diff options
author | Marek Olšák <[email protected]> | 2015-10-10 22:05:58 +0200 |
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committer | Marek Olšák <[email protected]> | 2015-10-17 21:40:03 +0200 |
commit | 529c5e77402012aa0c0a11ee71a1a65e51edb496 (patch) | |
tree | 9f77a16ce5121379847fef4beb465e97a38d5a53 /src | |
parent | a2197cac7fd6ce3f897a89121f5e499d28a3888e (diff) |
gallivm: implement the correct version of LRP
The previous version has precision issues. This can be a problem
with tessellation. Sadly, I can't find the article where I read it
anymore. I'm not sure if the unsafe-fp-math flag would be enough to revert
this.
v2: added the comment
Diffstat (limited to 'src')
-rw-r--r-- | src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c | 19 |
1 files changed, 13 insertions, 6 deletions
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c index 0ad78b0ace2..3d5e2cb316b 100644 --- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c +++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c @@ -538,12 +538,19 @@ lrp_emit( struct lp_build_tgsi_context * bld_base, struct lp_build_emit_data * emit_data) { - LLVMValueRef tmp; - tmp = lp_build_emit_llvm_binary(bld_base, TGSI_OPCODE_SUB, - emit_data->args[1], - emit_data->args[2]); - emit_data->output[emit_data->chan] = lp_build_emit_llvm_ternary(bld_base, - TGSI_OPCODE_MAD, emit_data->args[0], tmp, emit_data->args[2]); + struct lp_build_context *bld = &bld_base->base; + LLVMValueRef inv, a, b; + + /* This uses the correct version: (1 - t)*a + t*b + * + * An alternative version is "a + t*(b-a)". The problem is this version + * doesn't return "b" for t = 1, because "a + (b-a)" isn't equal to "b" + * because of the floating-point rounding. + */ + inv = lp_build_sub(bld, bld_base->base.one, emit_data->args[0]); + a = lp_build_mul(bld, emit_data->args[1], emit_data->args[0]); + b = lp_build_mul(bld, emit_data->args[2], inv); + emit_data->output[emit_data->chan] = lp_build_add(bld, a, b); } /* TGSI_OPCODE_MAD */ |