src/util/format_rgb9e5.h

   1 /*
   2  * Copyright (C) 2011 Marek Olšák <maraeo@gmail.com>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  21  * DEALINGS IN THE SOFTWARE.
  22  */
  23
  24 /* Copied from EXT_texture_shared_exponent and edited, getting rid of
  25  * expensive float math bits too. */
  26
  27 #ifndef RGB9E5_H
  28 #define RGB9E5_H
  29
  30 #include <assert.h>
  31 #include <stdint.h>
  32
  33 #include "c99_math.h"
  34
  35 #define RGB9E5_EXPONENT_BITS          5
  36 #define RGB9E5_MANTISSA_BITS          9
  37 #define RGB9E5_EXP_BIAS               15
  38 #define RGB9E5_MAX_VALID_BIASED_EXP   31
  39
  40 #define MAX_RGB9E5_EXP               (RGB9E5_MAX_VALID_BIASED_EXP - RGB9E5_EXP_BIAS)
  41 #define RGB9E5_MANTISSA_VALUES       (1<<RGB9E5_MANTISSA_BITS)
  42 #define MAX_RGB9E5_MANTISSA          (RGB9E5_MANTISSA_VALUES-1)
  43 #define MAX_RGB9E5                   (((float)MAX_RGB9E5_MANTISSA)/RGB9E5_MANTISSA_VALUES * (1<<MAX_RGB9E5_EXP))
  44
  45 static inline int rgb9e5_ClampRange(float x)
  46 {
  47    union { float f; uint32_t u; } f, max;
  48    f.f = x;
  49    max.f = MAX_RGB9E5;
  50
  51    if (f.u > 0x7f800000)
  52   /* catches neg, NaNs */
  53       return 0;
  54    else if (f.u >= max.u)
  55       return max.u;
  56    else
  57       return f.u;
  58 }
  59
  60 static inline uint32_t float3_to_rgb9e5(const float rgb[3])
  61 {
  62    int rm, gm, bm, exp_shared;
  63    uint32_t revdenom_biasedexp;
  64    union { float f; uint32_t u; } rc, bc, gc, maxrgb, revdenom;
  65
  66    rc.u = rgb9e5_ClampRange(rgb[0]);
  67    gc.u = rgb9e5_ClampRange(rgb[1]);
  68    bc.u = rgb9e5_ClampRange(rgb[2]);
  69    maxrgb.u = MAX3(rc.u, gc.u, bc.u);
  70
  71    /*
  72     * Compared to what the spec suggests, instead of conditionally adjusting
  73     * the exponent after the fact do it here by doing the equivalent of +0.5 -
  74     * the int add will spill over into the exponent in this case.
  75     */
  76    maxrgb.u += maxrgb.u & (1 << (23-9));
  77    exp_shared = MAX2((maxrgb.u >> 23), -RGB9E5_EXP_BIAS - 1 + 127) +
  78                 1 + RGB9E5_EXP_BIAS - 127;
  79    revdenom_biasedexp = 127 - (exp_shared - RGB9E5_EXP_BIAS -
  80                                RGB9E5_MANTISSA_BITS) + 1;
  81    revdenom.u = revdenom_biasedexp << 23;
  82    assert(exp_shared <= RGB9E5_MAX_VALID_BIASED_EXP);
  83
  84    /*
  85     * The spec uses strict round-up behavior (d3d10 disagrees, but in any case
  86     * must match what is done above for figuring out exponent).
  87     * We avoid the doubles ((int) rc * revdenom + 0.5) by doing the rounding
  88     * ourselves (revdenom was adjusted by +1, above).
  89     */
  90    rm = (int) (rc.f * revdenom.f);
  91    gm = (int) (gc.f * revdenom.f);
  92    bm = (int) (bc.f * revdenom.f);
  93    rm = (rm & 1) + (rm >> 1);
  94    gm = (gm & 1) + (gm >> 1);
  95    bm = (bm & 1) + (bm >> 1);
  96
  97    assert(rm <= MAX_RGB9E5_MANTISSA);
  98    assert(gm <= MAX_RGB9E5_MANTISSA);
  99    assert(bm <= MAX_RGB9E5_MANTISSA);
 100    assert(rm >= 0);
 101    assert(gm >= 0);
 102    assert(bm >= 0);
 103
 104    return (exp_shared << 27) | (bm << 18) | (gm << 9) | rm;
 105 }
 106
 107 static inline void rgb9e5_to_float3(uint32_t rgb, float retval[3])
 108 {
 109    int exponent;
 110    union { float f; uint32_t u; } scale;
 111
 112    exponent = (rgb >> 27) - RGB9E5_EXP_BIAS - RGB9E5_MANTISSA_BITS;
 113    scale.u = (exponent + 127) << 23;
 114
 115    retval[0] = ( rgb        & 0x1ff) * scale.f;
 116    retval[1] = ((rgb >> 9)  & 0x1ff) * scale.f;
 117    retval[2] = ((rgb >> 18) & 0x1ff) * scale.f;
 118 }
 119
 120 #endif