jpeglib画质的代码分析

    int scale_factor = quality;
    /* Safety limit on quality factor.  Convert 0 to 1 to avoid zero divide. */
    if (scale_factor <= 0)
    {
        scale_factor = 1;
    }
    else if (scale_factor > 100)
    {
        scale_factor = 100;
    }
    if (scale_factor < 50)
    {
        scale_factor = 5000 / scale_factor;
    }
    else
    {
        scale_factor = 200 - scale_factor*2;
    }

//force_baseline=TRUE
//DCTSIZE2=8*8
//scale_factor：质量，整数，0-100
//basic_table:std_luminance_quant_tbl和std_chrominance_quant_tbl
for (i = 0; i < DCTSIZE2; i++)
{
    //原来代码是long，并无必要。这里的50，是指默认表质量为50的意思
    int temp = ( basic_table[i] * scale_factor + 50) / 100;
    /* limit the values to the valid range */
    if (temp <= 0)
    {
        temp = 1;
    }
    else if (temp > 0x8FFFF)
    {
        //32767不如使用0x8FFF
        temp = 0x8FFFF; /* max quantizer needed for 12 bits */
    }
    if (force_baseline && temp > 0xFF)
    {
        //255L不如使用0xFF
        temp = 0xFF;    /* limit to baseline range if requested */
    }
    (*qtblptr)->quantval[i] = (UINT16) temp;
}

/* These are the sample quantization tables given in JPEG spec section K.1.
 * The spec says that the values given produce "good" quality, and
 * when divided by 2, "very good" quality.
 */
static const unsigned int std_luminance_quant_tbl[DCTSIZE2] = {
  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
};
static const unsigned int std_chrominance_quant_tbl[DCTSIZE2] = {
  17,  18,  24,  47,  99,  99,  99,  99,
  18,  21,  26,  66,  99,  99,  99,  99,
  24,  26,  56,  99,  99,  99,  99,  99,
  47,  66,  99,  99,  99,  99,  99,  99,
  99,  99,  99,  99,  99,  99,  99,  99,
  99,  99,  99,  99,  99,  99,  99,  99,
  99,  99,  99,  99,  99,  99,  99,  99,
  99,  99,  99,  99,  99,  99,  99,  99
};