[Mesa-dev] [PATCH] mesa/msaa: Allow X and Y flips in multisampled blits.

[Paul Berry]

>From the GL 4.3 spec, section 18.3.1 "Blitting Pixel Rectangles":

    If SAMPLE_BUFFERS for either the read framebuffer or draw
    framebuffer is greater than zero, no copy is performed and an
    INVALID_OPERATION error is generated if the dimensions of the
    source and destination rectangles provided to BlitFramebuffer are
    not identical, or if the formats of the read and draw framebuffers
    are not identical.

It is not clear from the spec whether "dimensions" should mean both
sign and magnitude, or just magnitude.

Previously, Mesa interpreted "dimensions" as meaning both sign and
magnitude, so any multisampled blit that attempted to flip the image
in the X and/or Y direction would fail.

However, Y flips are likely to be commonplace in OpenGL applications
that have been ported from DirectX applications, as a result of the
fact that DirectX and OpenGL differ in their orientation of the Y
axis.  Furthermore, at least one commercial driver (nVidia) permits Y
filps, and L4D2 relies on them being permitted.  So it seems prudent
for Mesa to permit them.

This patch changes Mesa to allow both X and Y flips, since there is no
language in the spec to indicate that X and Y flips should be treated
differently.

NOTE: This is a candidate for stable release branches.
---
 src/mesa/main/fbobject.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/mesa/main/fbobject.c b/src/mesa/main/fbobject.c
index 59a5ec3..abc9d83 100644
--- a/src/mesa/main/fbobject.c
+++ b/src/mesa/main/fbobject.c
@@ -2979,8 +2979,8 @@ _mesa_BlitFramebufferEXT(GLint srcX0, GLint srcY0, GLint srcX1, GLint srcY1,
    /* extra checks for multisample copies... */
    if (readFb->Visual.samples > 0 || drawFb->Visual.samples > 0) {
       /* src and dest region sizes must be the same */
-      if (srcX1 - srcX0 != dstX1 - dstX0 ||
-          srcY1 - srcY0 != dstY1 - dstY0) {
+      if (abs(srcX1 - srcX0) != abs(dstX1 - dstX0) ||
+          abs(srcY1 - srcY0) != abs(dstY1 - dstY0)) {
          _mesa_error(ctx, GL_INVALID_OPERATION,
                 "glBlitFramebufferEXT(bad src/dst multisample region sizes)");
          return;
-- 
1.7.7.6