[Mesa-dev] [PATCH] i965: fix textureGrad for cubemaps

Iago Toral itoral at igalia.com
Thu Sep 17 04:10:18 PDT 2015


Hi Tapani, Kevin,

awesome work! I was curious about how to fix this, at least when I was
looking at the specs for this stuff it was not obvious that the Math
involved for this was so different, I only recall seeing the reference
that texure coordinates had to be normalized to a [-1, 1] space after
selecting the face in the cube, but I did not see formulas to implement
all this like they had for the normal case. It looks like the Math
involved is quite different.

I added some minor comments below:

On Thu, 2015-09-17 at 08:12 +0300, Tapani Pälli wrote:
> Fixes regression caused by commit
> 2b1cdb0eddb73f62e4848d4b64840067f1f70865 in:
>    ES3-CTS.gtf.GL3Tests.shadow.shadow_execution_frag
> 
> No regressions observed in deqp, CTS or Piglit.
> 
> Signed-off-by: Tapani Pälli <tapani.palli at intel.com>
> Signed-off-by: Kevin Rogovin <kevin.rogovin at intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91114
> Cc: "11.0 10.7" <mesa-stable at lists.freedesktop.org>
> ---
>  .../dri/i965/brw_lower_texture_gradients.cpp       | 172 ++++++++++++++++++++-
>  1 file changed, 169 insertions(+), 3 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
> index 7a5f983..f8a31b7 100644
> --- a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
> @@ -48,6 +48,7 @@ public:
>  
>  private:
>     void emit(ir_variable *, ir_rvalue *);
> +   ir_variable *temp(void *ctx, const glsl_type *type, const char *name);
>  };
>  
>  /**
> @@ -60,6 +61,17 @@ lower_texture_grad_visitor::emit(ir_variable *var, ir_rvalue *value)
>     base_ir->insert_before(assign(var, value));
>  }
>  
> +/**
> + * Emit a temporary variable declaration
> + */
> +ir_variable *
> +lower_texture_grad_visitor::temp(void *ctx, const glsl_type *type, const char *name)
> +{
> +   ir_variable *var = new(ctx) ir_variable(type, name, ir_var_temporary);
> +   base_ir->insert_before(var);
> +   return var;
> +}
> +
>  static const glsl_type *
>  txs_type(const glsl_type *type)
>  {
> @@ -162,9 +174,163 @@ lower_texture_grad_visitor::visit_leave(ir_texture *ir)
>      */
>     ir->op = ir_txl;
>     if (ir->sampler->type->sampler_dimensionality == GLSL_SAMPLER_DIM_CUBE) {
> -      ir->lod_info.lod = expr(ir_binop_add,
> -                              expr(ir_unop_log2, rho),
> -                              new(mem_ctx) ir_constant(-1.0f));

It seems that in this case we don't need rho at all, so we should
probably move the rho computation to the else branch entirely.

> +      /* Cubemap texture lookups first generate a texture coordinate normalized
> +         to [-1, 1] on the appropiate face. The appropiate face is determined
> +         by which component has largest magnitude and its sign. The texture
> +         coordinate is the quotient of the remaining texture coordinates against
> +         that absolute value of the component of largest magnitude. This division
> +         requires that the computing of the derivative of the texel coordinate
> +         must use the quotient rule. The high level GLSL code is as follows:

Great comment! Where did you get this from? Is this detailed somwhere in
the spec? In that case maybe we want to add a reference to that as well.

> +         Step 1: selection
> +
> +         vec3 abs_p, Q, dQdx, dQdy;
> +         abs_p = abs(ir->coordinate);
> +         if (abs_p.x >= max(abs_p.y, abs_p.z)) {
> +            Q = ir->coordinate.yzx;
> +            dQdx = ir->lod_info.grad.dPdx.yzx;
> +            dQdy = ir->lod_info.grad.dPdy.yzx;
> +         }
> +         if (abs_p.y >= max(abs_p.x, abs_p.z)) {
> +            Q = ir->coordinate.xzy;
> +            dQdx = ir->lod_info.grad.dPdx.xzy;
> +            dQdy = ir->lod_info.grad.dPdy.xzy;
> +         }
> +         if (abs_p.z >= max(abs_p.x, abs_p.y)) {
> +            Q = ir->coordinate;
> +            dQdx = ir->lod_info.grad.dPdx;
> +            dQdy = ir->lod_info.grad.dPdy;
> +         }

This is a nitpick: you use 'Q, dQdx and dQdy' above, and 'q, dqdx, dqdy'
below... you probably want to be consistent with the capitalization and
use Q everywhere, since that is what you use in the actual
implementation.

> +         Step 2: use quotient rule to compute derivative. The normalized to [-1, 1]
> +         texel coordinate is given by Q.xy / (sign(Q.z) * Q.z). We are only concerned
> +         with the magnitudes of the derivatives whose values are not affected by the
> +         sign. We drop the sign from the computation.
> +
> +         vec2 dx, dy;
> +         float recip;
> +
> +         recip = 1.0 / Q.z;
> +         dx = recip * ( dqdx.xy - q.xy * (dqdx.z * recip) );
> +         dy = recip * ( dqdy.xy - q.xy * (dqdy.z * recip) );
> +
> +         Step 3: compute LOD. At this point we have the derivatives of the
> +         texture coordinates normalized to [-1,1]. We take the LOD to be
> +          result = log2( max(sqrt(dot(dx, dx)), sqrt(dy, dy)) * 0.5 * L)

                          ^ Remove this blank space here

> +                 = -1.0 + log2(max(sqrt(dot(dx, dx)), sqrt(dy, dy)) * L)
> +                 = -1.0 + log2(sqrt(max(dot(dx, dx), dot(dy,dy))) * L)
> +                 = -1.0 + log2(sqrt(l * l * max(dot(dx, dx), dot(dy,dy))))
                                      ^^^^^^ 'L' instead of 'l'
> +                 = -1.0 + 0.5 * log2(L * L * max(dot(dx, dx), dot(dy,dy)))
> +         where L is the dimension of the cubemap. The code is:
> +
> +         float m, result;
> +         m = max(dot(dx, dx), dot(dy, dy));

Maybe you want to use 'M' instead of 'm' for consistency.

> +         L = textureSize(sampler, 0).x;

Is it always going to be .x (the width) that we need here?

> +         result = -1.0 + 0.5 * log2(l * l * m);
                                      ^^^^^^ 'L' instead of 'l'
> +       */
> +
> +/* Helpers to make code more human readable. */
> +#define EMIT(instr) base_ir->insert_before(instr)
> +#define THEN(irif, instr) irif->then_instructions.push_tail(instr)
> +#define CLONE(x) x->clone(mem_ctx, NULL)
> +
> +      ir_variable *abs_p = temp(mem_ctx, glsl_type::vec3_type, "abs_p");
> +
> +      EMIT(assign(abs_p, swizzle_for_size(abs(CLONE(ir->coordinate)), 3)));
> +
> +      ir_variable *Q = temp(mem_ctx, glsl_type::vec3_type, "Q");
> +      ir_variable *dQdx = temp(mem_ctx, glsl_type::vec3_type, "dQdx");
> +      ir_variable *dQdy = temp(mem_ctx, glsl_type::vec3_type, "dQdy");
> +
> +      /* unmodified dPdx, dPdy values */
> +      ir_rvalue *dPdx = ir->lod_info.grad.dPdx;
> +      ir_rvalue *dPdy = ir->lod_info.grad.dPdy;
> +
> +      /* 1. compute selector */
> +
> +      /* if (abs_p.x >= max(abs_p.y, abs_p.z))  ... */
> +      ir_if *branch_x =
> +         new(mem_ctx) ir_if(gequal(swizzle_x(abs_p),
> +                                   max2(swizzle_y(abs_p), swizzle_z(abs_p))));
> +
> +      /* q = p.yzx;
> +       * dqdx = dpdx.yzx;
> +       * dqdy = dpdy.yzx;
> +       */
> +      int yzx = MAKE_SWIZZLE4(SWIZZLE_Y, SWIZZLE_Z, SWIZZLE_X, 0);

Isn't that 0 on the 4th component actually the same as SWIZZLE_X? I
suppose that you just want to make clear that we don't care about that
component... in that case, could we use something like SWIZZLE_NIL
instead? If not I guess 0 is good enough.

> +      THEN(branch_x, assign(Q, swizzle(CLONE(ir->coordinate), yzx, 3)));
> +      THEN(branch_x, assign(dQdx, swizzle(CLONE(dPdx), yzx, 3)));
> +      THEN(branch_x, assign(dQdy, swizzle(CLONE(dPdy), yzx, 3)));
> +      EMIT(branch_x);
> +
> +      /* if (abs_p.y >= max(abs_p.x, abs_p.z)) */
> +      ir_if *branch_y =
> +         new(mem_ctx) ir_if(gequal(swizzle_y(abs_p),
> +                                   max2(swizzle_x(abs_p), swizzle_z(abs_p))));
> +
> +      /* q = p.xzy;
> +       * dqdx = dpdx.xzy;
> +       * dqdy = dpdy.xzy;
> +       */
> +      int xzy = MAKE_SWIZZLE4(SWIZZLE_X, SWIZZLE_Z, SWIZZLE_Y, 0);
> +      THEN(branch_y, assign(Q, swizzle(CLONE(ir->coordinate), xzy, 3)));
> +      THEN(branch_y, assign(dQdx, swizzle(CLONE(dPdx), xzy, 3)));
> +      THEN(branch_y, assign(dQdy, swizzle(CLONE(dPdy), xzy, 3)));
> +      EMIT(branch_y);
> +
> +      /* if (abs_p.z >= max(abs_p.x, abs_p.y)) */
> +      ir_if *branch_z =
> +         new(mem_ctx) ir_if(gequal(swizzle_z(abs_p),
> +                            max2(swizzle_x(abs_p), swizzle_y(abs_p))));
> +
> +      /* q = p;
> +       * dqdx = dpdx;
> +       * dqdy = dpdy;
> +       */
> +      THEN(branch_z, assign(Q, swizzle_for_size(CLONE(ir->coordinate), 3)));
> +      THEN(branch_z, assign(dQdx, CLONE(dPdx)));
> +      THEN(branch_z, assign(dQdy, CLONE(dPdy)));
> +      EMIT(branch_z);
> +
> +      /* 2. quotient rule */
> +      ir_variable *recip = temp(mem_ctx, glsl_type::float_type, "recip");
> +      EMIT(assign(recip, div(new(mem_ctx) ir_constant(1.0f), swizzle_z(Q))));
> +
> +      ir_variable *dx = temp(mem_ctx, glsl_type::vec(2), "dx");
> +      ir_variable *dy = temp(mem_ctx, glsl_type::vec(2), "dy");

I think we usually use glsl_type::vec2_type instead of the
glsl_type::vec(components) version?

> +      /* dx = recip * ( dqdx.xy - q.xy * (dqdx.z * recip) );
> +       * dy = recip * ( dqdy.xy - q.xy * (dqdy.z * recip) );
> +       */
> +      EMIT(assign(dx, mul(recip, sub(swizzle_xy(dQdx),
> +                                     mul(swizzle_xy(Q), mul(swizzle_z(dQdx),
> +                                                            recip))))));
> +      EMIT(assign(dy, mul(recip, sub(swizzle_xy(dQdy),
> +                                     mul(swizzle_xy(Q), mul(swizzle_z(dQdy),
> +                                                            recip))))));

What if, instead of q.xy * (dqdx.z * recip), we do (q.xy * recip) *
dqdx.z? In that case the (q.xy * recip) can be emitted only once and its
result reused for both EMITs.

Iago

> +      /* m = max(dot(dx, dx), dot(dy, dy)); */
> +      ir_variable *M = temp(mem_ctx, glsl_type::float_type, "m");
> +      EMIT(assign(M, max2(dot(dx, dx), dot(dy, dy))));
> +
> +      /* size has textureSize() of LOD 0 */
> +      ir_variable *L = temp(mem_ctx, glsl_type::float_type, "L");
> +      EMIT(assign(L, swizzle_x(size)));
> +
> +      ir_variable *result = temp(mem_ctx, glsl_type::float_type, "result");
> +
> +      /* result = -1.0 + 0.5 * log2(L * L * m); */
> +      EMIT(assign(result,
> +                  add(new(mem_ctx)ir_constant(-1.0f),
> +                      mul(new(mem_ctx)ir_constant(0.5f),
> +                          expr(ir_unop_log2, mul(mul(L, L), M))))));
> +
> +      /* 3. final assignment of parameters to textureLod call */
> +      ir->lod_info.lod = new (mem_ctx) ir_dereference_variable(result);
> +
> +#undef THEN
> +#undef EMIT
> +
>     } else {
>        ir->lod_info.lod = expr(ir_unop_log2, rho);
>     }




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