Fix broken YCbCr subpixel positioning. Caught by the unit tests.

[movit] / ycbcr.cpp

diff --git a/ycbcr.cpp b/ycbcr.cpp
index f0124eaab178048a2ae3a00c758355fa275668a2..277ea9c7b3a3de1c97cf95a51b1941661656bcbd 100644 (file)
--- a/ycbcr.cpp
+++ b/ycbcr.cpp
@@ -1,3 +1,6 @@
+// Note: This file does not have its own unit test; it is tested mainly
+// through YCbCrInput's unit tests.
+
 #include <Eigen/Core>
 #include <Eigen/LU>
 
@@ -45,7 +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 (0.5 - local_chroma_pos) / resolution;
+       }
 }
 
 // Given <ycbcr_format>, compute the values needed to turn Y'CbCr into R'G'B';