[Mesa-dev] [PATCH 1/2] main/buffers: update error handling on DrawBuffers for 4.5

[Alejandro Piñeiro]

Before 4.5, GL_BACK was not allowed as a value of bufs. Since 4.5 it
is allowed under some circunstances:

>From the OpenGL 4.5 specification, Section 17.4.1 "Selecting Buffers
for Writing", page 493 (page 515 of the PDF):
 "An INVALID_ENUM error is generated if any value in bufs is FRONT ,
  LEFT , RIGHT , or FRONT_AND_BACK . This restriction applies to both
  the de- fault framebuffer and framebuffer objects, and exists
  because these constants may themselves refer to multiple buffers, as
  shown in table 17.4."

And on page 492 (page 514 of the PDF):
 "If the default framebuffer is affected, then each of the constants
  must be one of the values listed in table 17.6 or the special value
  BACK . When BACK is used, n must be 1 and color values are written
  into the left buffer for single-buffered contexts, or into the back
  left buffer for double-buffered contexts."

This patch keeps the same behaviour if OpenGL version is < 4. We
assume that for 4.x this is the intended behaviour, so a fix, but for
3.x the intended behaviour is the already in place.

Part of the fix for:
GL45-CTS.direct_state_access.framebuffers_draw_read_buffers_errors
---

Note that this change would make piglit test gl-3.1-draw-buffers to fail.

I have just sent patches to update that test, and a gl-4.5 equivalent
(using NamedFramebufferDrawBuffers) to the piglit list.

 src/mesa/main/buffers.c | 47 ++++++++++++++++++++++++++++++++++-------------
 1 file changed, 34 insertions(+), 13 deletions(-)

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index 2b24e5a..92c1839 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -343,7 +343,9 @@ _mesa_NamedFramebufferDrawBuffer(GLuint framebuffer, GLenum buf)
  * \param n  number of outputs
  * \param buffers  array [n] of renderbuffer names.  Unlike glDrawBuffer, the
  *                 names cannot specify more than one buffer.  For example,
- *                 GL_FRONT_AND_BACK is illegal.
+ *                 GL_FRONT_AND_BACK is illegal. The only exception is GL_BACK
+ *                 that is considered special and allowed as far as n is one
+ *                 since 4.5.
  */
 static void
 draw_buffers(struct gl_context *ctx, struct gl_framebuffer *fb,
@@ -401,20 +403,39 @@ draw_buffers(struct gl_context *ctx, struct gl_framebuffer *fb,
          return;
       }
 
-      /* From the OpenGL 4.0 specification, page 256:
-       * "For both the default framebuffer and framebuffer objects, the
-       *  constants FRONT, BACK, LEFT, RIGHT, and FRONT_AND_BACK are not
-       *  valid in the bufs array passed to DrawBuffers, and will result in
-       *  the error INVALID_ENUM. This restriction is because these
-       *  constants may themselves refer to multiple buffers, as shown in
-       *  table 4.4."
-       *  Previous versions of the OpenGL specification say INVALID_OPERATION,
-       *  but the Khronos conformance tests expect INVALID_ENUM.
+      /* From the OpenGL 4.5 specification, page 493 (page 515 of the PDF)
+       * "An INVALID_ENUM error is generated if any value in bufs is FRONT
+       *  , LEFT , RIGHT , or FRONT_AND_BACK . This restriction applies to
+       *  both the de- fault framebuffer and framebuffer objects, and
+       *  exists because these constants may themselves refer to multiple
+       *  buffers, as shown in table 17.4."
+       *
+       * And on page 492 (page 514 of the PDF):
+       * "If the default framebuffer is affected, then each of the
+       *  constants must be one of the values listed in table 17.6 or the
+       *  special value BACK . When BACK is used, n must be 1 and color
+       *  values are written into the left buffer for single-buffered
+       *  contexts, or into the back left buffer for double-buffered
+       *  contexts."
+       *
+       * Note "special value BACK". GL_BACK also refers to multiple buffers,
+       * but it is consider a special case here. This is a change on 4.5. For
+       * OpenGL 4.x we check that behaviour. For any previous version we keep
+       * considering it wrong (as INVALID_ENUM).
        */
       if (_mesa_bitcount(destMask[output]) > 1) {
-         _mesa_error(ctx, GL_INVALID_ENUM, "%s(invalid buffer %s)",
-                     caller, _mesa_enum_to_string(buffers[output]));
-         return;
+         if (_mesa_is_winsys_fbo(fb) && ctx->Version >= 40 && buffers[output] == GL_BACK) {
+            printf("default framebuffer! with buffer %s\n", _mesa_enum_to_string(buffers[output]));
+            if (n != 1) {
+               _mesa_error(ctx, GL_INVALID_OPERATION, "%s(with GL_BACK n must be 1)",
+                           caller);
+               return;
+            }
+         } else {
+            _mesa_error(ctx, GL_INVALID_ENUM, "%s(invalid buffer %s)",
+                        caller, _mesa_enum_to_string(buffers[output]));
+            return;
+         }
       }
 
       /* Section 4.2 (Whole Framebuffer Operations) of the OpenGL ES 3.0
-- 
2.9.3