Document why an FBO of 0 needs to be bound in QGLPaintBuffer sometimes

Reviewed-by: trustme

1 files changed, 6 insertions, 0 deletions

diff --git a/src/opengl/qglpaintdevice.cpp b/src/opengl/qglpaintdevice.cpp
index 7dac55d..8863b01 100644
--- a/src/opengl/qglpaintdevice.cpp
+++ b/src/opengl/qglpaintdevice.cpp
@@ -70,6 +70,12 @@ void QGLPaintDevice::beginPaint()
 
     // Record the currently bound FBO so we can restore it again
     // in endPaint() and bind this device's FBO
+    //
+    // Note: m_thisFBO could be zero if the paint device is not
+    // backed by an FBO (e.g. window back buffer).  But there could
+    // be a previous FBO bound to the context which we need to
+    // explicitly unbind.  Otherwise the painting will go into
+    // the previous FBO instead of to the window.
     m_previousFBO = ctx->d_func()->current_fbo;
     if (m_previousFBO != m_thisFBO) {
         ctx->d_ptr->current_fbo = m_thisFBO;