src/util/format_srgb.h

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/**
 * @file
 * SRGB translation.
 *
 * @author Brian Paul <brianp@vmware.com>
 * @author Michal Krol <michal@vmware.com>
 * @author Jose Fonseca <jfonseca@vmware.com>
 */

#ifndef U_FORMAT_SRGB_H_
#define U_FORMAT_SRGB_H_

#include <stdint.h>
#include <math.h>
#include "c99_compat.h"

extern const float
util_format_srgb_8unorm_to_linear_float_table[256];

extern const uint8_t
util_format_srgb_to_linear_8unorm_table[256];

extern const uint8_t
util_format_linear_to_srgb_8unorm_table[256];

extern const unsigned
util_format_linear_to_srgb_helper_table[104];


static inline float
util_format_srgb_to_linear_float(float cs)
{
   if (cs <= 0.0f)
      return 0.0f;
   else if (cs <= 0.04045f)
      return cs / 12.92f;
   else if (cs < 1.0f)
      return powf((cs + 0.055) / 1.055f, 2.4f);
   else
      return 1.0f;
}


static inline float
util_format_linear_to_srgb_float(float cl)
{
   if (cl <= 0.0f)
      return 0.0f;
   else if (cl < 0.0031308f)
      return 12.92f * cl;
   else if (cl < 1.0f)
      return 1.055f * powf(cl, 0.41666f) - 0.055f;
   else
      return 1.0f;
}


/**
 * Convert a unclamped linear float to srgb value in the [0,255].
 */
static inline uint8_t
util_format_linear_float_to_srgb_8unorm(float x)
{
   /*
    * This is taken from https://gist.github.com/rygorous/2203834
    * Use LUT and do linear interpolation.
    */
   union {
      uint32_t ui;
      float f;
   } almostone, minval, f;
   unsigned tab, bias, scale, t;

   almostone.ui = 0x3f7fffff;
   minval.ui = (127-13) << 23;

   /*
    * Clamp to [2^(-13), 1-eps]; these two values map to 0 and 1, respectively.
    * The tests are carefully written so that NaNs map to 0, same as in the
    * reference implementation.
    */
   if (!(x > minval.f))
      x = minval.f;
   if (x > almostone.f)
      x = almostone.f;

   /* Do the table lookup and unpack bias, scale */
   f.f = x;
   tab = util_format_linear_to_srgb_helper_table[(f.ui - minval.ui) >> 20];
   bias = (tab >> 16) << 9;
   scale = tab & 0xffff;

   /* Grab next-highest mantissa bits and perform linear interpolation */
   t = (f.ui >> 12) & 0xff;
   return (uint8_t) ((bias + scale*t) >> 16);
}


/**
 * Convert an 8-bit sRGB value from non-linear space to a
 * linear RGB value in [0, 1].
 * Implemented with a 256-entry lookup table.
 */
static inline float
util_format_srgb_8unorm_to_linear_float(uint8_t x)
{
   return util_format_srgb_8unorm_to_linear_float_table[x];
}


/*
 * XXX These 2 functions probably don't make a lot of sense (but lots
 * of potential callers which most likely all don't make sense neither)
 */

/**
 * Convert a 8bit normalized value from linear to srgb.
 */
static inline uint8_t
util_format_linear_to_srgb_8unorm(uint8_t x)
{
   return util_format_linear_to_srgb_8unorm_table[x];
}


/**
 * Convert a 8bit normalized value from srgb to linear.
 */
static inline uint8_t
util_format_srgb_to_linear_8unorm(uint8_t x)
{
   return util_format_srgb_to_linear_8unorm_table[x];
}


#endif /* U_FORMAT_SRGB_H_ */