1 files changed, 37 insertions, 11 deletions

diff --git a/src/gui/opengl/qopenglframebufferobject.cpp b/src/gui/opengl/qopenglframebufferobject.cpp
index a6feda8732..231e475111 100644
--- a/src/gui/opengl/qopenglframebufferobject.cpp
+++ b/src/gui/opengl/qopenglframebufferobject.cpp
@@ -1119,6 +1119,14 @@ Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format,
 
     Will try to return a premultiplied ARBG32 or RGB32 image. Since 5.2 it will fall back to
     a premultiplied RGBA8888 or RGBx8888 image when reading to ARGB32 is not supported.
+
+    For multisampled framebuffer objects the samples are resolved using the
+    \c{GL_EXT_framebuffer_blit} extension. If the extension is not available, the contents
+    of the returned image is undefined.
+
+    For singlesampled framebuffers the contents is retrieved via \c glReadPixels. This is
+    a potentially expensive and inefficient operation. Therefore it is recommended that
+    this function is used as seldom as possible.
 */
 QImage QOpenGLFramebufferObject::toImage() const
 {
@@ -1126,6 +1134,19 @@ QImage QOpenGLFramebufferObject::toImage() const
     if (!d->valid)
         return QImage();
 
+    QOpenGLContext *ctx = QOpenGLContext::currentContext();
+    if (!ctx) {
+        qWarning("QOpenGLFramebufferObject::toImage() called without a current context");
+        return QImage();
+    }
+
+    GLuint prevFbo = 0;
+    ctx->functions()->glGetIntegerv(GL_FRAMEBUFFER_BINDING, (GLint *) &prevFbo);
+
+    if (prevFbo != d->fbo())
+        const_cast<QOpenGLFramebufferObject *>(this)->bind();
+
+    QImage image;
     // qt_gl_read_framebuffer doesn't work on a multisample FBO
     if (format().samples() != 0) {
         QOpenGLFramebufferObject temp(size(), QOpenGLFramebufferObjectFormat());
@@ -1133,15 +1154,13 @@ QImage QOpenGLFramebufferObject::toImage() const
         QRect rect(QPoint(0, 0), size());
         blitFramebuffer(&temp, rect, const_cast<QOpenGLFramebufferObject *>(this), rect);
 
-        return temp.toImage();
+        image = temp.toImage();
+    } else {
+        image = qt_gl_read_framebuffer(d->size, format().internalTextureFormat() != GL_RGB, true);
     }
 
-    bool wasBound = isBound();
-    if (!wasBound)
-        const_cast<QOpenGLFramebufferObject *>(this)->bind();
-    QImage image = qt_gl_read_framebuffer(d->size, format().internalTextureFormat() != GL_RGB, true);
-    if (!wasBound)
-        const_cast<QOpenGLFramebufferObject *>(this)->release();
+    if (prevFbo != d->fbo())
+        ctx->functions()->glBindFramebuffer(GL_FRAMEBUFFER, prevFbo);
 
     return image;
 }
@@ -1213,6 +1232,8 @@ QOpenGLFramebufferObject::Attachment QOpenGLFramebufferObject::attachment() cons
 
     This can be used to free or reattach the depth and stencil buffer
     attachments as needed.
+
+    \note This function alters the current framebuffer binding.
  */
 void QOpenGLFramebufferObject::setAttachment(QOpenGLFramebufferObject::Attachment attachment)
 {
@@ -1296,6 +1317,9 @@ void QOpenGLFramebufferObject::blitFramebuffer(QOpenGLFramebufferObject *target,
     If \a source or \a target is 0, the default framebuffer will be used
     instead of a framebuffer object as source or target respectively.
 
+    This function will have no effect unless hasOpenGLFramebufferBlit() returns
+    true.
+
     The \a buffers parameter should be a mask consisting of any combination of
     \c GL_COLOR_BUFFER_BIT, \c GL_DEPTH_BUFFER_BIT, and
     \c GL_STENCIL_BUFFER_BIT.  Any buffer type that is not present both
@@ -1312,10 +1336,7 @@ void QOpenGLFramebufferObject::blitFramebuffer(QOpenGLFramebufferObject *target,
     have different sizes. The sizes must also be the same if any of the
     framebuffer objects are multisample framebuffers.
 
-    Note that the scissor test will restrict the blit area if enabled.
-
-    This function will have no effect unless hasOpenGLFramebufferBlit() returns
-    true.
+    \note The scissor test will restrict the blit area if enabled.
 
     \sa hasOpenGLFramebufferBlit()
 */
@@ -1332,6 +1353,9 @@ void QOpenGLFramebufferObject::blitFramebuffer(QOpenGLFramebufferObject *target,
     if (!extensions.hasOpenGLExtension(QOpenGLExtensions::FramebufferBlit))
         return;
 
+    GLuint prevFbo = 0;
+    ctx->functions()->glGetIntegerv(GL_FRAMEBUFFER_BINDING, (GLint *) &prevFbo);
+
     const int sx0 = sourceRect.left();
     const int sx1 = sourceRect.left() + sourceRect.width();
     const int sy0 = sourceRect.top();
@@ -1348,6 +1372,8 @@ void QOpenGLFramebufferObject::blitFramebuffer(QOpenGLFramebufferObject *target,
     extensions.glBlitFramebuffer(sx0, sy0, sx1, sy1,
                                  tx0, ty0, tx1, ty1,
                                  buffers, filter);
+
+    ctx->functions()->glBindFramebuffer(GL_FRAMEBUFFER, prevFbo); // sets both READ and DRAW
 }
 
 QT_END_NAMESPACE