QOpenGLWidget: use (and prefer) InvalidateFramebuffer

Desktop GL does not have the GL_EXT_discard_framebuffer extension at
all. Instead, it may have GL_ARB_invalidate_subdata.

Furthermore, GLES >= 3.0 and GL >= 4.3 both support FBO invalidation
through glInvalidateFramebuffer.

Extend the semantics of OpenGLExtension::DiscardFramebuffer to mean that
*any* possibility above is supported, and wrap it in a helper function
that calls whatever support is present. (This is all private API
anyhow.) Technically speaking glInvalidateFramebuffer supports a
superset of what glDiscardFramebufferEXT supports, but we never use such
superset, and the two APIs are otherwise identical.

Then, make QOpenGLWidget call that wrapper.

Change-Id: I64e042daf51493d3834c3ba1b53ae6e224bbc4ed
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/opengl/qopenglextensions_p.h

@@ -70,9 +70,9 @@ public:
 
     GLvoid *glMapBuffer(GLenum target, GLenum access);
     void glGetBufferSubData(GLenum target, qopengl_GLintptr offset, qopengl_GLsizeiptr size, GLvoid *data);
-    void glDiscardFramebufferEXT (GLenum target, GLsizei numAttachments, const GLenum *attachments);
 
     void flushShared();
+    void discardFramebuffer(GLenum target, GLsizei numAttachments, const GLenum *attachments);
 
     QOpenGLExtensionsPrivate *d() const;
 
@@ -117,14 +117,6 @@ inline void QOpenGLExtensions::glGetBufferSubData(GLenum target, qopengl_GLintpt
     Q_OPENGL_FUNCTIONS_DEBUG
 }
 
-
-inline void QOpenGLExtensions::glDiscardFramebufferEXT (GLenum target, GLsizei numAttachments, const GLenum *attachments)
-{
-    Q_D(QOpenGLExtensions);
-    Q_ASSERT(QOpenGLExtensions::isInitialized(d));
-    d->DiscardFramebuffer(target,numAttachments, attachments);
-    Q_OPENGL_FUNCTIONS_DEBUG
-}
 QT_END_NAMESPACE
 
 #endif // QOPENGL_EXTENSIONS_P_H

src/gui/opengl/qopenglfunctions.cpp

@@ -365,6 +365,7 @@ static int qt_gl_resolve_extensions()
                 | QOpenGLExtensions::FramebufferBlit
                 | QOpenGLExtensions::FramebufferMultisample
                 | QOpenGLExtensions::Sized8Formats
+                | QOpenGLExtensions::DiscardFramebuffer
                 | QOpenGLExtensions::StandardDerivatives
                 | QOpenGLExtensions::ETC2TextureCompression
                 | QOpenGLExtensions::HalfFloatVertex;
@@ -450,6 +451,9 @@ static int qt_gl_resolve_extensions()
         if (format.version() >= qMakePair(3, 3))
             extensions |= QOpenGLExtensions::TextureSwizzle;
 
+        if (format.version() >= qMakePair(4, 3) || extensionMatcher.match("GL_ARB_invalidate_subdata"))
+            extensions |= QOpenGLExtensions::DiscardFramebuffer;
+
         if (extensionMatcher.match("GL_ARB_map_buffer_range"))
             extensions |= QOpenGLExtensions::MapBufferRange;
 
@@ -5051,7 +5055,23 @@ QOpenGLExtensionsPrivate::QOpenGLExtensionsPrivate(QOpenGLContext *ctx)
     MapBuffer = RESOLVE(MapBuffer);
     GetBufferSubData = RESOLVE(GetBufferSubData);
     DiscardFramebuffer = RESOLVE(DiscardFramebuffer);
- }
+}
+
+void QOpenGLExtensions::discardFramebuffer(GLenum target, GLsizei numAttachments, const GLenum *attachments)
+{
+    Q_D(QOpenGLExtensions);
+    Q_ASSERT(QOpenGLExtensions::isInitialized(d));
+    Q_ASSERT(d->f.InvalidateFramebuffer || d->DiscardFramebuffer);
+
+    // On GLES >= 3 we prefer glInvalidateFramebuffer, even if the
+    // discard extension is present
+    if (d->f.InvalidateFramebuffer)
+        d->f.InvalidateFramebuffer(target, numAttachments, attachments);
+    else
+        d->DiscardFramebuffer(target, numAttachments, attachments);
+
+    Q_OPENGL_FUNCTIONS_DEBUG
+}
 
 void QOpenGLExtensions::flushShared()
 {

src/openglwidgets/qopenglwidget.cpp

@@ -504,9 +504,8 @@ QT_BEGIN_NAMESPACE
     benefits on certain hardware architectures common in the mobile and
     embedded space when a framebuffer object is used as the rendering target.
     The framebuffer object is invalidated between frames with
-    glDiscardFramebufferEXT if supported or a glClear. Please see the
-    documentation of EXT_discard_framebuffer for more information:
-    https://www.khronos.org/registry/gles/extensions/EXT/EXT_discard_framebuffer.txt
+    glInvalidateFramebuffer (if supported), or, as fallbacks,
+    glDiscardFramebufferEXT (if supported) or a call to glClear.
 
     \value PartialUpdate The framebuffer objects color buffer and ancillary
     buffers are not invalidated between frames.
@@ -1000,7 +999,7 @@ void QOpenGLWidgetPrivate::invalidateFbo()
             gl_color_attachment0, gl_depth_attachment, gl_stencil_attachment
         };
 #endif
-        f->glDiscardFramebufferEXT(GL_FRAMEBUFFER, sizeof attachments / sizeof *attachments, attachments);
+        f->discardFramebuffer(GL_FRAMEBUFFER, GLsizei(std::size(attachments)), attachments);
     } else {
         f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
     }