axmol/core/base/s3tc.cpp

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 Copyright (c) 2017-2018 Xiamen Yaji Software Co., Ltd.

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#include "base/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
}