[Mesa-dev] [PATCH] Fixed bug in unclamped float to ubyte conversion.

[Manfred Ernst]

Problem: The IEEE float optimized version of UNCLAMPED_FLOAT_TO_UBYTE in macros.h
computed incorrect results for inputs in the range 0x3f7f0000 (=0.99609375) to
0x3f7f7f80 (=0.99803924560546875) inclusive.  0x3f7f7f80 is the IEEE float
value that results in 254.5 when multiplied by 255.  With rounding mode
"round to closest even integer", this is the largest float in the range 0.0-1.0
that is converted to 254 by the generic implementation of
UNCLAMPED_FLOAT_TO_UBYTE.  The IEEE float optimized version incorrectly defined
the cut-off for mapping to 255 as 0x3f7f0000 (=255.0/256.0). The same bug was
present in the function float_to_ubyte in u_math.h.

Fix: The proposed fix replaces the incorrect cut-off value by 0x3f800000, which
is the IEEE float representation of 1.0f. 0x3f7f7f81 (or any value in between)
would also work, but 1.0f is probably cleaner.

The patch does not regress piglit on llvmpipe and on i965 on sandy bridge.
Tested-by Stéphane Marchesin <marcheu at chromium.org>
Reviewed-by Stéphane Marchesin <marcheu at chromium.org>
---
 src/gallium/auxiliary/util/u_math.h | 3 +--
 src/mesa/main/macros.h              | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/gallium/auxiliary/util/u_math.h b/src/gallium/auxiliary/util/u_math.h
index 607fbec..64d16cb 100644
--- a/src/gallium/auxiliary/util/u_math.h
+++ b/src/gallium/auxiliary/util/u_math.h
@@ -540,14 +540,13 @@ ubyte_to_float(ubyte ub)
 static INLINE ubyte
 float_to_ubyte(float f)
 {
-   const int ieee_0996 = 0x3f7f0000;   /* 0.996 or so */
    union fi tmp;
 
    tmp.f = f;
    if (tmp.i < 0) {
       return (ubyte) 0;
    }
-   else if (tmp.i >= ieee_0996) {
+   else if (tmp.i >= 0x3f800000 /* 1.0f */) {
       return (ubyte) 255;
    }
    else {
diff --git a/src/mesa/main/macros.h b/src/mesa/main/macros.h
index ac24672..8f5b5ae 100644
--- a/src/mesa/main/macros.h
+++ b/src/mesa/main/macros.h
@@ -142,7 +142,6 @@ extern GLfloat _mesa_ubyte_to_float_color_tab[256];
  *** CLAMPED_FLOAT_TO_UBYTE: map float known to be in [0,1] to ubyte in [0,255]
  ***/
 #if defined(USE_IEEE) && !defined(DEBUG)
-#define IEEE_0996 0x3f7f0000	/* 0.996 or so */
 /* This function/macro is sensitive to precision.  Test very carefully
  * if you change it!
  */
@@ -152,7 +151,7 @@ extern GLfloat _mesa_ubyte_to_float_color_tab[256];
            __tmp.f = (F);						\
            if (__tmp.i < 0)						\
               UB = (GLubyte) 0;						\
-           else if (__tmp.i >= IEEE_0996)				\
+           else if (__tmp.i >= IEEE_ONE)				\
               UB = (GLubyte) 255;					\
            else {							\
               __tmp.f = __tmp.f * (255.0F/256.0F) + 32768.0F;		\
-- 
1.8.2.1