DXT1-3-5 decoders

// 4 bit alpha
function setRgba8888Dxt3(dst, i, int565, a) {
  dst[i] = Math.floor(((int565 >> 11) & 31) / 31 * 255);
  dst[i + 1] = Math.floor(((int565 >> 5) & 63) / 63 * 255);
  dst[i + 2] = Math.floor((int565 & 31) / 31 * 255);
  dst[i + 3] = Math.floor((a / 15) * 255);
}

// 8 bit alpha
function setRgba8888Dxt5(dst, i, int565, a) {
  dst[i] = Math.floor(((int565 >> 11) & 31) / 31 * 255);
  dst[i + 1] = Math.floor(((int565 >> 5) & 63) / 63 * 255);
  dst[i + 2] = Math.floor((int565 & 31) / 31 * 255);
  dst[i + 3] = Math.floor(a);
}

function dxt1ToRgb565(src, width, height) {
  var c = new Uint16Array(4);
  var dst = new Uint16Array(width * height);
  var m;
  var dstI;
  var i ;
  var r0, g0, b0, r1, g1, b1;
  var blockWidth = width / 4;
  var blockHeight = height / 4;
  
  for (var blockY = 0; blockY < blockHeight; blockY++) {
    for (var blockX = 0; blockX < blockWidth; blockX++) {
      i = 4 * (blockY * blockWidth + blockX);
      c[0] = src[i];
      c[1] = src[i + 1];
      r0 = c[0] & 0x1f;
      g0 = c[0] & 0x7e0;
      b0 = c[0] & 0xf800;
      r1 = c[1] & 0x1f;
      g1 = c[1] & 0x7e0;
      b1 = c[1] & 0xf800;
      
      if (c[0] > c[1]) {
        c[2] = ((5 * r0 + 3 * r1) >> 3) | (((5 * g0 + 3 * g1) >> 3) & 0x7e0) | (((5 * b0 + 3 * b1) >> 3) & 0xf800);
        c[3] = ((5 * r1 + 3 * r0) >> 3) | (((5 * g1 + 3 * g0) >> 3) & 0x7e0) | (((5 * b1 + 3 * b0) >> 3) & 0xf800);
      } else {
        c[2] = (c[0] + c[1]) >> 1;
        c[3] = 0;
      }
      
      m = src[i + 2];
      dstI = (blockY * 4) * width + blockX * 4;
      dst[dstI] = c[m & 0x3];
      dst[dstI + 1] = c[(m >> 2) & 0x3];
      dst[dstI + 2] = c[(m >> 4) & 0x3];
      dst[dstI + 3] = c[(m >> 6) & 0x3];
      dstI += width;
      dst[dstI] = c[(m >> 8) & 0x3];
      dst[dstI + 1] = c[(m >> 10) & 0x3];
      dst[dstI + 2] = c[(m >> 12) & 0x3];
      dst[dstI + 3] = c[(m >> 14)];
      m = src[i + 3];
      dstI += width;
      dst[dstI] = c[m & 0x3];
      dst[dstI + 1] = c[(m >> 2) & 0x3];
      dst[dstI + 2] = c[(m >> 4) & 0x3];
      dst[dstI + 3] = c[(m >> 6) & 0x3];
      dstI += width;
      dst[dstI] = c[(m >> 8) & 0x3];
      dst[dstI + 1] = c[(m >> 10) & 0x3];
      dst[dstI + 2] = c[(m >> 12) & 0x3];
      dst[dstI + 3] = c[(m >> 14)];
    }
  }
  
  return dst;
}
  
function dxt3ToRgba8888(src, width, height) {
  var c = new Uint16Array(4);
  var dst = new Uint8Array(width * height * 4);
  var m;
  var a;
  var dstI;
  var i ;
  var r0, g0, b0, r1, g1, b1;
  var blockWidth = width / 4;
  var blockHeight = height / 4;
  
  for (var blockY = 0; blockY < blockHeight; blockY++) {
    for (var blockX = 0; blockX < blockWidth; blockX++) {
      i = 8 * (blockY * blockWidth + blockX);
      c[0] = src[i + 4];
      c[1] = src[i + 5];
      r0 = c[0] & 0x1f;
      g0 = c[0] & 0x7e0;
      b0 = c[0] & 0xf800;
      r1 = c[1] & 0x1f;
      g1 = c[1] & 0x7e0;
      b1 = c[1] & 0xf800;
      c[2] = ((5 * r0 + 3 * r1) >> 3) | (((5 * g0 + 3 * g1) >> 3) & 0x7e0) | (((5 * b0 + 3 * b1) >> 3) & 0xf800);
      c[3] = ((5 * r1 + 3 * r0) >> 3) | (((5 * g1 + 3 * g0) >> 3) & 0x7e0) | (((5 * b1 + 3 * b0) >> 3) & 0xf800);
      
      m = src[i + 6];
      a = src[i];
      dstI = (blockY * 16) * width + blockX * 16;
      setRgba8888Dxt3(dst, dstI, c[bits & 0x3], a & 0xf);
      setRgba8888Dxt3(dst, dstI + 4, c[(bits >> 2) & 0x3], (a >> 4) & 0xf);
      setRgba8888Dxt3(dst, dstI + 8, c[(bits >> 4) & 0x3], (a >> 8) & 0xf);
      setRgba8888Dxt3(dst, dstI + 12, c[(bits >> 6) & 0x3], (a >> 12) & 0xf);
      dstI += width * 4;
      a = src[i + 1];
      setRgba8888Dxt3(dst, dstI, c[(bits >> 8) & 0x3], a & 0xf);
      setRgba8888Dxt3(dst, dstI + 4, c[(bits >> 10) & 0x3], (a >> 4) & 0xf);
      setRgba8888Dxt3(dst, dstI + 8, c[(bits >> 12) & 0x3], (a >> 8) & 0xf);
      setRgba8888Dxt3(dst, dstI + 12, c[bits >> 14], (a >> 12) & 0xf);
      m = src[i + 7];
      a = src[i + 2];
      dstI += width * 4;
      setRgba8888Dxt3(dst, dstI, c[bits & 0x3], a & 0xf);
      setRgba8888Dxt3(dst, dstI + 4, c[(bits >> 2) & 0x3], (a >> 4) & 0xf);
      setRgba8888Dxt3(dst, dstI + 8, c[(bits >> 4) & 0x3], (a >> 8) & 0xf);
      setRgba8888Dxt3(dst, dstI + 12, c[(bits >> 6) & 0x3], (a >> 12) & 0xf);
      dstI += width * 4;
      a = src[i + 3];
      setRgba8888Dxt3(dst, dstI, c[(bits >> 8) & 0x3], a & 0xf);
      setRgba8888Dxt3(dst, dstI + 4, c[(bits >> 10) & 0x3], (a >> 4) & 0xf);
      setRgba8888Dxt3(dst, dstI + 8, c[(bits >> 12) & 0x3], (a >> 8) & 0xf);
      setRgba8888Dxt3(dst, dstI + 12, c[bits >> 14], (a >> 12) & 0xf);
    }
  }
  
  return dst;
}

function dxt5ToRgba8888(src, width, height) {
  var c = new Uint16Array(4);
  var a = new Uint8Array(8);
  var alphaBits;
  var dst = new Uint8Array(width * height * 4);
  var m;
  var a;
  var dstI;
  var i ;
  var r0, g0, b0, r1, g1, b1;
  var blockWidth = width / 4;
  var blockHeight = height / 4;
  
  for (var blockY = 0; blockY < blockHeight; blockY++) {
    for (var blockX = 0; blockX < blockWidth; blockX++) {
      i = 8 * (blockY * blockWidth + blockX);
      c[0] = src[i + 4];
      c[1] = src[i + 5];
      r0 = c[0] & 0x1f;
      g0 = c[0] & 0x7e0;
      b0 = c[0] & 0xf800;
      r1 = c[1] & 0x1f;
      g1 = c[1] & 0x7e0;
      b1 = c[1] & 0xf800;
      c[2] = ((5 * r0 + 3 * r1) >> 3) | (((5 * g0 + 3 * g1) >> 3) & 0x7e0) | (((5 * b0 + 3 * b1) >> 3) & 0xf800);
      c[3] = ((5 * r1 + 3 * r0) >> 3) | (((5 * g1 + 3 * g0) >> 3) & 0x7e0) | (((5 * b1 + 3 * b0) >> 3) & 0xf800);
      alphaBits = src[i + 1] + 65536 * (src[i + 2] + 65536 * src[i + 3]);
      a[0] = src[i] & 0xff;
      a[1] = src[i]  >> 8;
      
      if (a[0] > a[1]) {
        a[2] = (6 * a[0] + a[1]) / 7;
        a[3] = (5 * a[0] + 2 * a[1]) / 7;
        a[4] = (4 * a[0] + 3 * a[1]) / 7;
        a[5] = (3 * a[0] + 4 * a[1]) / 7;
        a[6] = (2 * a[0] + 5 * a[1]) / 7;
        a[7] = (a[0] + 6 * a[1]) / 7;
      } else {
        a[2] = (4 * a[0] + a[1]) / 5;
        a[3] = (3 * a[0] + 2 * a[1]) / 5;
        a[4] = (2 * a[0] + 3 * a[1]) / 5;
        a[5] = (a[0] + 4 * a[1]) / 5;
        a[6] = 0;
        a[7] = 1;
      }
      
      m = src[i + 6];
      dstI = (blockY * 16) * width + blockX * 16;
      setRgba8888Dxt5(dst, dstI, c[m & 0x3], a[alphaBits & 0x7]);
      setRgba8888Dxt5(dst, dstI + 4, c[(m >> 2) & 0x3], a[(alphaBits >> 3) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 8, c[(m >> 4) & 0x3], a[(alphaBits >> 6) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 12, c[(m >> 6) & 0x3], a[(alphaBits >> 9) & 0x7]);
      dstI += width * 4;
      setRgba8888Dxt5(dst, dstI, c[(m >> 8) & 0x3], a[(alphaBits >> 12) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 4, c[(m >> 10) & 0x3], a[(alphaBits >> 15) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 8, c[(m >> 12) & 0x3], a[(alphaBits >> 18) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 12, c[m >> 14], a[(alphaBits >> 21) & 0x7]);
      m = src[i + 7];
      dstI += width * 4;
      setRgba8888Dxt5(dst, dstI, c[m & 0x3], a[(alphaBits >> 24) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 4, c[(m >> 2) & 0x3], a[(alphaBits >> 27) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 8, c[(m >> 4) & 0x3], a[(alphaBits >> 30) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 12, c[(m >> 6) & 0x3],a[(alphaBits >> 33) & 0x7]);
      dstI += width * 4;
      setRgba8888Dxt5(dst, dstI, c[(m >> 8) & 0x3], a[(alphaBits >> 36) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 4, c[(m >> 10) & 0x3], a[(alphaBits >> 39) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 8, c[(m >> 12) & 0x3], a[(alphaBits >> 42) & 0x7]);
      setRgba8888Dxt5(dst, dstI + 12, c[m >> 14], a[(alphaBits >> 45) & 0x7]);
    }
  }
  
  return dst;
}