Add support for custom texture format in QOpenGLWidget

...in order to support sRGB framebuffers.

Add a --srgb option to the qopenglwidget example to allow testing.

[ChangeLog][QtWidgets][QOpenGLWidget] Added support for specifying
custom internal texture formats in QOpenGLWidget in order to make it
possible to have the widget backed by an sRGB-capable framebuffer.

Task-number: QTBUG-50987
Change-Id: I112e2f0ab0b1478c69e601031aa0bafaa87fa847
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

examples/opengl/qopenglwidget/glwidget.cpp

@@ -61,6 +61,10 @@
 
 const int bubbleNum = 8;
 
+#ifndef GL_SRGB8_ALPHA8
+#define GL_SRGB8_ALPHA8 0x8C43
+#endif
+
 GLWidget::GLWidget(MainWindow *mw, bool button, const QColor &background)
     : m_mainWindow(mw),
       m_showBubbles(true),
@@ -75,6 +79,8 @@ GLWidget::GLWidget(MainWindow *mw, bool button, const QColor &background)
       m_background(background)
 {
     setMinimumSize(300, 250);
+    if (QCoreApplication::arguments().contains(QStringLiteral("--srgb")))
+        setTextureFormat(GL_SRGB8_ALPHA8);
 }
 
 GLWidget::~GLWidget()

examples/opengl/qopenglwidget/main.cpp

@@ -61,6 +61,8 @@ int main( int argc, char ** argv )
     QSurfaceFormat format;
     format.setDepthBufferSize(24);
     format.setStencilBufferSize(8);
+    if (QCoreApplication::arguments().contains(QStringLiteral("--srgb")))
+        format.setColorSpace(QSurfaceFormat::sRGBColorSpace);
     if (QCoreApplication::arguments().contains(QStringLiteral("--multisample")))
         format.setSamples(4);
     QSurfaceFormat::setDefaultFormat(format);

src/widgets/kernel/qopenglwidget.cpp

@@ -566,7 +566,8 @@ public:
           paintDevice(0),
           updateBehavior(QOpenGLWidget::NoPartialUpdate),
           requestedSamples(0),
-          inPaintGL(false)
+          inPaintGL(false),
+          textureFormat(0)
     {
         requestedFormat = QSurfaceFormat::defaultFormat();
     }
@@ -610,6 +611,7 @@ public:
     QOpenGLWidget::UpdateBehavior updateBehavior;
     int requestedSamples;
     bool inPaintGL;
+    GLenum textureFormat;
 };
 
 void QOpenGLWidgetPaintDevicePrivate::beginPaint()
@@ -703,12 +705,16 @@ void QOpenGLWidgetPrivate::recreateFbo()
     QOpenGLFramebufferObjectFormat format;
     format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
     format.setSamples(samples);
+    if (textureFormat)
+        format.setInternalTextureFormat(textureFormat);
 
     const QSize deviceSize = q->size() * q->devicePixelRatioF();
     fbo = new QOpenGLFramebufferObject(deviceSize, format);
     if (samples > 0)
         resolvedFbo = new QOpenGLFramebufferObject(deviceSize);
 
+    textureFormat = fbo->format().internalTextureFormat();
+
     fbo->bind();
     context->functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
 
@@ -767,7 +773,9 @@ void QOpenGLWidgetPrivate::initialize()
         return;
     }
 
-    // Propagate settings that make sense only for the tlw.
+    // Propagate settings that make sense only for the tlw. Note that this only
+    // makes sense for properties that get picked up even after the native
+    // window is created.
     QSurfaceFormat tlwFormat = tlw->windowHandle()->format();
     if (requestedFormat.swapInterval() != tlwFormat.swapInterval()) {
         // Most platforms will pick up the changed swap interval on the next
@@ -990,7 +998,6 @@ QOpenGLWidget::UpdateBehavior QOpenGLWidget::updateBehavior() const
  */
 void QOpenGLWidget::setFormat(const QSurfaceFormat &format)
 {
-    Q_UNUSED(format);
     Q_D(QOpenGLWidget);
     if (Q_UNLIKELY(d->initialized)) {
         qWarning("QOpenGLWidget: Already initialized, setting the format has no effect");
@@ -1022,6 +1029,47 @@ QSurfaceFormat QOpenGLWidget::format() const
     return d->initialized ? d->context->format() : d->requestedFormat;
 }
 
+/*!
+    Sets a custom internal texture format.
+
+    When working with sRGB framebuffers, it will be necessary to specify a
+    format like \c{GL_SRGB8_ALPHA8}. This can be achieved by calling this
+    function.
+
+    \note This function has no effect if called after the widget has already
+    been shown and thus it performed initialization.
+
+    \note This function will typically have to be used in combination with a
+    QSurfaceFormat::setDefaultFormat() call that sets the color space to
+    QSurfaceFormat::sRGBColorSpace.
+
+    \since 5.10
+ */
+void QOpenGLWidget::setTextureFormat(GLenum texFormat)
+{
+    Q_D(QOpenGLWidget);
+    if (Q_UNLIKELY(d->initialized)) {
+        qWarning("QOpenGLWidget: Already initialized, setting the internal texture format has no effect");
+        return;
+    }
+
+    d->textureFormat = texFormat;
+}
+
+/*!
+    \return the active internal texture format if the widget has already
+    initialized, the requested format if one was set but the widget has not yet
+    been made visible, or 0 if setTextureFormat() was not called and the widget
+    has not yet been made visible.
+
+    \since 5.10
+ */
+GLenum QOpenGLWidget::textureFormat() const
+{
+    Q_D(const QOpenGLWidget);
+    return d->textureFormat;
+}
+
 /*!
   \return \e true if the widget and OpenGL resources, like the context, have
   been successfully initialized. Note that the return value is always false

src/widgets/kernel/qopenglwidget.h

@@ -72,6 +72,9 @@ public:
     void setFormat(const QSurfaceFormat &format);
     QSurfaceFormat format() const;
 
+    GLenum textureFormat() const;
+    void setTextureFormat(GLenum texFormat);
+
     bool isValid() const;
 
     void makeCurrent();

tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp

@@ -64,6 +64,7 @@ void tst_QOpenGLWidget::create()
 {
     QScopedPointer<QOpenGLWidget> w(new QOpenGLWidget);
     QVERIFY(!w->isValid());
+    QVERIFY(w->textureFormat() == 0);
     QSignalSpy frameSwappedSpy(w.data(), SIGNAL(frameSwapped()));
     w->show();
     QTest::qWaitForWindowExposed(w.data());
@@ -73,6 +74,7 @@ void tst_QOpenGLWidget::create()
     QVERIFY(w->context());
     QCOMPARE(w->context()->format(), w->format());
     QVERIFY(w->defaultFramebufferObject() != 0);
+    QVERIFY(w->textureFormat() != 0);
 }
 
 class ClearWidget : public QOpenGLWidget, protected QOpenGLFunctions