jpeg压缩源代码阅读笔记

1.将图像分为若干个8*8的块

2.对于每个块中的每个值(YCbCr?)转换为整型，并且减去平均值128，再进行DCT运算

temp = 0.0;
for (u = 0; u < 8; u++)
    for (v = 0; v < 8; v++)
    {
        temp = 0.0;
        for (x = 0; x < 8; x++)
            for (y = 0; y < 8; y++)
                temp += (double)inblock[x][y] * cos((2 * x + 1)*u*pi / 16)*cos((2 * y + 1)*v*pi / 16);
        adct[u][v] = temp * c[u] * c[v] / 4;
    }

我的优化：

            double temps[8][8];
            for (int i = 0; i < 8; i++) {
                for (int j = 0; j < 8; j++) {
                    temps[i][j] = cos((2 * i + 1)*j*pi / 16);
                }
            }
            temp = 0.0;
            for (u = 0; u < 8; u++)
                for (v = 0; v < 8; v++)
                {
                    temp = 0.0;
                    for (x = 0; x < 8; x++)
                        for (y = 0; y < 8; y++)
                            temp += (double)inblock[x][y] * temps[x][u] * temps[y][v];
                    adct[u][v] = temp * c[u] * c[v] / 4;
                }

3.量化块，将块中每个值除以给定的量化值表中对应的值

//给定的量化值表，可以对其缩放/扩大进行压缩图像质量的控制

int qtbl[8][8] = { 16,11,10,16,24,40,51,61,
12,12,14,19,26,58,60,55,
14,13,16,24,40,57,69,56,
14,17,22,29,51,87,80,62,
18,22,37,56,68,109,103,77,
24,35,55,64,81,104,113,92,
49,64,78,87,103,121,120,101,
72,92,95,98,112,100,103,99 };


for (u = 0; u < 8; u++){
  for (v = 0; v < 8; v++)
  {
          temp1 = adct[u][v];
          if (temp1 < 0)
          {
            temp1 = -temp1;
            temp1 += qtbl[u][v] / 2;
            temp2 = (int)temp1 / qtbl[u][v];
            aqant[u][v] = -temp2;
          }
          else
          {
             temp1 += qtbl[u][v] / 2;
             temp2 = (int)temp1 / qtbl[u][v];
             aqant[u][v] = temp2;
          }
   }
}

4.将块改为zigzag顺序

short int zigzag[8][8] = { 0, 1, 5, 6,14,15,27,28,
                          2, 4, 7,13,16,26,29,42,
                          3, 8,12,17,25,30,41,43,
                          9,11,18,24,31,40,44,53,
                         10,19,23,32,39,45,52,54,
                         20,22,33,38,46,51,55,60,
                         21,34,37,47,50,56,59,61,
                         35,36,48,49,57,58,62,63 };
/* change the 8x8 block to zigzag sequence form  ZIGZAG扫描*/
for (u = 0; u < 8; u++)
    for (v = 0; v < 8; v++)
        zz[zigzag[u][v]] = aqant[u][v];

5.DC值减去最后一个块的DC之差

            /* replace DC value of each block as the difference between curent DC and the DC of last block */
            tempdc = zz[0];
            zz[0] = zz[0] - last_dc_value;
            last_dc_value = tempdc;