[Mesa-dev] [PATCH] gallivm/util: implement requested ubyte to float lookup function

[Maxence Le Doré]

This was a suggested rework as a todo. The only thing that annoy me is 
that the table is initialized in math init function with logarithm and 
exponential tables. And ubyte_to_float is not a math function in a way 
comparable to exp and log. Feel free to suggest me or do by yourself a 
modification that could place this init in a more appropriate place.



 From d04055ee06f2c21747607d865dce086e705babcf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Maxence=20Le=20Dor=C3=A9?= <maxence.ledore at gmail.com>
Date: Sun, 10 Mar 2013 18:15:47 +0100
Subject: [PATCH] util: implement ubyte to float lookup function

---
  src/gallium/auxiliary/util/u_math.c |   10 ++++++++++
  src/gallium/auxiliary/util/u_math.h |    9 +++++++--
  2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/gallium/auxiliary/util/u_math.c 
b/src/gallium/auxiliary/util/u_math.c
index 2811475..87d42e4 100644
--- a/src/gallium/auxiliary/util/u_math.c
+++ b/src/gallium/auxiliary/util/u_math.c
@@ -56,6 +56,15 @@ init_log2_table(void)
  }


+static void
+init_ubyte_to_float_table(void)
+{
+   int i;
+   for (i = 0; i < UBYTE_TO_FLOAT_TABLE_SIZE; i++)
+      ubyte_to_float_table[i] = (float) i * (1.0f / 255.0f);
+}
+
+
  /**
   * One time init for math utilities.
   */
@@ -66,6 +75,7 @@ util_init_math(void)
     if (!initialized) {
        init_pow2_table();
        init_log2_table();
+      init_ubyte_to_float_table();
        initialized = TRUE;
     }
  }
diff --git a/src/gallium/auxiliary/util/u_math.h 
b/src/gallium/auxiliary/util/u_math.h
index 607fbec..6795a54 100644
--- a/src/gallium/auxiliary/util/u_math.h
+++ b/src/gallium/auxiliary/util/u_math.h
@@ -523,14 +523,19 @@ fui( float f )
  }


+#define UBYTE_TO_FLOAT_TABLE_SIZE 256
+extern float ubyte_to_float_table[UBYTE_TO_FLOAT_TABLE_SIZE];
+
+
  /**
   * Convert ubyte to float in [0, 1].
- * XXX a 256-entry lookup table would be slightly faster.
   */
+
+
  static INLINE float
  ubyte_to_float(ubyte ub)
  {
-   return (float) ub * (1.0f / 255.0f);
+   return ubyte_to_float_table[ub];
  }


-- 
1.7.9.5