Add swapInterval to QSurfaceFormat

Implement swap interval support for EGL, GLX and WGL.

The environment variable QT_QPA_EGLFS_SWAPINTERVAL is renamed to
QT_QPA_EGL_SWAPINTERVAL and can be used to override the applications'
setting of the swap interval.

Task-number: QTBUG-31939

Change-Id: I644325d5d3306b7604bffd7efccda3c00ed37d36
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

2 files changed, 50 insertions, 2 deletions

diff --git a/src/gui/kernel/qsurfaceformat.cpp b/src/gui/kernel/qsurfaceformat.cpp
index ebfd71b4d3..b8fe55adc0 100644
--- a/src/gui/kernel/qsurfaceformat.cpp
+++ b/src/gui/kernel/qsurfaceformat.cpp
@@ -72,6 +72,7 @@ public:
         , profile(QSurfaceFormat::NoProfile)
         , major(2)
         , minor(0)
+        , swapInterval(1) // default to vsync
     {
     }
 
@@ -89,7 +90,8 @@ public:
           renderableType(other->renderableType),
           profile(other->profile),
           major(other->major),
-          minor(other->minor)
+          minor(other->minor),
+          swapInterval(other->swapInterval)
     {
     }
 
@@ -107,6 +109,7 @@ public:
     QSurfaceFormat::OpenGLContextProfile profile;
     int major;
     int minor;
+    int swapInterval;
 };
 
 /*!
@@ -676,6 +679,46 @@ void QSurfaceFormat::setVersion(int major, int minor)
 }
 
 /*!
+    Sets the preferred swap interval. The swap interval specifies the
+    minimum number of video frames that are displayed before a buffer
+    swap occurs. This can be used to sync the GL drawing into a window
+    to the vertical refresh of the screen.
+
+    Setting an \a interval value of 0 will turn the vertical refresh
+    syncing off, any value higher than 0 will turn the vertical
+    syncing on. Setting \a interval to a higher value, for example 10,
+    results in having 10 vertical retraces between every buffer swap.
+
+    The default interval is 1.
+
+    Changing the swap interval may not be supported by the underlying
+    platform. In this case, the request will be silently ignored.
+
+    \since 5.3
+
+    \sa swapInterval()
+ */
+void QSurfaceFormat::setSwapInterval(int interval)
+{
+    if (d->swapInterval != interval) {
+        detach();
+        d->swapInterval = interval;
+    }
+}
+
+/*!
+    Returns the swap interval.
+
+    \since 5.3
+
+    \sa setSwapInterval()
+*/
+int QSurfaceFormat::swapInterval() const
+{
+    return d->swapInterval;
+}
+
+/*!
     Returns \c true if all the options of the two QSurfaceFormat objects
     \a a and \a b are equal.
 
@@ -694,7 +737,8 @@ bool operator==(const QSurfaceFormat& a, const QSurfaceFormat& b)
         && a.d->swapBehavior == b.d->swapBehavior
         && a.d->profile == b.d->profile
         && a.d->major == b.d->major
-        && a.d->minor == b.d->minor);
+        && a.d->minor == b.d->minor
+        && a.d->swapInterval == b.d->swapInterval);
 }
 
 /*!
@@ -724,6 +768,7 @@ QDebug operator<<(QDebug dbg, const QSurfaceFormat &f)
                   << ", stencilBufferSize " << d->stencilSize
                   << ", samples " << d->numSamples
                   << ", swapBehavior " << d->swapBehavior
+                  << ", swapInterval " << d->swapInterval
                   << ", profile  " << d->profile
                   << ')';
 
diff --git a/src/gui/kernel/qsurfaceformat.h b/src/gui/kernel/qsurfaceformat.h
index d182aa9f13..453beac5cd 100644
--- a/src/gui/kernel/qsurfaceformat.h
+++ b/src/gui/kernel/qsurfaceformat.h
@@ -135,6 +135,9 @@ public:
     bool testOption(FormatOption option) const;
     QSurfaceFormat::FormatOptions options() const;
 
+    int swapInterval() const;
+    void setSwapInterval(int interval);
+
 private:
     QSurfaceFormatPrivate *d;