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i965: Compute dFdy() correctly for FBOs.

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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 <kenneth@whitecape.org>
This commit is contained in:
Paul Berry 2012-06-20 13:40:45 -07:00
parent d988ea5e81
commit 82d25963a8
6 changed files with 46 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

10
src/mesa/drivers/dri/i965/brw_fs.cpp
View file

@ -1848,6 +1848,13 @@ brw_fs_precompile(struct gl_context *ctx, struct gl_shader_program *prog)
struct brw_context *brw = brw_context(ctx);
struct brw_wm_prog_key key;
/* 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;
if (!prog->_LinkedShaders[MESA_SHADER_FRAGMENT])
return true;
@ -1892,6 +1899,9 @@ brw_fs_precompile(struct gl_context *ctx, struct gl_shader_program *prog)
if (fp->Base.InputsRead & FRAG_BIT_WPOS) {
key.drawable_height = ctx->DrawBuffer->Height;
}
if ((fp->Base.InputsRead & FRAG_BIT_WPOS) || program_uses_dfdy) {
key.render_to_fbo = _mesa_is_user_fbo(ctx->DrawBuffer);
}

3
src/mesa/drivers/dri/i965/brw_fs.h
View file

@ -534,7 +534,8 @@ public:
struct brw_reg src);
void generate_discard(fs_inst *inst);
void generate_ddx(fs_inst *inst, struct brw_reg dst, struct brw_reg src);
void generate_ddy(fs_inst *inst, struct brw_reg dst, struct brw_reg src);
void generate_ddy(fs_inst *inst, struct brw_reg dst, struct brw_reg src,
bool negate_value);
void generate_spill(fs_inst *inst, struct brw_reg src);
void generate_unspill(fs_inst *inst, struct brw_reg dst);
void generate_pull_constant_load(fs_inst *inst, struct brw_reg dst);

14
src/mesa/drivers/dri/i965/brw_fs_emit.cpp
View file

@ -441,8 +441,13 @@ fs_visitor::generate_ddx(fs_inst *inst, struct brw_reg dst, struct brw_reg src)
brw_ADD(p, dst, src0, negate(src1));
}
/* The negate_value boolean is used to negate the derivative computation for
* FBOs, since they place the origin at the upper left instead of the lower
* left.
*/
void
fs_visitor::generate_ddy(fs_inst *inst, struct brw_reg dst, struct brw_reg src)
fs_visitor::generate_ddy(fs_inst *inst, struct brw_reg dst, struct brw_reg src,
bool negate_value)
{
struct brw_reg src0 = brw_reg(src.file, src.nr, 0,
BRW_REGISTER_TYPE_F,
@ -456,7 +461,10 @@ fs_visitor::generate_ddy(fs_inst *inst, struct brw_reg dst, struct brw_reg src)
BRW_WIDTH_4,
BRW_HORIZONTAL_STRIDE_0,
BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
brw_ADD(p, dst, src0, negate(src1));
if (negate_value)
brw_ADD(p, dst, src1, negate(src0));
else
brw_ADD(p, dst, src0, negate(src1));
}
void
@ -902,7 +910,7 @@ fs_visitor::generate_code()
generate_ddx(inst, dst, src[0]);
break;
case FS_OPCODE_DDY:
generate_ddy(inst, dst, src[0]);
generate_ddy(inst, dst, src[0], c->key.render_to_fbo);
break;
case FS_OPCODE_SPILL:

10
src/mesa/drivers/dri/i965/brw_wm.c
View file

@ -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);
}

3
src/mesa/drivers/dri/i965/brw_wm.h
View file

@ -346,7 +346,8 @@ void emit_ddxy(struct brw_compile *p,
const struct brw_reg *dst,
GLuint mask,
bool is_ddx,
const struct brw_reg *arg0);
const struct brw_reg *arg0,
bool negate_value);
void emit_delta_xy(struct brw_compile *p,
const struct brw_reg *dst,
GLuint mask,

15
src/mesa/drivers/dri/i965/brw_wm_emit.c
View file

@ -457,12 +457,16 @@ void emit_frontfacing(struct brw_compile *p,
* between each other. We could probably do it like ddx and swizzle the right
* order later, but bail for now and just produce
* ((ss0.tl - ss0.bl)x4 (ss1.tl - ss1.bl)x4)
*
* The negate_value boolean is used to negate the d/dy computation for FBOs,
* since they place the origin at the upper left instead of the lower left.
*/
void emit_ddxy(struct brw_compile *p,
const struct brw_reg *dst,
GLuint mask,
bool is_ddx,
const struct brw_reg *arg0)
const struct brw_reg *arg0,
bool negate_value)
{
int i;
struct brw_reg src0, src1;
@ -498,7 +502,10 @@ void emit_ddxy(struct brw_compile *p,
BRW_HORIZONTAL_STRIDE_0,
BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
}
brw_ADD(p, dst[i], src0, negate(src1));
if (negate_value)
brw_ADD(p, dst[i], src1, negate(src0));
else
brw_ADD(p, dst[i], src0, negate(src1));
}
}
if (mask & SATURATE)
@ -1746,11 +1753,11 @@ void brw_wm_emit( struct brw_wm_compile *c )
break;
case OPCODE_DDX:
emit_ddxy(p, dst, dst_flags, true, args[0]);
emit_ddxy(p, dst, dst_flags, true, args[0], false);
break;
case OPCODE_DDY:
emit_ddxy(p, dst, dst_flags, false, args[0]);
emit_ddxy(p, dst, dst_flags, false, args[0], c->key.render_to_fbo);
break;
case OPCODE_DP2: