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>

1 files changed, 21 insertions, 1 deletions

diff --git a/src/gui/opengl/qopenglfunctions.cpp b/src/gui/opengl/qopenglfunctions.cpp
index 0b760eb2dd..c9352fbc31 100644
--- a/src/gui/opengl/qopenglfunctions.cpp
+++ b/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()
 {