Mesa (master): i965/blorp: Modify manual_blend() to avoid unnecessary loss of precision.
Paul Berry
stereotype441 at kemper.freedesktop.org
Fri Jul 20 16:38:03 UTC 2012
Module: Mesa
Branch: master
Commit: b961d37e613b8b14927c42e09d16d09d70ebcb77
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=b961d37e613b8b14927c42e09d16d09d70ebcb77
Author: Paul Berry <stereotype441 at gmail.com>
Date: Sat Jul 7 08:02:48 2012 -0700
i965/blorp: Modify manual_blend() to avoid unnecessary loss of precision.
When downsampling from an MSAA image to a single-sampled image, it is
inevitable that some loss of numerical precision will occur, since we
have to use 32-bit floating point registers to hold the intermediate
results while blending. However, it seems reasonable to expect that
when all samples corresponding to a given pixel have the exact same
color value, there will be no loss of precision.
Previously, we averaged samples as follows:
blend = (((sample[0] + sample[1]) + sample[2]) + sample[3]) / 4
This had the potential to lose numerical precision when all samples
have the same color value, since ((sample[0] + sample[1]) + sample[2])
may not be precisely representable as a 32-bit float, even if the
individual samples are.
This patch changes the formula to:
blend = ((sample[0] + sample[1]) + (sample[2] + sample[3])) / 4
This avoids any loss of precision in the event that all samples are
the same, by ensuring that each addition operation adds two equal
values.
As a side benefit, this puts the formula in the form we will need in
order to implement correct blending of integer formats.
Reviewed-by: Anuj Phogat <anuj.phogat at gmail.com>
---
src/mesa/drivers/dri/i965/brw_blorp_blit.cpp | 117 ++++++++++++++++++++------
1 files changed, 90 insertions(+), 27 deletions(-)
diff --git a/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp b/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
index 6954733..74ae52d 100644
--- a/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
+++ b/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
@@ -434,6 +434,11 @@ private:
const sampler_message_arg *args, int num_args);
void render_target_write();
+ /**
+ * Base-2 logarithm of the maximum number of samples that can be blended.
+ */
+ static const unsigned LOG2_MAX_BLEND_SAMPLES = 2;
+
void *mem_ctx;
struct brw_context *brw;
const brw_blorp_blit_prog_key *key;
@@ -455,13 +460,8 @@ private:
struct brw_reg offset;
} x_transform, y_transform;
- /* Data to be written to render target (4 vec16's) */
- struct brw_reg result;
-
- /* Auxiliary storage for data returned by a sampling operation when
- * blending (4 vec16's)
- */
- struct brw_reg texture_data;
+ /* Data read from texture (4 vec16's per array element) */
+ struct brw_reg texture_data[LOG2_MAX_BLEND_SAMPLES + 1];
/* Auxiliary storage for the contents of the MCS surface.
*
@@ -622,7 +622,7 @@ brw_blorp_blit_program::compile(struct brw_context *brw,
if (brw->intel.gen == 6) {
/* Gen6 hardware an automatically blend using the SAMPLE message */
single_to_blend();
- sample(result);
+ sample(texture_data[0]);
} else {
/* Gen7+ hardware doesn't automaticaly blend. */
manual_blend();
@@ -656,7 +656,7 @@ brw_blorp_blit_program::compile(struct brw_context *brw,
*/
if (key->tex_layout == INTEL_MSAA_LAYOUT_CMS)
mcs_fetch();
- texel_fetch(result);
+ texel_fetch(texture_data[0]);
}
/* Finally, write the fetched (or blended) value to the render target and
@@ -695,8 +695,9 @@ brw_blorp_blit_program::alloc_regs()
prog_data.first_curbe_grf = reg;
alloc_push_const_regs(reg);
reg += BRW_BLORP_NUM_PUSH_CONST_REGS;
- this->result = vec16(brw_vec8_grf(reg, 0)); reg += 8;
- this->texture_data = vec16(brw_vec8_grf(reg, 0)); reg += 8;
+ for (unsigned i = 0; i < ARRAY_SIZE(texture_data); ++i) {
+ this->texture_data[i] = vec16(brw_vec8_grf(reg, 0)); reg += 8;
+ }
this->mcs_data =
retype(brw_vec8_grf(reg, 0), BRW_REGISTER_TYPE_UD); reg += 8;
for (int i = 0; i < 2; ++i) {
@@ -711,6 +712,9 @@ brw_blorp_blit_program::alloc_regs()
this->t1 = vec16(retype(brw_vec8_grf(reg++, 0), BRW_REGISTER_TYPE_UW));
this->t2 = vec16(retype(brw_vec8_grf(reg++, 0), BRW_REGISTER_TYPE_UW));
+ /* Make sure we didn't run out of registers */
+ assert(reg <= GEN7_MRF_HACK_START);
+
int mrf = 2;
this->base_mrf = mrf;
}
@@ -1061,6 +1065,24 @@ brw_blorp_blit_program::single_to_blend()
SWAP_XY_AND_XPYP();
}
+
+/**
+ * Count the number of trailing 1 bits in the given value. For example:
+ *
+ * count_trailing_one_bits(0) == 0
+ * count_trailing_one_bits(7) == 3
+ * count_trailing_one_bits(11) == 2
+ */
+inline int count_trailing_one_bits(unsigned value)
+{
+#if defined(__GNUC__) && ((__GNUC__ * 100 + __GNUC_MINOR__) >= 304) /* gcc 3.4 or later */
+ return __builtin_ctz(~value);
+#else
+ return _mesa_bitcount(value & ~(value + 1));
+#endif
+}
+
+
void
brw_blorp_blit_program::manual_blend()
{
@@ -1070,27 +1092,68 @@ brw_blorp_blit_program::manual_blend()
if (key->tex_layout == INTEL_MSAA_LAYOUT_CMS)
mcs_fetch();
- /* Gather sample 0 data first */
- s_is_zero = true;
- texel_fetch(result);
-
- /* Gather data for remaining samples and accumulate it into result. */
- s_is_zero = false;
- for (int i = 1; i < num_samples; ++i) {
- brw_MOV(&func, S, brw_imm_uw(i));
- texel_fetch(texture_data);
-
- /* TODO: should use a smaller loop bound for non-RGBA formats */
- for (int j = 0; j < 4; ++j) {
- brw_ADD(&func, offset(result, 2*j), offset(vec8(result), 2*j),
- offset(vec8(texture_data), 2*j));
+ /* We add together samples using a binary tree structure, e.g. for 4x MSAA:
+ *
+ * result = ((sample[0] + sample[1]) + (sample[2] + sample[3])) / 4
+ *
+ * This ensures that when all samples have the same value, no numerical
+ * precision is lost, since each addition operation always adds two equal
+ * values, and summing two equal floating point values does not lose
+ * precision.
+ *
+ * We perform this computation by treating the texture_data array as a
+ * stack and performing the following operations:
+ *
+ * - push sample 0 onto stack
+ * - push sample 1 onto stack
+ * - add top two stack entries
+ * - push sample 2 onto stack
+ * - push sample 3 onto stack
+ * - add top two stack entries
+ * - add top two stack entries
+ * - divide top stack entry by 4
+ *
+ * Note that after pushing sample i onto the stack, the number of add
+ * operations we do is equal to the number of trailing 1 bits in i. This
+ * works provided the total number of samples is a power of two, which it
+ * always is for i965.
+ */
+ unsigned stack_depth = 0;
+ for (int i = 0; i < num_samples; ++i) {
+ assert(stack_depth == _mesa_bitcount(i)); /* Loop invariant */
+
+ /* Push sample i onto the stack */
+ assert(stack_depth < ARRAY_SIZE(texture_data));
+ if (i == 0) {
+ s_is_zero = true;
+ } else {
+ s_is_zero = false;
+ brw_MOV(&func, S, brw_imm_uw(i));
+ }
+ texel_fetch(texture_data[stack_depth++]);
+
+ /* Do count_trailing_one_bits(i) times */
+ for (int j = count_trailing_one_bits(i); j-- > 0; ) {
+ assert(stack_depth >= 2);
+ --stack_depth;
+
+ /* TODO: should use a smaller loop bound for non_RGBA formats */
+ for (int k = 0; k < 4; ++k) {
+ brw_ADD(&func, offset(texture_data[stack_depth - 1], 2*k),
+ offset(vec8(texture_data[stack_depth - 1]), 2*k),
+ offset(vec8(texture_data[stack_depth]), 2*k));
+ }
}
}
+ /* We should have just 1 sample on the stack now. */
+ assert(stack_depth == 1);
+
/* Scale the result down by a factor of num_samples */
/* TODO: should use a smaller loop bound for non-RGBA formats */
for (int j = 0; j < 4; ++j) {
- brw_MUL(&func, offset(result, 2*j), offset(vec8(result), 2*j),
+ brw_MUL(&func, offset(texture_data[0], 2*j),
+ offset(vec8(texture_data[0]), 2*j),
brw_imm_f(1.0/num_samples));
}
}
@@ -1274,7 +1337,7 @@ brw_blorp_blit_program::render_target_write()
for (int i = 0; i < 4; ++i) {
/* E.g. mov(16) m2.0<1>:f r2.0<8;8,1>:f { Align1, H1 } */
brw_MOV(&func, offset(mrf_rt_write, mrf_offset),
- offset(vec8(result), 2*i));
+ offset(vec8(texture_data[0]), 2*i));
mrf_offset += 2;
}
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