dxtn.c

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/*
 * DXTn codec
 * Version:  1.1
 *
 * Copyright (C) 2004  Daniel Borca   All Rights Reserved.
 *
 * this is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 *
 * this is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNU Make; see the file COPYING.  If not, write to
 * the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.    
 */


#include <stdlib.h>
#include <string.h>

#include "types.h"
#include "internal.h"
#include "dxtn.h"


/***************************************************************************\
 * DXTn encoder
 *
 * The encoder was built by reversing the decoder,
 * and is vaguely based on FXT1 codec. Note that this code
 * is merely a proof of concept, since it is highly UNoptimized!
\***************************************************************************/


#define MAX_COMP 4 /* ever needed maximum number of components in texel */
#define MAX_VECT 4 /* ever needed maximum number of base vectors to find */
#define N_TEXELS 16 /* number of texels in a block (always 16) */
#define COLOR565(v) (word)((((v)[RCOMP] & 0xf8) << 8) | (((v)[GCOMP] & 0xfc) << 3) | ((v)[BCOMP] >> 3))


static const int dxtn_color_tlat[2][4] = {
    { 0, 2, 3, 1 },
    { 0, 2, 1, 3 }
};

static const int dxtn_alpha_tlat[2][8] = {
    { 0, 2, 3, 4, 5, 6, 7, 1 },
    { 0, 2, 3, 4, 5, 1, 6, 7 }
};


static void
dxt1_rgb_quantize (dword *cc, const byte *lines[], int comps)
{
    float b, iv[MAX_COMP];   /* interpolation vector */

    dword hi; /* high doubleword */
    int color0, color1;
    int n_vect;
    const int n_comp = 3;
    int black = 0;

    int minSum = 2000; /* big enough */
    int maxSum = -1; /* small enough */
    int minCol = 0; /* phoudoin: silent compiler! */
    int maxCol = 0; /* phoudoin: silent compiler! */

    byte input[N_TEXELS][MAX_COMP];
    int i, k, l;

    /* make the whole block opaque */
    /* we will NEVER reference ACOMP of any pixel */

    /* 4 texels each line */
    for (l = 0; l < 4; l++) {
    for (k = 0; k < 4; k++) {
        for (i = 0; i < comps; i++) {
        input[k + l * 4][i] = *lines[l]++;
        }
    }
    }

    /* Our solution here is to find the darkest and brightest colors in
     * the 4x4 tile and use those as the two representative colors.
     * There are probably better algorithms to use (histogram-based).
     */
    for (k = 0; k < N_TEXELS; k++) {
    int sum = 0;
    for (i = 0; i < n_comp; i++) {
        sum += input[k][i];
    }
    if (minSum > sum) {
        minSum = sum;
        minCol = k;
    }
    if (maxSum < sum) {
        maxSum = sum;
        maxCol = k;
    }
    if (sum == 0) {
        black = 1;
    }
    }

    color0 = COLOR565(input[minCol]);
    color1 = COLOR565(input[maxCol]);

    if (color0 == color1) {
    /* we'll use 3-vector */
    cc[0] = color0 | (color1 << 16);
    hi = black ? -1 : 0;
    } else {
    if (black && ((color0 == 0) || (color1 == 0))) {
        /* we still can use 4-vector */
        black = 0;
    }

    if (black ^ (color0 <= color1)) {
        int aux;
        aux = color0;
        color0 = color1;
        color1 = aux;
        aux = minCol;
        minCol = maxCol;
        maxCol = aux;
    }
    n_vect = (color0 <= color1) ? 2 : 3;

    MAKEIVEC(n_vect, n_comp, iv, b, input[minCol], input[maxCol]);

    /* add in texels */
    cc[0] = color0 | (color1 << 16);
    hi = 0;
    for (k = N_TEXELS - 1; k >= 0; k--) {
        int texel = 3;
        int sum = 0;
        if (black) {
        for (i = 0; i < n_comp; i++) {
            sum += input[k][i];
        }
        }
        if (!black || sum) {
        /* interpolate color */
        CALCCDOT(texel, n_vect, n_comp, iv, b, input[k]);
        texel = dxtn_color_tlat[black][texel];
        }
        /* add in texel */
        hi <<= 2;
        hi |= texel;
    }
    }
    cc[1] = hi;
}


static void
dxt1_rgba_quantize (dword *cc, const byte *lines[], int comps)
{
    float b, iv[MAX_COMP];  /* interpolation vector */

    dword hi;       /* high doubleword */
    int color0, color1;
    int n_vect;
    const int n_comp = 3;
    int transparent = 0;

    int minSum = 2000;      /* big enough */
    int maxSum = -1;        /* small enough */
    int minCol = 0;     /* phoudoin: silent compiler! */
    int maxCol = 0;     /* phoudoin: silent compiler! */

    byte input[N_TEXELS][MAX_COMP];
    int i, k, l;

    if (comps == 3) {
    /* make the whole block opaque */
    memset(input, -1, sizeof(input));
    }

    /* 4 texels each line */
    for (l = 0; l < 4; l++) {
    for (k = 0; k < 4; k++) {
        for (i = 0; i < comps; i++) {
        input[k + l * 4][i] = *lines[l]++;
        }
    }
    }

    /* Our solution here is to find the darkest and brightest colors in
     * the 4x4 tile and use those as the two representative colors.
     * There are probably better algorithms to use (histogram-based).
     */
    for (k = 0; k < N_TEXELS; k++) {
    int sum = 0;
    for (i = 0; i < n_comp; i++) {
        sum += input[k][i];
    }
    if (minSum > sum) {
        minSum = sum;
        minCol = k;
    }
    if (maxSum < sum) {
        maxSum = sum;
        maxCol = k;
    }
    if (input[k][ACOMP] < 128) {
        transparent = 1;
    }
    }

    color0 = COLOR565(input[minCol]);
    color1 = COLOR565(input[maxCol]);

    if (color0 == color1) {
    /* we'll use 3-vector */
    cc[0] = color0 | (color1 << 16);
    hi = transparent ? -1 : 0;
    } else {
    if (transparent ^ (color0 <= color1)) {
        int aux;
        aux = color0;
        color0 = color1;
        color1 = aux;
        aux = minCol;
        minCol = maxCol;
        maxCol = aux;
    }
    n_vect = (color0 <= color1) ? 2 : 3;

    MAKEIVEC(n_vect, n_comp, iv, b, input[minCol], input[maxCol]);

    /* add in texels */
    cc[0] = color0 | (color1 << 16);
    hi = 0;
    for (k = N_TEXELS - 1; k >= 0; k--) {
        int texel = 3;
        if (input[k][ACOMP] >= 128) {
        /* interpolate color */
        CALCCDOT(texel, n_vect, n_comp, iv, b, input[k]);
        texel = dxtn_color_tlat[transparent][texel];
        }
        /* add in texel */
        hi <<= 2;
        hi |= texel;
    }
    }
    cc[1] = hi;
}


static void
dxt3_rgba_quantize (dword *cc, const byte *lines[], int comps)
{
    float b, iv[MAX_COMP];  /* interpolation vector */

    dword lolo, lohi;   /* low quadword: lo dword, hi dword */
    dword hihi;     /* high quadword: high dword */
    int color0, color1;
    const int n_vect = 3;
    const int n_comp = 3;

    int minSum = 2000;      /* big enough */
    int maxSum = -1;        /* small enough */
    int minCol = 0;     /* phoudoin: silent compiler! */
    int maxCol = 0;     /* phoudoin: silent compiler! */

    byte input[N_TEXELS][MAX_COMP];
    int i, k, l;

    if (comps == 3) {
    /* make the whole block opaque */
    memset(input, -1, sizeof(input));
    }

    /* 4 texels each line */
    for (l = 0; l < 4; l++) {
    for (k = 0; k < 4; k++) {
        for (i = 0; i < comps; i++) {
        input[k + l * 4][i] = *lines[l]++;
        }
    }
    }

    /* Our solution here is to find the darkest and brightest colors in
     * the 4x4 tile and use those as the two representative colors.
     * There are probably better algorithms to use (histogram-based).
     */
    for (k = 0; k < N_TEXELS; k++) {
    int sum = 0;
    for (i = 0; i < n_comp; i++) {
        sum += input[k][i];
    }
    if (minSum > sum) {
        minSum = sum;
        minCol = k;
    }
    if (maxSum < sum) {
        maxSum = sum;
        maxCol = k;
    }
    }

    /* add in alphas */
    lolo = lohi = 0;
    for (k = N_TEXELS - 1; k >= N_TEXELS / 2; k--) {
    /* add in alpha */
    lohi <<= 4;
    lohi |= input[k][ACOMP] >> 4;
    }
    cc[1] = lohi;
    for (; k >= 0; k--) {
    /* add in alpha */
    lolo <<= 4;
    lolo |= input[k][ACOMP] >> 4;
    }
    cc[0] = lolo;

    color0 = COLOR565(input[minCol]);
    color1 = COLOR565(input[maxCol]);
    cc[2] = color0 | (color1 << 16);

    hihi = 0;
    if (color0 != color1) {
    MAKEIVEC(n_vect, n_comp, iv, b, input[minCol], input[maxCol]);

    /* add in texels */
    for (k = N_TEXELS - 1; k >= 0; k--) {
        int texel;
        /* interpolate color */
        CALCCDOT(texel, n_vect, n_comp, iv, b, input[k]);
        texel = dxtn_color_tlat[0][texel];
        /* add in texel */
        hihi <<= 2;
        hihi |= texel;
    }
    }
    cc[3] = hihi;
}


static void
dxt5_rgba_quantize (dword *cc, const byte *lines[], int comps)
{
    float b, iv[MAX_COMP];  /* interpolation vector */

    qword lo;           /* low quadword */
    dword hihi;     /* high quadword: high dword */
    int color0, color1;
    const int n_vect = 3;
    const int n_comp = 3;

    int minSum = 2000;      /* big enough */
    int maxSum = -1;        /* small enough */
    int minCol = 0;     /* phoudoin: silent compiler! */
    int maxCol = 0;     /* phoudoin: silent compiler! */
    int alpha0 = 2000;      /* big enough */
    int alpha1 = -1;        /* small enough */
    int anyZero = 0, anyOne = 0;
    int a_vect;

    byte input[N_TEXELS][MAX_COMP];
    int i, k, l;

    if (comps == 3) {
    /* make the whole block opaque */
    memset(input, -1, sizeof(input));
    }

    /* 4 texels each line */
    for (l = 0; l < 4; l++) {
    for (k = 0; k < 4; k++) {
        for (i = 0; i < comps; i++) {
        input[k + l * 4][i] = *lines[l]++;
        }
    }
    }

    /* Our solution here is to find the darkest and brightest colors in
     * the 4x4 tile and use those as the two representative colors.
     * There are probably better algorithms to use (histogram-based).
     */
    for (k = 0; k < N_TEXELS; k++) {
    int sum = 0;
    for (i = 0; i < n_comp; i++) {
        sum += input[k][i];
    }
    if (minSum > sum) {
        minSum = sum;
        minCol = k;
    }
    if (maxSum < sum) {
        maxSum = sum;
        maxCol = k;
    }
    if (alpha0 > input[k][ACOMP]) {
        alpha0 = input[k][ACOMP];
    }
    if (alpha1 < input[k][ACOMP]) {
        alpha1 = input[k][ACOMP];
    }
    if (input[k][ACOMP] == 0) {
        anyZero = 1;
    }
    if (input[k][ACOMP] == 255) {
        anyOne = 1;
    }
    }

    /* add in alphas */
    if (alpha0 == alpha1) {
    /* we'll use 6-vector */
    cc[0] = alpha0 | (alpha1 << 8);
    cc[1] = 0;
    } else {
    if (anyZero && ((alpha0 == 0) || (alpha1 == 0))) {
        /* we still might use 8-vector */
        anyZero = 0;
    }
    if (anyOne && ((alpha0 == 255) || (alpha1 == 255))) {
        /* we still might use 8-vector */
        anyOne = 0;
    }
    if ((anyZero | anyOne) ^ (alpha0 <= alpha1)) {
        int aux;
        aux = alpha0;
        alpha0 = alpha1;
        alpha1 = aux;
    }
    a_vect = (alpha0 <= alpha1) ? 5 : 7;

    /* compute interpolation vector */
    iv[ACOMP] = (float)a_vect / (alpha1 - alpha0);
    b = -iv[ACOMP] * alpha0 + 0.5F;

    /* add in alphas */
    Q_MOV32(lo, 0);
    for (k = N_TEXELS - 1; k >= 0; k--) {
        int texel = -1;
        if (anyZero | anyOne) {
        if (input[k][ACOMP] == 0) {
            texel = 6;
        } else if (input[k][ACOMP] == 255) {
            texel = 7;
        }
        }
        /* interpolate alpha */
        if (texel == -1) {
        float dot = input[k][ACOMP] * iv[ACOMP];
        texel = (int)(dot + b);
#if SAFECDOT
        if (texel < 0) {
            texel = 0;
        } else if (texel > a_vect) {
            texel = a_vect;
        }
#endif
        texel = dxtn_alpha_tlat[anyZero | anyOne][texel];
        }
        /* add in texel */
        Q_SHL(lo, 3);
        Q_OR32(lo, texel);
    }
    Q_SHL(lo, 16);
    Q_OR32(lo, alpha0 | (alpha1 << 8));
    ((qword *)cc)[0] = lo;
    }

    color0 = COLOR565(input[minCol]);
    color1 = COLOR565(input[maxCol]);
    cc[2] = color0 | (color1 << 16);

    hihi = 0;
    if (color0 != color1) {
    MAKEIVEC(n_vect, n_comp, iv, b, input[minCol], input[maxCol]);

    /* add in texels */
    for (k = N_TEXELS - 1; k >= 0; k--) {
        int texel;
        /* interpolate color */
        CALCCDOT(texel, n_vect, n_comp, iv, b, input[k]);
        texel = dxtn_color_tlat[0][texel];
        /* add in texel */
        hihi <<= 2;
        hihi |= texel;
    }
    }
    cc[3] = hihi;
}


#define ENCODER(dxtn, n)                        \
int TAPIENTRY                               \
dxtn##_encode (int width, int height, int comps,            \
           const void *source, int srcRowStride,            \
           void *dest, int destRowStride)               \
{                                   \
    int x, y;                               \
    const byte *data;                           \
    dword *encoded = (dword *)dest;                 \
    void *newSource = NULL;                     \
                                    \
    /* Replicate image if width is not M4 or height is not M4 */    \
    if ((width & 3) | (height & 3)) {                   \
    int newWidth = (width + 3) & ~3;                \
    int newHeight = (height + 3) & ~3;              \
    newSource = malloc(comps * newWidth * newHeight * sizeof(byte *));\
    _mesa_upscale_teximage2d(width, height, newWidth, newHeight,    \
                               comps, (const byte *)source,     \
                   srcRowStride, (byte *)newSource);    \
    source = newSource;                     \
    width = newWidth;                       \
    height = newHeight;                     \
    srcRowStride = comps * newWidth;                \
    }                                   \
                                    \
    data = (const byte *)source;                    \
    destRowStride = (destRowStride - width * n) / 4;            \
    for (y = 0; y < height; y += 4) {                   \
    unsigned int offs = 0 + (y + 0) * srcRowStride;         \
    for (x = 0; x < width; x += 4) {                \
        const byte *lines[4];                   \
        lines[0] = &data[offs];                 \
        lines[1] = lines[0] + srcRowStride;             \
        lines[2] = lines[1] + srcRowStride;             \
        lines[3] = lines[2] + srcRowStride;             \
        offs += 4 * comps;                      \
        dxtn##_quantize(encoded, lines, comps);         \
        /* 4x4 block */                     \
        encoded += n;                       \
    }                               \
    encoded += destRowStride;                   \
    }                                   \
                                    \
    if (newSource != NULL) {                        \
    free(newSource);                        \
    }                                   \
                                    \
    return 0;                               \
}

ENCODER(dxt1_rgb,  2)
ENCODER(dxt1_rgba, 2)
ENCODER(dxt3_rgba, 4)
ENCODER(dxt5_rgba, 4)


/***************************************************************************\
 * DXTn decoder
 *
 * The decoder is based on GL_EXT_texture_compression_s3tc
 * specification and serves as a concept for the encoder.
\***************************************************************************/


/* lookup table for scaling 4 bit colors up to 8 bits */
static const byte _rgb_scale_4[] = {
    0,   17,  34,  51,  68,  85,  102, 119,
    136, 153, 170, 187, 204, 221, 238, 255
};

/* lookup table for scaling 5 bit colors up to 8 bits */
static const byte _rgb_scale_5[] = {
    0,   8,   16,  25,  33,  41,  49,  58,
    66,  74,  82,  90,  99,  107, 115, 123,
    132, 140, 148, 156, 165, 173, 181, 189,
    197, 206, 214, 222, 230, 239, 247, 255
};

/* lookup table for scaling 6 bit colors up to 8 bits */
static const byte _rgb_scale_6[] = {
    0,   4,   8,   12,  16,  20,  24,  28,
    32,  36,  40,  45,  49,  53,  57,  61,
    65,  69,  73,  77,  81,  85,  89,  93,
    97,  101, 105, 109, 113, 117, 121, 125,
    130, 134, 138, 142, 146, 150, 154, 158,
    162, 166, 170, 174, 178, 182, 186, 190,
    194, 198, 202, 206, 210, 215, 219, 223,
    227, 231, 235, 239, 243, 247, 251, 255
};


#define CC_SEL(cc, which) (((dword *)(cc))[(which) / 32] >> ((which) & 31))
#define UP4(c) _rgb_scale_4[(c) & 15]
#define UP5(c) _rgb_scale_5[(c) & 31]
#define UP6(c) _rgb_scale_6[(c) & 63]
#define ZERO_4UBV(v) *((dword *)(v)) = 0


void TAPIENTRY
dxt1_rgb_decode_1 (const void *texture, int stride,
           int i, int j, byte *rgba)
{
    const byte *src = (const byte *)texture
               + ((j / 4) * ((stride + 3) / 4) + i / 4) * 8;
    const int code = (src[4 + (j & 3)] >> ((i & 3) * 2)) & 0x3;
    if (code == 0) {
    rgba[RCOMP] = UP5(CC_SEL(src, 11));
    rgba[GCOMP] = UP6(CC_SEL(src,  5));
    rgba[BCOMP] = UP5(CC_SEL(src,  0));
    } else if (code == 1) {
    rgba[RCOMP] = UP5(CC_SEL(src, 27));
    rgba[GCOMP] = UP6(CC_SEL(src, 21));
    rgba[BCOMP] = UP5(CC_SEL(src, 16));
    } else {
    const word col0 = src[0] | (src[1] << 8);
    const word col1 = src[2] | (src[3] << 8);
    if (col0 > col1) {
        if (code == 2) {
        rgba[RCOMP] = (UP5(col0 >> 11) * 2 + UP5(col1 >> 11)) / 3;
        rgba[GCOMP] = (UP6(col0 >>  5) * 2 + UP6(col1 >>  5)) / 3;
        rgba[BCOMP] = (UP5(col0      ) * 2 + UP5(col1      )) / 3;
        } else {
        rgba[RCOMP] = (UP5(col0 >> 11) + 2 * UP5(col1 >> 11)) / 3;
        rgba[GCOMP] = (UP6(col0 >>  5) + 2 * UP6(col1 >>  5)) / 3;
        rgba[BCOMP] = (UP5(col0      ) + 2 * UP5(col1      )) / 3;
        }
    } else {
        if (code == 2) {
        rgba[RCOMP] = (UP5(col0 >> 11) + UP5(col1 >> 11)) / 2;
        rgba[GCOMP] = (UP6(col0 >>  5) + UP6(col1 >>  5)) / 2;
        rgba[BCOMP] = (UP5(col0      ) + UP5(col1      )) / 2;
        } else {
        ZERO_4UBV(rgba);
        }
    }
    }
    rgba[ACOMP] = 255;
}


void TAPIENTRY
dxt1_rgba_decode_1 (const void *texture, int stride,
            int i, int j, byte *rgba)
{
    /* Same as rgb_dxt1 above, except alpha=0 if col0<=col1 and code=3. */
    const byte *src = (const byte *)texture
               + ((j / 4) * ((stride + 3) / 4) + i / 4) * 8;
    const int code = (src[4 + (j & 3)] >> ((i & 3) * 2)) & 0x3;
    if (code == 0) {
    rgba[RCOMP] = UP5(CC_SEL(src, 11));
    rgba[GCOMP] = UP6(CC_SEL(src,  5));
    rgba[BCOMP] = UP5(CC_SEL(src,  0));
    rgba[ACOMP] = 255;
    } else if (code == 1) {
    rgba[RCOMP] = UP5(CC_SEL(src, 27));
    rgba[GCOMP] = UP6(CC_SEL(src, 21));
    rgba[BCOMP] = UP5(CC_SEL(src, 16));
    rgba[ACOMP] = 255;
    } else {
    const word col0 = src[0] | (src[1] << 8);
    const word col1 = src[2] | (src[3] << 8);
    if (col0 > col1) {
        if (code == 2) {
        rgba[RCOMP] = (UP5(col0 >> 11) * 2 + UP5(col1 >> 11)) / 3;
        rgba[GCOMP] = (UP6(col0 >>  5) * 2 + UP6(col1 >>  5)) / 3;
        rgba[BCOMP] = (UP5(col0      ) * 2 + UP5(col1      )) / 3;
        } else {
        rgba[RCOMP] = (UP5(col0 >> 11) + 2 * UP5(col1 >> 11)) / 3;
        rgba[GCOMP] = (UP6(col0 >>  5) + 2 * UP6(col1 >>  5)) / 3;
        rgba[BCOMP] = (UP5(col0      ) + 2 * UP5(col1      )) / 3;
        }
        rgba[ACOMP] = 255;
    } else {
        if (code == 2) {
        rgba[RCOMP] = (UP5(col0 >> 11) + UP5(col1 >> 11)) / 2;
        rgba[GCOMP] = (UP6(col0 >>  5) + UP6(col1 >>  5)) / 2;
        rgba[BCOMP] = (UP5(col0      ) + UP5(col1      )) / 2;
        rgba[ACOMP] = 255;
        } else {
        ZERO_4UBV(rgba);
        }
    }
    }
}


void TAPIENTRY
dxt3_rgba_decode_1 (const void *texture, int stride,
            int i, int j, byte *rgba)
{
    const byte *src = (const byte *)texture
               + ((j / 4) * ((stride + 3) / 4) + i / 4) * 16;
    const int code = (src[12 + (j & 3)] >> ((i & 3) * 2)) & 0x3;
    const dword *cc = (const dword *)(src + 8);
    if (code == 0) {
    rgba[RCOMP] = UP5(CC_SEL(cc, 11));
    rgba[GCOMP] = UP6(CC_SEL(cc,  5));
    rgba[BCOMP] = UP5(CC_SEL(cc,  0));
    } else if (code == 1) {
    rgba[RCOMP] = UP5(CC_SEL(cc, 27));
    rgba[GCOMP] = UP6(CC_SEL(cc, 21));
    rgba[BCOMP] = UP5(CC_SEL(cc, 16));
    } else if (code == 2) {
    /* (col0 * (4 - code) + col1 * (code - 1)) / 3 */
    rgba[RCOMP] = (UP5(CC_SEL(cc, 11)) * 2 + UP5(CC_SEL(cc, 27))) / 3;
    rgba[GCOMP] = (UP6(CC_SEL(cc,  5)) * 2 + UP6(CC_SEL(cc, 21))) / 3;
    rgba[BCOMP] = (UP5(CC_SEL(cc,  0)) * 2 + UP5(CC_SEL(cc, 16))) / 3;
    } else {
    rgba[RCOMP] = (UP5(CC_SEL(cc, 11)) + 2 * UP5(CC_SEL(cc, 27))) / 3;
    rgba[GCOMP] = (UP6(CC_SEL(cc,  5)) + 2 * UP6(CC_SEL(cc, 21))) / 3;
    rgba[BCOMP] = (UP5(CC_SEL(cc,  0)) + 2 * UP5(CC_SEL(cc, 16))) / 3;
    }
    rgba[ACOMP] = UP4(src[((j & 3) * 4 + (i & 3)) / 2] >> ((i & 1) * 4));
}


void TAPIENTRY
dxt5_rgba_decode_1 (const void *texture, int stride,
            int i, int j, byte *rgba)
{
    const byte *src = (const byte *)texture
               + ((j / 4) * ((stride + 3) / 4) + i / 4) * 16;
    const int code = (src[12 + (j & 3)] >> ((i & 3) * 2)) & 0x3;
    const dword *cc = (const dword *)(src + 8);
    const byte alpha0 = src[0];
    const byte alpha1 = src[1];
    const int alphaShift = (((j & 3) * 4) + (i & 3)) * 3 + 16;
    const int acode = ((alphaShift == 31)
            ? CC_SEL(src + 2, alphaShift - 16)
            : CC_SEL(src, alphaShift)) & 0x7;
    if (code == 0) {
    rgba[RCOMP] = UP5(CC_SEL(cc, 11));
    rgba[GCOMP] = UP6(CC_SEL(cc,  5));
    rgba[BCOMP] = UP5(CC_SEL(cc,  0));
    } else if (code == 1) {
    rgba[RCOMP] = UP5(CC_SEL(cc, 27));
    rgba[GCOMP] = UP6(CC_SEL(cc, 21));
    rgba[BCOMP] = UP5(CC_SEL(cc, 16));
    } else if (code == 2) {
    /* (col0 * (4 - code) + col1 * (code - 1)) / 3 */
    rgba[RCOMP] = (UP5(CC_SEL(cc, 11)) * 2 + UP5(CC_SEL(cc, 27))) / 3;
    rgba[GCOMP] = (UP6(CC_SEL(cc,  5)) * 2 + UP6(CC_SEL(cc, 21))) / 3;
    rgba[BCOMP] = (UP5(CC_SEL(cc,  0)) * 2 + UP5(CC_SEL(cc, 16))) / 3;
    } else {
    rgba[RCOMP] = (UP5(CC_SEL(cc, 11)) + 2 * UP5(CC_SEL(cc, 27))) / 3;
    rgba[GCOMP] = (UP6(CC_SEL(cc,  5)) + 2 * UP6(CC_SEL(cc, 21))) / 3;
    rgba[BCOMP] = (UP5(CC_SEL(cc,  0)) + 2 * UP5(CC_SEL(cc, 16))) / 3;
    }
    if (acode == 0) {
    rgba[ACOMP] = alpha0;
    } else if (acode == 1) {
    rgba[ACOMP] = alpha1;
    } else if (alpha0 > alpha1) {
    rgba[ACOMP] = ((8 - acode) * alpha0 + (acode - 1) * alpha1) / 7;
    } else if (acode == 6) {
    rgba[ACOMP] = 0;
    } else if (acode == 7) {
    rgba[ACOMP] = 255;
    } else {
    rgba[ACOMP] = ((6 - acode) * alpha0 + (acode - 1) * alpha1) / 5;
    }
}