src/common/mathutil.cpp

//
// Copyright (c) 2013 The ANGLE Project Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//

// mathutil.cpp: Math and bit manipulation functions.

#include "common/mathutil.h"
#include <algorithm>
#include <math.h>

namespace gl
{

struct RGB9E5Data
{
    unsigned int R : 9;
    unsigned int G : 9;
    unsigned int B : 9;
    unsigned int E : 5;
};

// B is the exponent bias (15)
static const int g_sharedexp_bias = 15;

// N is the number of mantissa bits per component (9)
static const int g_sharedexp_mantissabits = 9;

// Emax is the maximum allowed biased exponent value (31)
static const int g_sharedexp_maxexponent = 31;

static const float g_sharedexp_max = ((pow(2.0f, g_sharedexp_mantissabits) - 1) /
                                       pow(2.0f, g_sharedexp_mantissabits)) *
                                     pow(2.0f, g_sharedexp_maxexponent - g_sharedexp_bias);

unsigned int convertRGBFloatsTo999E5(float red, float green, float blue)
{
    const float red_c = std::max<float>(0, std::min(g_sharedexp_max, red));
    const float green_c = std::max<float>(0, std::min(g_sharedexp_max, green));
    const float blue_c = std::max<float>(0, std::min(g_sharedexp_max, blue));

    const float max_c = std::max<float>(std::max<float>(red_c, green_c), blue_c);
    const float exp_p = std::max<float>(-g_sharedexp_bias - 1, floor(log(max_c))) + 1 + g_sharedexp_bias;
    const int max_s = floor((max_c / (pow(2.0f, exp_p - g_sharedexp_bias - g_sharedexp_mantissabits))) + 0.5f);
    const int exp_s = (max_s < pow(2.0f, g_sharedexp_mantissabits)) ? exp_p : exp_p + 1;

    RGB9E5Data output;
    output.R = floor((red_c / (pow(2.0f, exp_s - g_sharedexp_bias - g_sharedexp_mantissabits))) + 0.5f);
    output.G = floor((green_c / (pow(2.0f, exp_s - g_sharedexp_bias - g_sharedexp_mantissabits))) + 0.5f);
    output.B = floor((blue_c / (pow(2.0f, exp_s - g_sharedexp_bias - g_sharedexp_mantissabits))) + 0.5f);
    output.E = exp_s;

    return *reinterpret_cast<unsigned int*>(&output);
}

void convert999E5toRGBFloats(unsigned int input, float *red, float *green, float *blue)
{
    const RGB9E5Data *inputData = reinterpret_cast<const RGB9E5Data*>(&input);

    *red = inputData->R * pow(2.0f, (int)inputData->E - g_sharedexp_bias - g_sharedexp_mantissabits);
    *green = inputData->G * pow(2.0f, (int)inputData->E - g_sharedexp_bias - g_sharedexp_mantissabits);
    *blue = inputData->B * pow(2.0f, (int)inputData->E - g_sharedexp_bias - g_sharedexp_mantissabits);
}

}