From ae80fa4731b729d7e546af212d08a93a555ff3c1 Mon Sep 17 00:00:00 2001
From: Matthew Waters <matthew@centricular.com>
Date: Fri, 25 Oct 2019 22:03:18 +1100
Subject: [PATCH] qmlglsrc: read from the back buffer when use-default-fbo =
 TRUE

glReadBuffer(GL_COLOR_ATTACHMENT0) on the default framebuffer (0) is
invalid GL API usage and would result in a GL error being thrown.
---
 ext/qt/qtwindow.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/ext/qt/qtwindow.cc b/ext/qt/qtwindow.cc
index 2143ab57e7..9360c3337f 100644
--- a/ext/qt/qtwindow.cc
+++ b/ext/qt/qtwindow.cc
@@ -242,14 +242,17 @@ QtGLWindow::afterRendering()
       GST_ERROR ("FBO errors");
       goto errors;
     }
-    gl->ReadBuffer (GL_COLOR_ATTACHMENT0);
+    if (this->priv->useDefaultFbo)
+      gl->ReadBuffer (GL_BACK);
+    else
+      gl->ReadBuffer (GL_COLOR_ATTACHMENT0);
     gl->BlitFramebuffer (0, 0, width, height,
         0, 0, width, height,
         GL_COLOR_BUFFER_BIT, GL_LINEAR);
   } else {
     gl->CopyTexImage2D (GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, width, height, 0);
   }
-  
+
   GST_DEBUG ("rendering finished");
 
 errors: