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      1 /**************************************************************************
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      3  * Copyright 2010 VMware, Inc.
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     27 
     28 /**
     29  * @file
     30  * SRGB translation.
     31  *
     32  * @author Brian Paul <brianp (at) vmware.com>
     33  * @author Michal Krol <michal (at) vmware.com>
     34  * @author Jose Fonseca <jfonseca (at) vmware.com>
     35  */
     36 
     37 #ifndef U_FORMAT_SRGB_H_
     38 #define U_FORMAT_SRGB_H_
     39 
     40 #include <stdint.h>
     41 #include <math.h>
     42 #include "c99_compat.h"
     43 
     44 extern const float
     45 util_format_srgb_8unorm_to_linear_float_table[256];
     46 
     47 extern const uint8_t
     48 util_format_srgb_to_linear_8unorm_table[256];
     49 
     50 extern const uint8_t
     51 util_format_linear_to_srgb_8unorm_table[256];
     52 
     53 extern const unsigned
     54 util_format_linear_to_srgb_helper_table[104];
     55 
     56 
     57 static inline float
     58 util_format_linear_to_srgb_float(float cl)
     59 {
     60    if (cl <= 0.0f)
     61       return 0.0f;
     62    else if (cl < 0.0031308f)
     63       return 12.92f * cl;
     64    else if (cl < 1.0f)
     65       return 1.055f * powf(cl, 0.41666f) - 0.055f;
     66    else
     67       return 1.0f;
     68 }
     69 
     70 
     71 /**
     72  * Convert a unclamped linear float to srgb value in the [0,255].
     73  */
     74 static inline uint8_t
     75 util_format_linear_float_to_srgb_8unorm(float x)
     76 {
     77    /*
     78     * This is taken from https://gist.github.com/rygorous/2203834
     79     * Use LUT and do linear interpolation.
     80     */
     81    union {
     82       uint32_t ui;
     83       float f;
     84    } almostone, minval, f;
     85    unsigned tab, bias, scale, t;
     86 
     87    almostone.ui = 0x3f7fffff;
     88    minval.ui = (127-13) << 23;
     89 
     90    /*
     91     * Clamp to [2^(-13), 1-eps]; these two values map to 0 and 1, respectively.
     92     * The tests are carefully written so that NaNs map to 0, same as in the
     93     * reference implementation.
     94     */
     95    if (!(x > minval.f))
     96       x = minval.f;
     97    if (x > almostone.f)
     98       x = almostone.f;
     99 
    100    /* Do the table lookup and unpack bias, scale */
    101    f.f = x;
    102    tab = util_format_linear_to_srgb_helper_table[(f.ui - minval.ui) >> 20];
    103    bias = (tab >> 16) << 9;
    104    scale = tab & 0xffff;
    105 
    106    /* Grab next-highest mantissa bits and perform linear interpolation */
    107    t = (f.ui >> 12) & 0xff;
    108    return (uint8_t) ((bias + scale*t) >> 16);
    109 }
    110 
    111 
    112 /**
    113  * Convert an 8-bit sRGB value from non-linear space to a
    114  * linear RGB value in [0, 1].
    115  * Implemented with a 256-entry lookup table.
    116  */
    117 static inline float
    118 util_format_srgb_8unorm_to_linear_float(uint8_t x)
    119 {
    120    return util_format_srgb_8unorm_to_linear_float_table[x];
    121 }
    122 
    123 
    124 /*
    125  * XXX These 2 functions probably don't make a lot of sense (but lots
    126  * of potential callers which most likely all don't make sense neither)
    127  */
    128 
    129 /**
    130  * Convert a 8bit normalized value from linear to srgb.
    131  */
    132 static inline uint8_t
    133 util_format_linear_to_srgb_8unorm(uint8_t x)
    134 {
    135    return util_format_linear_to_srgb_8unorm_table[x];
    136 }
    137 
    138 
    139 /**
    140  * Convert a 8bit normalized value from srgb to linear.
    141  */
    142 static inline uint8_t
    143 util_format_srgb_to_linear_8unorm(uint8_t x)
    144 {
    145    return util_format_srgb_to_linear_8unorm_table[x];
    146 }
    147 
    148 
    149 #endif /* U_FORMAT_SRGB_H_ */
    150