Reduce double-copying of textures when flipping upside down

bindTexture() flipped images in-place, to reduce data copying.
But there is one case where the in-place is worse: when the
QImage is not detached.  In that case, the flip was copying
the entire image and then flipping the lines, effectively
processing the contents twice.  The new version uses mirrored()
to reduce the overhead for non-detached images.

Reviewed-by: Samuel

1 files changed, 15 insertions, 7 deletions

diff --git a/src/opengl/qgl.cpp b/src/opengl/qgl.cpp
index d5ca218..5ada125 100644
--- a/src/opengl/qgl.cpp
+++ b/src/opengl/qgl.cpp
@@ -2277,13 +2277,21 @@ QGLTexture* QGLContextPrivate::bindTexture(const QImage &image, GLenum target, G
 #ifdef QGL_BIND_TEXTURE_DEBUG
             printf(" - flipping bits over y (%d ms)\n", time.elapsed());
 #endif
-        int ipl = img.bytesPerLine() / 4;
-        int h = img.height();
-        for (int y=0; y<h/2; ++y) {
-            int *a = (int *) img.scanLine(y);
-            int *b = (int *) img.scanLine(h - y - 1);
-            for (int x=0; x<ipl; ++x)
-                qSwap(a[x], b[x]);
+        if (img.isDetached()) {
+            int ipl = img.bytesPerLine() / 4;
+            int h = img.height();
+            for (int y=0; y<h/2; ++y) {
+                int *a = (int *) img.scanLine(y);
+                int *b = (int *) img.scanLine(h - y - 1);
+                for (int x=0; x<ipl; ++x)
+                    qSwap(a[x], b[x]);
+            }
+        } else {
+            // Create a new image and copy across.  If we use the
+            // above in-place code then a full copy of the image is
+            // made before the lines are swapped, which processes the
+            // data twice.  This version should only do it once.
+            img = img.mirrored();
         }
     }