Start supporting purely offscreen QOpenGLWidget

Due to popular demand. It does have it benefits (especially when
it comes to convenience) to allow grabbing QOpenGLWidgets even
when they are not part of an actual window and are not actually
visible.

Does not involve much more than dropping the warnings and bailouts
when there is active native window (because the QOpenGLWidget/its
parents are still hidden).

In addition the device pixel ratio from metric() has to be fixed
as well.

[ChangeLog][Qt Widgets] QOpenGLWidget is now able to render and
return its content via grabFramebuffer(), QWidget::grab() or
QWidget::render() even when the widget has not been made visible.

Task-number: QTBUG-47185
Task-number: QTBUG-61280
Change-Id: Icc2b0b3ce9778a3eb6409d54744238568abb0f0d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 70 insertions, 0 deletions

diff --git a/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp b/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
index c7fca550e5..510576f6df 100644
--- a/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
+++ b/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
@@ -61,6 +61,8 @@ private slots:
     void showHide();
     void nativeWindow();
     void stackWidgetOpaqueChildIsVisible();
+    void offscreen();
+    void offscreenThenOnscreen();
 };
 
 void tst_QOpenGLWidget::initTestCase()
@@ -572,6 +574,74 @@ void tst_QOpenGLWidget::stackWidgetOpaqueChildIsVisible()
     #undef VERIFY_COLOR
 }
 
+void tst_QOpenGLWidget::offscreen()
+{
+    {
+        QScopedPointer<ClearWidget> w(new ClearWidget(0, 800, 600));
+        w->resize(800, 600);
+
+        w->setClearColor(0, 0, 1);
+        QImage image = w->grabFramebuffer();
+
+        QVERIFY(!image.isNull());
+        QCOMPARE(image.width(), w->width());
+        QCOMPARE(image.height(), w->height());
+        QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
+    }
+
+    // QWidget::grab() should eventually end up in grabFramebuffer() as well
+    {
+        QScopedPointer<ClearWidget> w(new ClearWidget(0, 800, 600));
+        w->resize(800, 600);
+
+        w->setClearColor(0, 0, 1);
+        QPixmap pm = w->grab();
+        QImage image = pm.toImage();
+
+        QVERIFY(!image.isNull());
+        QCOMPARE(image.width(), w->width());
+        QCOMPARE(image.height(), w->height());
+        QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
+    }
+
+    // ditto for QWidget::render()
+    {
+        QScopedPointer<ClearWidget> w(new ClearWidget(0, 800, 600));
+        w->resize(800, 600);
+
+        w->setClearColor(0, 0, 1);
+        QImage image(800, 600, QImage::Format_ARGB32);
+        w->render(&image);
+
+        QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
+    }
+}
+
+void tst_QOpenGLWidget::offscreenThenOnscreen()
+{
+    QScopedPointer<ClearWidget> w(new ClearWidget(0, 800, 600));
+    w->resize(800, 600);
+
+    w->setClearColor(0, 0, 1);
+    QImage image = w->grabFramebuffer();
+
+    QVERIFY(!image.isNull());
+    QCOMPARE(image.width(), w->width());
+    QCOMPARE(image.height(), w->height());
+    QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
+
+    // now let's make things more challenging: show. Internally this needs
+    // recreating the context.
+    w->show();
+    QTest::qWaitForWindowExposed(w.data());
+
+    image = w->grabFramebuffer();
+    QVERIFY(!image.isNull());
+    QCOMPARE(image.width(), w->width());
+    QCOMPARE(image.height(), w->height());
+    QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
+}
+
 QTEST_MAIN(tst_QOpenGLWidget)
 
 #include "tst_qopenglwidget.moc"