGeometry(const QRect &rect)
 
 bool QVGWindowSurface::scroll(const QRegion &area, int dx, int dy)
 {
-    return QWindowSurface::scroll(area, dx, dy);
+    if (!d_ptr->scroll(window(), area, dx, dy))
+        return QWindowSurface::scroll(area, dx, dy);
+    return true;
 }
 
 void QVGWindowSurface::beginPaint(const QRegion &region)
diff --git a/src/openvg/qwindowsurface_vgegl.cpp b/src/openvg/qwindowsurface_vgegl.cpp
index 46a905f..99b614b 100644
--- a/src/openvg/qwindowsurface_vgegl.cpp
+++ b/src/openvg/qwindowsurface_vgegl.cpp
@@ -758,6 +758,33 @@ void QVGEGLWindowSurfaceDirect::endPaint
     }
 }
 
+bool QVGEGLWindowSurfaceDirect::supportsStaticContents() const
+{
+#if defined(QVG_BUFFER_SCROLLING) && !defined(QVG_NO_PRESERVED_SWAP)
+    return true;
+#else
+    return QVGEGLWindowSurfacePrivate::supportsStaticContents();
+#endif
+}
+
+bool QVGEGLWindowSurfaceDirect::scroll(QWidget *widget, const QRegion& area, int dx, int dy)
+{
+#ifdef QVG_BUFFER_SCROLLING
+    QEglContext *context = ensureContext(widget);
+    if (context) {
+        context->makeCurrent(windowSurface);
+        QRect scrollRect = area.boundingRect();
+        int sx = scrollRect.x();
+        int sy = size.height() - scrollRect.y() - scrollRect.height();
+        vgSeti(VG_SCISSORING, VG_FALSE);
+        vgCopyPixels(sx + dx, sy - dy, sx, sy, scrollRect.width(), scrollRect.height());
+        context->lazyDoneCurrent();
+        return true;
+    }
+#endif
+    return false;
+}
+
 QT_END_NAMESPACE
 
 #endif
diff --git a/src/openvg/qwindowsurface_vgegl_p.h b/src/openvg/qwindowsurface_vgegl_p.h
index aa0c648..892fd9d 100644
--- a/src/openvg/qwindowsurface_vgegl_p.h
+++ b/src/openvg/qwindowsurface_vgegl_p.h
@@ -77,6 +77,8 @@ public:
         (QWidget *widget, const QRegion& region, QImage *image = 0) = 0;
     virtual VGImage surfaceImage() const;
     virtual QSize surfaceSize() const = 0;
+    virtual bool supportsStaticContents() const { return false; }
+    virtual bool scroll(QWidget *, const QRegion&, int, int) { return false; }
 
 private:
     QVGPaintEngine *engine;
@@ -128,6 +130,8 @@ public:
     void beginPaint(QWidget *widget);
     void endPaint(QWidget *widget, const QRegion& region, QImage *image);
     QSize surfaceSize() const { return size; }
+    bool supportsStaticContents() const;
+    bool scroll(QWidget *widget, const QRegion& area, int dx, int dy);
 
 protected:
     QEglContext *context;