windows: gl: Fix WGL_SAMPLES reduction

The logic for writing 0 and false to the keys and values
is off by one.

While we are at it, unify the naming between the two settings
that are possible to reduce (samples and sRGB).

Amends d64f776a9a41a8d1144397b7b62efbdee8d24857

Pick-to: 6.5 6.4 6.2
Fixes: QTBUG-109453
Change-Id: I97f3a3c7175bcb555c70967056ab2de45b077f48
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 8 insertions, 7 deletions

diff --git a/src/plugins/platforms/windows/qwindowsglcontext.cpp b/src/plugins/platforms/windows/qwindowsglcontext.cpp
index 65740d69da..cfd73519fa 100644
--- a/src/plugins/platforms/windows/qwindowsglcontext.cpp
+++ b/src/plugins/platforms/windows/qwindowsglcontext.cpp
@@ -544,11 +544,14 @@ static int choosePixelFormat(HDC hdc,
         iAttributes[i++] = WGL_NUMBER_OVERLAYS_ARB;
         iAttributes[i++] = 1;
     }
+    // must be the one before the last one
     const int samples = format.samples();
     const bool sampleBuffersRequested = samples > 1
             && testFlag(staticContext.extensions, QOpenGLStaticContext::SampleBuffers);
+    int sampleBuffersKeyPosition = 0;
     int samplesValuePosition = 0;
     if (sampleBuffersRequested) {
+        sampleBuffersKeyPosition = i;
         iAttributes[i++] = WGL_SAMPLE_BUFFERS_ARB;
         iAttributes[i++] = TRUE;
         iAttributes[i++] = WGL_SAMPLES_ARB;
@@ -560,9 +563,9 @@ static int choosePixelFormat(HDC hdc,
     }
     // must be the last
     bool srgbRequested = format.colorSpace() == QColorSpace::SRgb;
-    int srgbValuePosition = 0;
+    int srgbCapableKeyPosition = 0;
     if (srgbRequested) {
-        srgbValuePosition = i;
+        srgbCapableKeyPosition = i;
         iAttributes[i++] = WGL_FRAMEBUFFER_SRGB_CAPABLE_EXT;
         iAttributes[i++] = TRUE;
     }
@@ -576,8 +579,9 @@ static int choosePixelFormat(HDC hdc,
                 && numFormats >= 1;
         if (valid || (!sampleBuffersRequested && !srgbRequested))
             break;
+        // NB reductions must be done in reverse order (disable the last first, then move on to the one before that, etc.)
         if (srgbRequested) {
-            iAttributes[srgbValuePosition] = 0;
+            iAttributes[srgbCapableKeyPosition] = 0;
             srgbRequested = false;
         } else if (sampleBuffersRequested) {
             if (iAttributes[samplesValuePosition] > 1) {
@@ -585,11 +589,8 @@ static int choosePixelFormat(HDC hdc,
             } else if (iAttributes[samplesValuePosition] == 1) {
                 // Fallback in case it is unable to initialize with any
                 // samples to avoid falling back to the GDI path
-                // NB: The sample attributes needs to be at the end for this
-                // to work correctly
-                iAttributes[samplesValuePosition - 1] = FALSE;
+                iAttributes[sampleBuffersKeyPosition] = 0;
                 iAttributes[samplesValuePosition] = 0;
-                iAttributes[samplesValuePosition + 1] = 0;
             } else {
                 break;
             }