alpha.cpp

/* -----------------------------------------------------------------------------

    Copyright (c) 2006 Simon Brown                          si@sjbrown.co.uk

    Permission is hereby granted, free of charge, to any person obtaining
    a copy of this software and associated documentation files (the
    "Software"), to    deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
    IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
    CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
    TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
    SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

   -------------------------------------------------------------------------- */

#include "alpha.h"

#include <climits>
#include <algorithm>

namespace squish {

static int FloatToInt( float a, int limit )
{
    // use ANSI round-to-zero behaviour to get round-to-nearest
    int i = ( int )( a + 0.5f );

    // clamp to the limit
    if( i < 0 )
        i = 0;
    else if( i > limit )
        i = limit;

    // done
    return i;
}

void CompressAlphaDxt3( float const* bgra, int mask, void* block )
{
    u8* bytes = reinterpret_cast< u8* >( block );

    // quantise and pack the alpha values pairwise
    for( int i = 0; i < 8; ++i )
    {
        // quantise down to 4 bits
        float alpha1 = bgra[8*i + 3] * 15.0f;
        float alpha2 = bgra[8*i + 7] * 15.0f;
        int quant1 = FloatToInt( alpha1, 15 );
        int quant2 = FloatToInt( alpha2, 15 );

        // set alpha to zero where masked
        int bit1 = 1 << ( 2*i );
        int bit2 = 1 << ( 2*i + 1 );
        if( ( mask & bit1 ) == 0 )
            quant1 = 0;
        if( ( mask & bit2 ) == 0 )
            quant2 = 0;

        // pack into the byte
        bytes[i] = ( u8 )( quant1 | ( quant2 << 4 ) );
    }
}

void DecompressAlphaDxt3( float* bgra, void const* block )
{
    u8 const* bytes = reinterpret_cast< u8 const* >( block );

    // unpack the alpha values pairwise
    for( int i = 0; i < 8; ++i )
    {
        // quantise down to 4 bits
        u8 quant = bytes[i];

        // unpack the values
        u8 lo = quant & 0x0f;
        u8 hi = quant & 0xf0;

        // convert back up to bytes
        bgra[8*i + 3] = ( lo | ( lo << 4 ) ) / 255.0f;
        bgra[8*i + 7] = ( hi | ( hi >> 4 ) ) / 255.0f;
    }
}

static void FixRange( int& min, int& max, int steps )
{
    if( max - min < steps )
        max = std::min( min + steps, 255 );
    if( max - min < steps )
        min = std::max( 0, max - steps );
}

static int FitCodes( float const* bgra, int mask, u8 const* codes, u8* indices )
{
    // fit each alpha value to the codebook
    int err = 0;
    for( int i = 0; i < 16; ++i )
    {
        // check this pixel is valid
        int bit = 1 << i;
        if( ( mask & bit ) == 0 )
        {
            // use the first code
            indices[i] = 0;
            continue;
        }

        // find the least error and corresponding index
        int value = ( int )( bgra[4*i + 3] * 255 );
        int least = INT_MAX;
        int index = 0;
        for( int j = 0; j < 8; ++j )
        {
            // get the squared error from this code
            int dist = ( int )value - ( int )codes[j];
            dist *= dist;

            // compare with the best so far
            if( dist < least )
            {
                least = dist;
                index = j;
            }
        }

        // save this index and accumulate the error
        indices[i] = ( u8 )index;
        err += least;
    }

    // return the total error
    return err;
}

static void WriteAlphaBlock( int alpha0, int alpha1, u8 const* indices, void* block )
{
    u8* bytes = reinterpret_cast< u8* >( block );

    // write the first two bytes
    bytes[0] = ( u8 )alpha0;
    bytes[1] = ( u8 )alpha1;

    // pack the indices with 3 bits each
    u8* dest = bytes + 2;
    u8 const* src = indices;
    for( int i = 0; i < 2; ++i )
    {
        // pack 8 3-bit values
        int value = 0;
        for( int j = 0; j < 8; ++j )
        {
            int index = *src++;
            value |= ( index << 3*j );
        }

        // store in 3 bytes
        for( int j = 0; j < 3; ++j )
        {
            int byte = ( value >> 8*j ) & 0xff;
            *dest++ = ( u8 )byte;
        }
    }
}

static void WriteAlphaBlock5( int alpha0, int alpha1, u8 const* indices, void* block )
{
    // check the relative values of the endpoints
    if( alpha0 > alpha1 )
    {
        // swap the indices
        u8 swapped[16];
        for( int i = 0; i < 16; ++i )
        {
            u8 index = indices[i];
            if( index == 0 )
                swapped[i] = 1;
            else if( index == 1 )
                swapped[i] = 0;
            else if( index <= 5 )
                swapped[i] = 7 - index;
            else
                swapped[i] = index;
        }

        // write the block
        WriteAlphaBlock( alpha1, alpha0, swapped, block );
    }
    else
    {
        // write the block
        WriteAlphaBlock( alpha0, alpha1, indices, block );
    }
}

static void WriteAlphaBlock7( int alpha0, int alpha1, u8 const* indices, void* block )
{
    // check the relative values of the endpoints
    if( alpha0 < alpha1 )
    {
        // swap the indices
        u8 swapped[16];
        for( int i = 0; i < 16; ++i )
        {
            u8 index = indices[i];
            if( index == 0 )
                swapped[i] = 1;
            else if( index == 1 )
                swapped[i] = 0;
            else
                swapped[i] = 9 - index;
        }

        // write the block
        WriteAlphaBlock( alpha1, alpha0, swapped, block );
    }
    else
    {
        // write the block
        WriteAlphaBlock( alpha0, alpha1, indices, block );
    }
}

void CompressAlphaDxt5( float const* bgra, int mask, void* block )
{
    // get the range for 5-alpha and 7-alpha interpolation
    int min5 = 255;
    int max5 = 0;
    int min7 = 255;
    int max7 = 0;
    for( int i = 0; i < 16; ++i )
    {
        // check this pixel is valid
        int bit = 1 << i;
        if( ( mask & bit ) == 0 )
            continue;

        // incorporate into the min/max
        int value = ( int )( bgra[4*i + 3] * 255 );
        if( value < min7 )
            min7 = value;
        if( value > max7 )
            max7 = value;
        if( value != 0 && value < min5 )
            min5 = value;
        if( value != 255 && value > max5 )
            max5 = value;
    }

    // handle the case that no valid range was found
    if( min5 > max5 )
        min5 = max5;
    if( min7 > max7 )
        min7 = max7;

    // fix the range to be the minimum in each case
    FixRange( min5, max5, 5 );
    FixRange( min7, max7, 7 );

    // set up the 5-alpha code book
    u8 codes5[8];
    codes5[0] = ( u8 )min5;
    codes5[1] = ( u8 )max5;
    for( int i = 1; i < 5; ++i )
        codes5[1 + i] = ( u8 )( ( ( 5 - i )*min5 + i*max5 )/5 );
    codes5[6] = 0;
    codes5[7] = 255;

    // set up the 7-alpha code book
    u8 codes7[8];
    codes7[0] = ( u8 )min7;
    codes7[1] = ( u8 )max7;
    for( int i = 1; i < 7; ++i )
        codes7[1 + i] = ( u8 )( ( ( 7 - i )*min7 + i*max7 )/7 );

    // fit the data to both code books
    u8 indices5[16];
    u8 indices7[16];
    int err5 = FitCodes( bgra, mask, codes5, indices5 );
    int err7 = FitCodes( bgra, mask, codes7, indices7 );

    // save the block with least error
    if( err5 <= err7 )
        WriteAlphaBlock5( min5, max5, indices5, block );
    else
        WriteAlphaBlock7( min7, max7, indices7, block );
}

void DecompressAlphaDxt5( float* bgra, void const* block )
{
    // get the two alpha values
    u8 const* bytes = reinterpret_cast< u8 const* >( block );
    int alpha0 = bytes[0];
    int alpha1 = bytes[1];

    // compare the values to build the codebook
    u8 codes[8];
    codes[0] = ( u8 )alpha0;
    codes[1] = ( u8 )alpha1;
    if( alpha0 <= alpha1 )
    {
        // use 5-alpha codebook
        for( int i = 1; i < 5; ++i )
            codes[1 + i] = ( u8 )( ( ( 5 - i )*alpha0 + i*alpha1 )/5 );
        codes[6] = 0;
        codes[7] = 255;
    }
    else
    {
        // use 7-alpha codebook
        for( int i = 1; i < 7; ++i )
            codes[1 + i] = ( u8 )( ( ( 7 - i )*alpha0 + i*alpha1 )/7 );
    }

    // decode the indices
    u8 indices[16];
    u8 const* src = bytes + 2;
    u8* dest = indices;
    for( int i = 0; i < 2; ++i )
    {
        // grab 3 bytes
        int value = 0;
        for( int j = 0; j < 3; ++j )
        {
            int byte = *src++;
            value |= ( byte << 8*j );
        }

        // unpack 8 3-bit values from it
        for( int j = 0; j < 8; ++j )
        {
            int index = ( value >> 3*j ) & 0x7;
            *dest++ = ( u8 )index;
        }
    }

    // write out the indexed codebook values
    for( int i = 0; i < 16; ++i )
        bgra[4*i + 3] = ( float )( codes[indices[i]] / 255.0f );
}

} // namespace squish