Partially reset GL global state before/after Qt/OCCT to avoid artifacts

Sample code for testing at the end of OcctQOpenGLWidgetViewer::paintGL():
```
{
    QPainter p(this);
    p.setPen(Qt::yellow);
    QFont font;
    font.setFamily("Arial");
    font.setPixelSize(32);
    p.setFont(font);
    p.drawText(QPoint(400, 50), QLatin1String("ABCDEFG"));
  }
```

Author: gkv311

occt-qopenglwidget/OcctQOpenGLWidgetViewer.cpp

@@ -480,10 +480,16 @@ void OcctQOpenGLWidgetViewer::paintGL()
 #endif
   }
 
+  // reset global GL state from Qt before redrawing OCCT
+  OcctGlTools::ResetGlStateBeforeOcct(myView);
+
   // flush pending input events and redraw the viewer
   Handle(V3d_View) aView = !myFocusView.IsNull() ? myFocusView : myView;
   aView->InvalidateImmediate();
   AIS_ViewController::FlushViewEvents(myContext, aView, true);
+
+  // reset global GL state after OCCT before redrawing Qt
+  OcctGlTools::ResetGlStateAfterOcct(myView);
 }
 
 // ================================================================

occt-qt-tools/OcctGlTools.cpp

@@ -3,6 +3,7 @@
 #include "OcctGlTools.h"
 
 #include <OpenGl_GraphicDriver.hxx>
+#include <OpenGl_GlCore20.hxx>
 #include <OpenGl_FrameBuffer.hxx>
 #include <OpenGl_View.hxx>
 #include <OpenGl_Window.hxx>
@@ -15,3 +16,54 @@ Handle(OpenGl_Context) OcctGlTools::GetGlContext(const Handle(V3d_View)& theView
   Handle(OpenGl_View) aGlView = Handle(OpenGl_View)::DownCast(theView->View());
   return aGlView->GlWindow()->GetGlContext();
 }
+
+// ================================================================
+// Function : ResetGlStateBeforeOcct
+// ================================================================
+void OcctGlTools::ResetGlStateBeforeOcct(const Handle(V3d_View)& theView)
+{
+  Handle(OpenGl_Context) aGlCtx = GetGlContext(theView);
+  if (aGlCtx.IsNull())
+    return;
+
+  if (aGlCtx->core20fwd != nullptr)
+  {
+    // shouldn't be a problem in most cases, but make sure to unbind active GLSL program
+    aGlCtx->core20fwd->glUseProgram(0);
+  }
+
+  // Qt leaves GL_BLEND enabled after drawing semitransparent elements,
+  // but OCCT doesn't reset its state before drawing opaque objects.
+  // Disable also texture bindings left by Qt.
+  aGlCtx->core11fwd->glBindTexture(GL_TEXTURE_2D, 0);
+  aGlCtx->core11fwd->glDisable(GL_BLEND);
+  if (aGlCtx->core11ffp != nullptr)
+  {
+    aGlCtx->core11fwd->glDisable(GL_ALPHA_TEST);
+    aGlCtx->core11fwd->glDisable(GL_TEXTURE_2D);
+  }
+}
+
+// ================================================================
+// Function : ResetGlStateAfterOcct
+// ================================================================
+void OcctGlTools::ResetGlStateAfterOcct(const Handle(V3d_View)& theView)
+{
+  Handle(OpenGl_Context) aGlCtx = GetGlContext(theView);
+  if (aGlCtx.IsNull())
+    return;
+
+  // Qt expects default OpenGL pack/unpack alignment setup,
+  // while OCCT manages it dynamically;
+  // without resetting alignment setup, Qt will draw
+  // some textures corrupted (like fonts)
+  aGlCtx->core11fwd->glPixelStorei(GL_PACK_ALIGNMENT, 4);
+  aGlCtx->core11fwd->glPixelStorei(GL_UNPACK_ALIGNMENT, 4);
+
+  if (aGlCtx->core15fwd != nullptr)
+  {
+    // Qt expects first texture object to be bound,
+    // but OCCT might leave another object bound within multi-texture mapping
+    aGlCtx->core15fwd->glActiveTexture(GL_TEXTURE0);
+  }
+}

occt-qt-tools/OcctGlTools.h

@@ -13,6 +13,13 @@ class OcctGlTools
 public:
   //! Return GL context.
   static Handle(OpenGl_Context) GetGlContext(const Handle(V3d_View)& theView);
+
+  //! Cleanup up global GL state after Qt before redrawing OCCT Viewer.
+  static void ResetGlStateBeforeOcct(const Handle(V3d_View)& theView);
+
+  //! Cleanup up global GL state after OCCT before redrawing Qt.
+  //! Alternative to QQuickOpenGLUtils::resetOpenGLState().
+  static void ResetGlStateAfterOcct(const Handle(V3d_View)& theView);
 };
 
 #endif // _OcctGlTools_HeaderFile

occt-qtquick/OcctQQuickFramebufferViewer.cpp

@@ -561,14 +561,20 @@ void OcctQQuickFramebufferViewer::render(QOpenGLFramebufferObject* theFbo)
     dumpGlInfo(true, false);
   }
 
+  // reset global GL state from Qt before redrawing OCCT
+  OcctGlTools::ResetGlStateBeforeOcct(myView);
+
   // flush pending input events and redraw the viewer
   myView->InvalidateImmediate();
   AIS_ViewController::FlushViewEvents(myContext, myView, true);
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
-  //QQuickOpenGLUtils::resetOpenGLState()
+  // reset global GL state after OCCT before redrawing Qt
+  // (alternative to QQuickOpenGLUtils::resetOpenGLState())
+  OcctGlTools::ResetGlStateAfterOcct(myView);
+/*#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+  QQuickOpenGLUtils::resetOpenGLState()
 #else
   if (aQWindow != nullptr)
     aQWindow->resetOpenGLState();
-#endif
+#endif*/
 }