From: Steinar H. Gunderson <sgunderson@bigfoot.com>
Date: Thu, 30 Jul 2015 11:35:20 +0000 (+0200)
Subject: Do some IEEE trickery to help the shader optimizer remove a sub or two in some YCbCr... 
X-Git-Tag: 1.2.0~36
X-Git-Url: https://git.sesse.net/?p=movit;a=commitdiff_plain;h=9580d67514ec138e65496aa1170c63b53f657207

Do some IEEE trickery to help the shader optimizer remove a sub or two in some YCbCr cases.
---

diff --git a/ycbcr.cpp b/ycbcr.cpp
index 08c8cd2..6dbcd9d 100644
--- a/ycbcr.cpp
+++ b/ycbcr.cpp
@@ -48,7 +48,12 @@ namespace movit {
 float compute_chroma_offset(float pos, unsigned subsampling_factor, unsigned resolution)
 {
 	float local_chroma_pos = (0.5 + pos * (subsampling_factor - 1)) / subsampling_factor;
-	return (0.5 - local_chroma_pos) / resolution;
+	if (fabs(local_chroma_pos - 0.5) < 1e-10) {
+		// x + (-0) can be optimized away freely, as opposed to x + 0.
+		return -0.0;
+	} else {
+		return (local_chroma_pos - 0.5) / resolution;
+	}
 }
 
 // Given <ycbcr_format>, compute the values needed to turn Y'CbCr into R'G'B';