axmol/cocos/base/s3tc.cpp

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#include "s3tc.h"

//Decode S3TC encode block to 4x4 RGB32 pixels
static void s3tc_decode_block(uint8_t **blockData,
                       uint32_t *decodeBlockData,
                       unsigned int stride,
                       bool oneBitAlphaFlag,
                       uint64_t alpha,
                       S3TCDecodeFlag decodeFlag)
{
    unsigned int colorValue0 = 0 , colorValue1 = 0, initAlpha = (!oneBitAlphaFlag * 255u) << 24;
    unsigned int rb0 = 0, rb1 = 0, rb2 = 0, rb3 = 0, g0 = 0, g1 = 0, g2 = 0, g3 = 0;
    
    uint32_t colors[4], pixelsIndex = 0;
    
    /* load the two color values*/
    memcpy((void *)&colorValue0, *blockData, 2);
    (*blockData) += 2;
    
    memcpy((void *)&colorValue1, *blockData, 2);
    (*blockData) += 2;
    
    /* the channel is r5g6b5 , 16 bits */
    rb0  = (colorValue0 << 19 | colorValue0 >> 8) & 0xf800f8;
    rb1  = (colorValue1 << 19 | colorValue1 >> 8) & 0xf800f8;
    g0   = (colorValue0 << 5) & 0x00fc00;
    g1   = (colorValue1 << 5) & 0x00fc00;
    g0  += (g0 >> 6) & 0x000300;
    g1  += (g1 >> 6) & 0x000300;
    
    colors[0] = rb0 + g0 + initAlpha;
    colors[1] = rb1 + g1 + initAlpha;
    
    /* interpolate the other two color values */
    if (colorValue0 > colorValue1 || oneBitAlphaFlag)
    {
        rb2 = (((2*rb0 + rb1) * 21) >> 6) & 0xff00ff;
        rb3 = (((2*rb1 + rb0) * 21) >> 6) & 0xff00ff;
        g2  = (((2*g0 + g1 ) * 21) >> 6) & 0x00ff00;
        g3  = (((2*g1 + g0 ) * 21) >> 6) & 0x00ff00;
        colors[3] = rb3 + g3 + initAlpha;
    }
    else
    {
        rb2 = ((rb0+rb1) >> 1) & 0xff00ff;
        g2  = ((g0 +g1 ) >> 1) & 0x00ff00;
        colors[3] = 0 ;
    }
    colors[2] = rb2 + g2 + initAlpha;
    
    /*read the pixelsIndex , 2bits per pixel, 4 bytes */
    memcpy((void*)&pixelsIndex, *blockData, 4);
    (*blockData) += 4;
    
    if (S3TCDecodeFlag::DXT5 == decodeFlag)
    {
        //dxt5 use interpolate alpha
        // 8-Alpha block: derive the other six alphas.
        // Bit code 000 = alpha0, 001 = alpha1, other are interpolated.
        
        unsigned int alphaArray[8];
        
        alphaArray[0] = (alpha ) & 0xff ;
        alphaArray[1] = (alpha >> 8) & 0xff ;
        
        if (alphaArray[0] >= alphaArray[1])
        {
            alphaArray[2] = (alphaArray[0]*6 + alphaArray[1]*1) / 7;
            alphaArray[3] = (alphaArray[0]*5 + alphaArray[1]*2) / 7;
            alphaArray[4] = (alphaArray[0]*4 + alphaArray[1]*3) / 7;
            alphaArray[5] = (alphaArray[0]*3 + alphaArray[1]*4) / 7;
            alphaArray[6] = (alphaArray[0]*2 + alphaArray[1]*5) / 7;
            alphaArray[7] = (alphaArray[0]*1 + alphaArray[1]*6) / 7;
        }
        else if (alphaArray[0] < alphaArray[1])
        {
            alphaArray[2] = (alphaArray[0]*4 + alphaArray[1]*1) / 5;
            alphaArray[3] = (alphaArray[0]*3 + alphaArray[1]*2) / 5;
            alphaArray[4] = (alphaArray[0]*2 + alphaArray[1]*3) / 5;
            alphaArray[5] = (alphaArray[0]*1 + alphaArray[1]*4) / 5;
            alphaArray[6] = 0;
            alphaArray[7] = 255;
        }
        
        // read the flowing 48bit indices (16*3)
        alpha >>= 16;
        
        for (int y = 0; y < 4; ++y)
        {
            for (int x = 0; x < 4; ++x)
            {
                decodeBlockData[x] = (alphaArray[alpha & 5] << 24) + colors[pixelsIndex & 3];
                pixelsIndex >>= 2;
                alpha >>= 3;
            }
            decodeBlockData += stride;
        }
    } //if (dxt5 == comFlag)
    else
    { //dxt1 dxt3 use explicit alpha
        for (int y = 0; y < 4; ++y)
        {
            for (int x = 0; x < 4; ++x)
            {
                initAlpha   = (static_cast<int>(alpha) & 0x0f) << 28;
                initAlpha   += initAlpha >> 4;
                decodeBlockData[x] = initAlpha + colors[pixelsIndex & 3];
                pixelsIndex >>= 2;
                alpha       >>= 4;
            }
            decodeBlockData += stride;
        }
    }
}

//Decode S3TC encode data to RGB32
void s3tc_decode(uint8_t *encodeData,             //in_data
                 uint8_t *decodeData,             //out_data
                 const int pixelsWidth,
                 const int pixelsHeight,
                 S3TCDecodeFlag decodeFlag)
{
    uint32_t *decodeBlockData = (uint32_t *)decodeData;
    for (int block_y = 0; block_y < pixelsHeight / 4; ++block_y, decodeBlockData += 3 * pixelsWidth)   //stride = 3*width
    {
        for(int block_x = 0; block_x < pixelsWidth / 4; ++block_x, decodeBlockData += 4)            //skip 4 pixels
        {
            uint64_t blockAlpha = 0;
            
            switch (decodeFlag)
            {
                case S3TCDecodeFlag::DXT1:
                {
                    s3tc_decode_block(&encodeData, decodeBlockData, pixelsWidth, 0, 0LL, S3TCDecodeFlag::DXT1);
                }
                    break;
                case S3TCDecodeFlag::DXT3:
                {
                    memcpy((void *)&blockAlpha, encodeData, 8);
                    encodeData += 8;
                    s3tc_decode_block(&encodeData, decodeBlockData, pixelsWidth, 1, blockAlpha, S3TCDecodeFlag::DXT3);
                }
                    break;
                case S3TCDecodeFlag::DXT5:
                {
                    memcpy((void *)&blockAlpha, encodeData, 8);
                    encodeData += 8;
                    s3tc_decode_block(&encodeData, decodeBlockData, pixelsWidth, 1, blockAlpha, S3TCDecodeFlag::DXT5);
                }
                    break;
                default:
                    break;
            }//switch
        }//for block_x
    }//for block_y
}