Introduced preserved swap buffer path in GL window surface.

When a buffer swap leaves the back buffer intact we don't have to use
an FBO or PB, but can render directly to the window's back buffer,
yielding higher performance and depending less on extensions such as
multisample FBOs and FBO blitting.

Reviewed-by: Trond

1 files changed, 64 insertions, 50 deletions

diff --git a/src/opengl/qwindowsurface_gl.cpp b/src/opengl/qwindowsurface_gl.cpp
index 6fce3e3..a3422b5 100644
--- a/src/opengl/qwindowsurface_gl.cpp
+++ b/src/opengl/qwindowsurface_gl.cpp
@@ -102,16 +102,18 @@ QGLGraphicsSystem::QGLGraphicsSystem()
         int spec[16];
         spec[i++] = GLX_RGBA;
         spec[i++] = GLX_DOUBLEBUFFER;
-#if 0
-        spec[i++] = GLX_DEPTH_SIZE;
-        spec[i++] = 8;
-        spec[i++] = GLX_STENCIL_SIZE;
-        spec[i++] = 8;
-        spec[i++] = GLX_SAMPLE_BUFFERS_ARB;
-        spec[i++] = 1;
-        spec[i++] = GLX_SAMPLES_ARB;
-        spec[i++] = 4;
-#endif
+
+        if (!qgetenv("QT_GL_SWAPBUFFER_PRESERVE").isNull()) {
+            spec[i++] = GLX_DEPTH_SIZE;
+            spec[i++] = 8;
+            spec[i++] = GLX_STENCIL_SIZE;
+            spec[i++] = 8;
+            spec[i++] = GLX_SAMPLE_BUFFERS_ARB;
+            spec[i++] = 1;
+            spec[i++] = GLX_SAMPLES_ARB;
+            spec[i++] = 4;
+        }
+
         spec[i++] = XNone;
 
         XVisualInfo *vi = glXChooseVisual(X11->display, X11->defaultScreen, spec);
@@ -229,6 +231,7 @@ struct QGLWindowSurfacePrivate
 
     int tried_fbo : 1;
     int tried_pb : 1;
+    int destructive_swap_buffers : 1;
 
     QGLContext *ctx;
 
@@ -252,6 +255,7 @@ QGLWindowSurface::QGLWindowSurface(QWidget *window)
     d_ptr->ctx = 0;
     d_ptr->tried_fbo = false;
     d_ptr->tried_pb = false;
+    d_ptr->destructive_swap_buffers = qgetenv("QT_GL_SWAPBUFFER_PRESERVE").isNull();
 }
 
 QGLWindowSurface::~QGLWindowSurface()
@@ -342,6 +346,9 @@ void QGLWindowSurface::flush(QWidget *widget, const QRegion &rgn, const QPoint &
     QWidget *parent = widget->internalWinId() ? widget : widget->nativeParentWidget();
     Q_ASSERT(parent);
 
+    if (!geometry().isValid())
+        return;
+
     hijackWindow(parent);
 
     QRect br = rgn.boundingRect().translated(offset);
@@ -356,48 +363,50 @@ void QGLWindowSurface::flush(QWidget *widget, const QRegion &rgn, const QPoint &
 
         if (context()->format().doubleBuffer()) {
 #if !defined(QT_OPENGL_ES_2)
-            glBindTexture(target, d_ptr->tex_id);
+            if (d_ptr->destructive_swap_buffers) {
+                glBindTexture(target, d_ptr->tex_id);
 
-            QVector<QRect> rects = d_ptr->paintedRegion.rects();
-            for (int i = 0; i < rects.size(); ++i) {
-                QRect br = rects.at(i);
-                if (br.isEmpty())
-                    continue;
+                QVector<QRect> rects = d_ptr->paintedRegion.rects();
+                for (int i = 0; i < rects.size(); ++i) {
+                    QRect br = rects.at(i);
+                    if (br.isEmpty())
+                        continue;
 
-                const uint bottom = window()->height() - (br.y() + br.height());
-                glCopyTexSubImage2D(target, 0, br.x(), bottom, br.x(), bottom, br.width(), br.height());
-            }
+                    const uint bottom = window()->height() - (br.y() + br.height());
+                    glCopyTexSubImage2D(target, 0, br.x(), bottom, br.x(), bottom, br.width(), br.height());
+                }
 
-            glBindTexture(target, 0);
+                glBindTexture(target, 0);
 
-            QRegion dirtyRegion = QRegion(window()->rect()) - d_ptr->paintedRegion;
+                QRegion dirtyRegion = QRegion(window()->rect()) - d_ptr->paintedRegion;
 
-            if (!dirtyRegion.isEmpty()) {
-                context()->makeCurrent();
+                if (!dirtyRegion.isEmpty()) {
+                    context()->makeCurrent();
 
-                glMatrixMode(GL_MODELVIEW);
-                glLoadIdentity();
+                    glMatrixMode(GL_MODELVIEW);
+                    glLoadIdentity();
 
-                glMatrixMode(GL_PROJECTION);
-                glLoadIdentity();
+                    glMatrixMode(GL_PROJECTION);
+                    glLoadIdentity();
 #ifndef QT_OPENGL_ES
-                glOrtho(0, window()->width(), window()->height(), 0, -999999, 999999);
+                    glOrtho(0, window()->width(), window()->height(), 0, -999999, 999999);
 #else
-                glOrthof(0, window()->width(), window()->height(), 0, -999999, 999999);
+                    glOrthof(0, window()->width(), window()->height(), 0, -999999, 999999);
 #endif
-                glViewport(0, 0, window()->width(), window()->height());
+                    glViewport(0, 0, window()->width(), window()->height());
 
-                QVector<QRect> rects = dirtyRegion.rects();
-                glColor4f(1, 1, 1, 1);
-                for (int i = 0; i < rects.size(); ++i) {
-                    QRect rect = rects.at(i);
-                    if (rect.isEmpty())
-                        continue;
+                    QVector<QRect> rects = dirtyRegion.rects();
+                    glColor4f(1, 1, 1, 1);
+                    for (int i = 0; i < rects.size(); ++i) {
+                        QRect rect = rects.at(i);
+                        if (rect.isEmpty())
+                            continue;
 
-                    drawTexture(rect, d_ptr->tex_id, window()->size(), rect);
+                        drawTexture(rect, d_ptr->tex_id, window()->size(), rect);
+                    }
                 }
-            }
 #endif
+            }
             d_ptr->paintedRegion = QRegion();
 
             context()->swapBuffers();
@@ -422,7 +431,7 @@ void QGLWindowSurface::flush(QWidget *widget, const QRegion &rgn, const QPoint &
     }
 
     QSize size = widget->rect().size();
-    if (ctx->format().doubleBuffer()) {
+    if (d_ptr->destructive_swap_buffers && ctx->format().doubleBuffer()) {
         rect = parent->rect();
         br = rect.translated(wOffset);
         size = parent->size();
@@ -501,13 +510,16 @@ void QGLWindowSurface::updateGeometry()
     d_ptr->size = rect.size();
 
     if (d_ptr->ctx) {
-        glBindTexture(target, d_ptr->tex_id);
-        glTexImage2D(target, 0, GL_RGBA, rect.width(), rect.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, 0);
-        glBindTexture(target, 0);
+        if (d_ptr->destructive_swap_buffers) {
+            glBindTexture(target, d_ptr->tex_id);
+            glTexImage2D(target, 0, GL_RGBA, rect.width(), rect.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, 0);
+            glBindTexture(target, 0);
+        }
         return;
     }
 
-    if ((QGLExtensions::glExtensions & QGLExtensions::FramebufferObject)
+    if (d_ptr->destructive_swap_buffers
+        && (QGLExtensions::glExtensions & QGLExtensions::FramebufferObject)
 #ifdef QT_OPENGL_ES_2
         && (QGLExtensions::glExtensions & QGLExtensions::FramebufferBlit)
 #endif
@@ -542,7 +554,7 @@ void QGLWindowSurface::updateGeometry()
     }
 
 #if !defined(QT_OPENGL_ES_2)
-    if (d_ptr->pb || !d_ptr->tried_pb) {
+    if (d_ptr->destructive_swap_buffers && (d_ptr->pb || !d_ptr->tried_pb)) {
         d_ptr->tried_pb = true;
 
         if (d_ptr->pb) {
@@ -590,13 +602,15 @@ void QGLWindowSurface::updateGeometry()
     QGLContext *ctx = reinterpret_cast<QGLContext *>(window()->d_func()->extraData()->glContext);
     ctx->makeCurrent();
 
-    glGenTextures(1, &d_ptr->tex_id);
-    glBindTexture(target, d_ptr->tex_id);
-    glTexImage2D(target, 0, GL_RGBA, rect.width(), rect.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, 0);
+    if (d_ptr->destructive_swap_buffers) {
+        glGenTextures(1, &d_ptr->tex_id);
+        glBindTexture(target, d_ptr->tex_id);
+        glTexImage2D(target, 0, GL_RGBA, rect.width(), rect.height(), 0, GL_RGB, GL_UNSIGNED_BYTE, 0);
 
-    glTexParameterf(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
-    glTexParameterf(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
-    glBindTexture(target, 0);
+        glTexParameterf(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+        glTexParameterf(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+        glBindTexture(target, 0);
+    }
 
     qDebug() << "QGLWindowSurface: Using plain widget as window surface" << this;;
     d_ptr->ctx = ctx;