From bd67962c5e8469062516da396e9c466a9446e00d Mon Sep 17 00:00:00 2001
From: Brian Paul <brianp@vmware.com>
Date: Fri, 21 Jan 2011 17:34:42 -0700
Subject: st/mesa: comments in update_viewport()

---
 src/mesa/state_tracker/st_atom_viewport.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/mesa/state_tracker/st_atom_viewport.c b/src/mesa/state_tracker/st_atom_viewport.c
index d10f1840df..133cc49eaf 100644
--- a/src/mesa/state_tracker/st_atom_viewport.c
+++ b/src/mesa/state_tracker/st_atom_viewport.c
@@ -47,10 +47,14 @@ update_viewport( struct st_context *st )
    /* _NEW_BUFFERS
     */
    if (st_fb_orientation(ctx->DrawBuffer) == Y_0_TOP) {
+      /* Drawing to a window.  The corresponding gallium surface uses
+       * Y=0=TOP but OpenGL is Y=0=BOTTOM.  So we need to invert the viewport.
+       */
       yScale = -1;
       yBias = (GLfloat)ctx->DrawBuffer->Height;
    }
    else {
+      /* Drawing to an FBO where Y=0=BOTTOM, like OpenGL - don't invert */
       yScale = 1.0;
       yBias = 0.0;
    }
-- 
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