Adds a few \warnings to the docs on do-not-use-for-performance-reasons

Reviewed-by: Trond

1 files changed, 16 insertions, 2 deletions

diff --git a/src/gui/image/qpixmap.cpp b/src/gui/image/qpixmap.cpp
index 8684a1b..efb8260 100644
--- a/src/gui/image/qpixmap.cpp
+++ b/src/gui/image/qpixmap.cpp
@@ -674,7 +674,7 @@ void QPixmap::resize_helper(const QSize &s)
     pixels black. The effect of this function is undefined when the pixmap is
     being painted on.
 
-    This is potentially an expensive operation.
+    \warning This is potentially an expensive operation.
 
     \sa mask(), {QPixmap#Pixmap Transformations}{Pixmap Transformations},
     QBitmap
@@ -1380,6 +1380,12 @@ void QPixmap::deref()
     If the given \a size is empty, this function returns a null
     pixmap.
 
+
+    In some cases it can be more beneficial to draw the pixmap to a
+    painter with a scale set rather than scaling the pixmap. This is
+    the case when the painter is for instance based on OpenGL or when
+    the scale factor changes rapidly.
+
     \sa isNull(), {QPixmap#Pixmap Transformations}{Pixmap
     Transformations}
 
@@ -1751,6 +1757,10 @@ int QPixmap::metric(PaintDeviceMetric metric) const
     The effect of this function is undefined when the pixmap is being
     painted on.
 
+    \warning This is potentially an expensive operation. Most usecases
+    for this function are covered by QPainter and compositionModes
+    which will normally execute faster.
+
     \sa alphaChannel(), {QPixmap#Pixmap Transformations}{Pixmap
     Transformations}
  */
@@ -1793,6 +1803,9 @@ void QPixmap::setAlphaChannel(const QPixmap &alphaChannel)
 
     \image alphachannelimage.png The pixmap and channelImage QPixmaps
 
+    \warning This is an expensive operation. The alpha channel of the
+    pixmap is extracted dynamically from the pixeldata.
+
     \sa setAlphaChannel(), {QPixmap#Pixmap Information}{Pixmap
     Information}
 */
@@ -1814,7 +1827,8 @@ QPaintEngine *QPixmap::paintEngine() const
 
     Extracts a bitmap mask from the pixmap's alphachannel.
 
-    This is potentially an expensive operation.
+    \warning This is potentially an expensive operation. The mask of
+    the pixmap is extracted dynamically from the pixeldata.
 
     \sa setMask(), {QPixmap#Pixmap Information}{Pixmap Information}
 */