i965: Compute dFdy() correctly for FBOs.

On i965, dFdx() and dFdy() are computed by taking advantage of the
fact that each consecutive set of 4 pixels dispatched to the fragment
shader always constitutes a contiguous 2x2 block of pixels in a fixed
arrangement known as a "sub-span".  So we calculate dFdx() by taking
the difference between the values computed for the left and right
halves of the sub-span, and we calculate dFdy() by taking the
difference between the values computed for the top and bottom halves
of the sub-span.

However, there's a subtlety when FBOs are in use: since FBOs use a
coordinate system where the origin is at the upper left, and window
system framebuffers use a coordinate system where the origin is at the
lower left, the computation of dFdy() needs to be negated for FBOs.

This patch modifies the fragment shader back-ends to negate the value
of dFdy() when an FBO is in use.  It also modifies the code that
populates the program key (brw_wm_populate_key() and
brw_fs_precompile()) so that they always record in the program key
whether we are rendering to an FBO or to a window system framebuffer;
this ensures that the fragment shader will get recompiled when
switching between FBO and non-FBO use.

This will result in unnecessary recompiles of fragment shaders that
don't use dFdy().  To fix that, we will need to adapt the GLSL and
NV_fragment_program front-ends to record whether or not a given shader
uses dFdy().  I plan to implement this in a future patch series; I've
left FIXME comments in the code as a reminder.

Fixes Piglit test "fbo-deriv".

NOTE: This is a candidate for stable release branches.

Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_wm.c b/src/mesa/drivers/dri/i965/brw_wm.c
index e919e5a3627..300e3bb1187 100644
--- a/src/mesa/drivers/dri/i965/brw_wm.c
+++ b/src/mesa/drivers/dri/i965/brw_wm.c
@@ -420,6 +420,13 @@ static void brw_wm_populate_key( struct brw_context *brw,
    GLuint line_aa;
    GLuint i;
 
+   /* As a temporary measure we assume that all programs use dFdy() (and hence
+    * need to be compiled differently depending on whether we're rendering to
+    * an FBO).  FIXME: set this bool correctly based on the contents of the
+    * program.
+    */
+   bool program_uses_dfdy = true;
+
    memset(key, 0, sizeof(*key));
 
    /* Build the index for table lookup
@@ -519,6 +526,9 @@ static void brw_wm_populate_key( struct brw_context *brw,
     */
    if (fp->program.Base.InputsRead & FRAG_BIT_WPOS) {
       key->drawable_height = ctx->DrawBuffer->Height;
+   }
+
+   if ((fp->program.Base.InputsRead & FRAG_BIT_WPOS) || program_uses_dfdy) {
       key->render_to_fbo = _mesa_is_user_fbo(ctx->DrawBuffer);
    }