C++ JPEG编码

依据上一篇的JPEG编码所得到的RGB信息，我们可以重新对RGB图片进行编码，也可对其他图片如BMP所得到的RGB信息进行编码,来得到*.jpg文件，注意我这里实现的JPEG编码不知道为啥编码出来的文件比原来大了好多。 还有要注意的地方，下面会着重写的

1.转换色彩空间

看了挺多博客，说是有不同的公式分别对应不同用途的图片。我这里使用的RGB转YCbCr的公式如下：

double y  = (0.299 * t.red + 0.587 * t.green + 0.114 * t.blue - 128);
double cb = (-0.1687 * t.red - 0.3313 * t.green + 0.500 * t.blue);
double cr = (0.500 * t.red - 0.4187 * t.green - 0.0813 * t.blue);

这里t的结构如下，如果要是还有Alpha这个值也可以添加进去

struct RGB{
	uint8_t red;
	uint8_t green;
	uint8_t blue;
};

2.离散余弦变化

使用矩阵乘法，转化那个DCT变换的公式，最后会得到 Y=AXA’ 这个公式A’是指A的转置，X是输入，Y是输出
矩阵A的获取方式如下：

double** JPEGData::createDCTAndIDCTArray(int row){
    double** res=new double*[row];
    for(int i=0;i<row;i++) res[i]=new double[row];
    for(int i=0;i<row;i++){
        for(int j=0;j<row;j++){
            double t=0;
            if(i==0) t=sqrt(1.0/row);
            else t=sqrt(2.0/row);
            res[i][j]=t*cos(M_PI*(j+0.5)*i/row);
        }
    }
    return res;
}

计算Y的方法如下：

void JPEGData::DCT(double** originMatrix){
    //原理 Y=A*X*A'
    vector<vector<double>> temp(ROW,vector<double>(COL,0));
    for(int i=0;i<ROW;i++){
        for(int j=0;j<COL;j++){
            double sum=0;
            for(int k=0;k<COL;k++){
                sum+=DCTAndIDCTArray[i][k]*originMatrix[k][j];
            }
            temp[i][j]=sum;
        }
    }
    for(int i=0;i<ROW;i++){
        for(int j=0;j<COL;j++){
            double sum=0;
            for(int k=0;k<COL;k++){
                sum+=temp[i][k]*DCTAndIDCTArray[j][k];
            }
            originMatrix[i][j]=sum;
        }
    }
}

3.zigzag编码

此编码的目的是为了使更多的0聚到一起，从而压缩文件大小（我打断点发现我这里0里面经常会出现1）。编码方式是将88的矩阵变成164的矩阵。

4.量化

我看到很多博客都没有详细说明，我这里说一下：
此步量化是对已经zigzag编码后的量化，就是对已经变成1×64的数组的变换，而对应的量化表如下，也是1×64的数组，对应项相乘即可

static uint8_t YQualityTable[64] = {
    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 uint8_t CQualityTable[64]={
	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
};

5.Huffman编码，（之前要差分编码和行程编码）

Huffman编码的直流和交流表如下：

static const uint8_t bits_dc_luminance[16] = {//亮度表
	0, 1, 5, 1, 1, 1, 1, 1, 
	1, 0, 0, 0, 0, 0, 0, 0 
};
static const uint8_t val_dc_luminance[] = { 
	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 
};

static const uint8_t bits_dc_chrominance[16] = {//色度表
	0, 3, 1, 1, 1, 1, 1, 1, 
	1, 1, 1, 0, 0, 0, 0, 0 
};
static const uint8_t val_dc_chrominance[] = { 
	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 
};

static const uint8_t bits_ac_luminance[16] = {
	0, 2, 1, 3, 3, 2, 4, 3, 
	5, 5, 4, 4, 0, 0, 1, 0x7d 
};
static const uint8_t val_ac_luminance[] = { 
	0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
	0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
	0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
	0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
	0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
	0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
	0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
	0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
	0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
	0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
	0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
	0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
	0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
	0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
	0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
	0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
	0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
	0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
	0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
	0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
	0xf9, 0xfa 
};

static const uint8_t bits_ac_chrominance[16] = {
	0, 2, 1, 2, 4, 4, 3, 4, 
	7, 5, 4, 4, 0, 1, 2, 0x77 
};
static const uint8_t val_ac_chrominance[] = { 
	0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
	0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
	0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,
	0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
	0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
	0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
	0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
	0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
	0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
	0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
	0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
	0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
	0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
	0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
	0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
	0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
	0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
	0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
	0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
	0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
	0xf9, 0xfa 
};

依据这几个表可以得出4个Huffman表，两个直流和两个交流表。
然后就要写数据了，真正压缩数据前面的部分我就不写了，看后面的代码吧，直接开始压缩数据如何编码。
到此步骤你已经得到了一个MCU的数据一次要写一个MCU，直流分量重置间隔默认为图片一行的MCU数量
这里要注意几点：
第一，要对直流分量进行差分编码，对交流分量进行形成编码
第二，如果写入的一个字节是0xFF，这是段标识所以要在此字节后面加入一个字节的0x00
第三，如果一行的MCU都写入完毕，接下来写入DRI，那就需要把上一个MCU没写入的部分写入，例如上一个MCU写入的字节多了两位11，那就写入一个字节0b11000000，然后写入两个字节的DRI标识
第四，写入的数据中有负数，比如写入-2，那对应就应该写入0b01，可使用如下方法得到对应的值

len = bitLength((int)abs(val))
pow(2, len) + val - 1

数据编码部分，要用写入数据有多少位查找对应的Huffman编码，先写入Huffman编码，再写入数据，直流交流均如此。
直流部分要使用差分编码，写入的数据是上一个DC值减去此DC值的差值
交流部分要使用行程编码，因为之前的操作大部分数据都变成0了，所以行程编码很合适，然后将有多少0和数据有多少位作为权重查找Huffman编码

代码如下

我将上述步骤都写到一起了，一次处理一个MCU并写入

bool JPEGData::writeJPEG(const char* filePath, int samp_factor[3][2], int quality_scale){
    auto _rgb = getRGBMatrix();
    max_h_samp_factor=0;
    max_v_samp_factor=0;
    for(int i=0;i<3;i++){
        write_samp_factor[i][0]=samp_factor[i][0];
        write_samp_factor[i][1]=samp_factor[i][1];
        max_h_samp_factor=max(max_h_samp_factor,write_samp_factor[i][0]);
        max_v_samp_factor=max(max_v_samp_factor,write_samp_factor[i][1]);
    }
    initQualityTable(quality_scale);
    fstream file(filePath,ios::out|ios::binary);
    if(file.fail()) return false;
    try{
        createDCEnHuffman();
        createACEnHuffman();
        writeTwoByte(file, (uint16_t)((FLAG << 8) + SOI));  // SOI
        for(int i=0;i<18;i++){              //APP
            writeByte(file, APP[i]);
        }
        JPEGQuality::write(file);                           // DQT
        writeTwoByte(file, (uint16_t)((FLAG << 8) + SOF0)); // SOF
        writeTwoByte(file, (uint16_t)0x0011);
        writeByte(file, (uint8_t)0x08);
        writeTwoByte(file, (uint16_t)height);
        writeTwoByte(file, (uint16_t)width);
        writeByte(file, (uint8_t)0x03);

        writeByte(file, (uint8_t)0x01);
        writeByte(file, (uint8_t)((samp_factor[0][0] << 4) | samp_factor[0][1]));
        writeByte(file, (uint8_t)0x00);

        writeByte(file, (uint8_t)0x02);
        writeByte(file, (uint8_t)((samp_factor[1][0] << 4) | samp_factor[1][1]));
        writeByte(file, (uint8_t)0x01);

        writeByte(file, (uint8_t)0x03);
        writeByte(file, (uint8_t)((samp_factor[2][0] << 4) | samp_factor[2][1]));
        writeByte(file, (uint8_t)0x01);

        JPEGHuffmanCode::write(file);   // DHT
        writeTwoByte(file, (uint16_t)((FLAG << 8) + DRI)); // DRI
        writeTwoByte(file, (uint16_t)0x0004);
        writeTwoByte(file, (uint16_t)ceil(_rgb.col * 1.0 / (ROW * max_h_samp_factor)));
        writeTwoByte(file, (uint16_t)((FLAG<<8)+SOS)); // SOS
        writeTwoByte(file, (uint16_t)0x000C);
        writeByte(file, (uint8_t)0x03);

        writeByte(file, (uint8_t)0x01);
        writeByte(file, (uint8_t)0x00);
        
        writeByte(file, (uint8_t)0x02);
        writeByte(file, (uint8_t)0x11);

        writeByte(file, (uint8_t)0x03);
        writeByte(file, (uint8_t)0x11);

        writeByte(file, (uint8_t)0);
        writeByte(file, (uint8_t)0x3F);
        writeByte(file, (uint8_t)0);
        RGBToYCbCr(_rgb, file);
        writeTwoByte(file, (uint16_t)((FLAG << 8) + (uint8_t)JPEGPType::EOI));  // EOI
    }
    catch(...){
        file.close();
        return false;
    }
    file.close();
    return true;
}
void JPEGData::RGBToYCbCr(Matrix<RGB> _rgb, fstream& file){
    // vector res;
    int mcu_width  = COL * max_h_samp_factor,
        mcu_height = ROW * max_v_samp_factor;
    int YUV[3] = {write_samp_factor[0][0] * write_samp_factor[0][1],
                  write_samp_factor[1][0] * write_samp_factor[1][1],
                  write_samp_factor[2][0] * write_samp_factor[2][1]};
    int y_h_param  = max_h_samp_factor * 1.0 / write_samp_factor[0][0],
        y_v_param  = max_v_samp_factor * 1.0 / write_samp_factor[0][1],
        cb_h_param = max_h_samp_factor * 1.0 / write_samp_factor[1][0],
        cb_v_param = max_v_samp_factor * 1.0 / write_samp_factor[1][1],
        cr_h_param = max_h_samp_factor * 1.0 / write_samp_factor[2][0],
        cr_v_param = max_v_samp_factor * 1.0 / write_samp_factor[2][1];
    double cb_h_samp_scale=write_samp_factor[1][0]*1.0/max_h_samp_factor,
           cb_v_samp_scale=write_samp_factor[1][1]*1.0/max_v_samp_factor,
           cr_h_samp_scale=write_samp_factor[2][0]*1.0/max_h_samp_factor,
           cr_v_samp_scale=write_samp_factor[2][1]*1.0/max_v_samp_factor;
    preDCValue[0] = preDCValue[1] = preDCValue[2] = 0;
    mcu_rows = ceil(_rgb.row * 1.0 / mcu_height);
    mcu_cols = ceil(_rgb.col * 1.0 / mcu_width);
    int DRI = mcu_cols, DriFLAG = 0xD0;
    for (int mcu_y = 0; mcu_y < mcu_rows; mcu_y++) {
        for (int mcu_x = 0; mcu_x < mcu_cols; mcu_x++) {
            double ***yuv = new double **[YUV[0] + YUV[1] + YUV[2]];
            int count=0;
            for (int i = 0; i < 3; i++) {
                for (int j = 0; j < YUV[i]; j++) {
                    yuv[count] = new double *[ROW];
                    for (int k = 0; k < ROW; k++) {
                        yuv[count][k] = new double[COL];
                        memset(yuv[count][k], 0, sizeof(double) * COL);
                    }
                    count++;
                }
            }
            int row = mcu_y * mcu_height,
                col = mcu_x * mcu_width;
            // cout<
            for (int i = 0; i < mcu_height; i++) {
                for (int j = 0; j < mcu_width; j++) {
                    RGB t = _rgb.getValue(row + i, col + j); // 得到的是一整个mcu，但是要把它分成多个8*8矩阵
                    double y  = YCbCrValueLimit(0.299 * t.red + 0.587 * t.green + 0.114 * t.blue - 128);
                    double cb = YCbCrValueLimit(-0.1687 * t.red - 0.3313 * t.green + 0.500 * t.blue);
                    double cr = YCbCrValueLimit(0.500 * t.red - 0.4187 * t.green - 0.0813 * t.blue);
                    int yPos = (i / ROW) * max_h_samp_factor + (j / COL);
                    int cbPos = YUV[0] + (int)((j / COL) * cb_v_samp_scale) +
                                (int)((i / ROW) * cb_h_samp_scale);
                    int crPos = YUV[0] + YUV[1] +
                                (int)((j / COL) * cr_v_samp_scale) +
                                (int)((i / ROW) * cr_h_samp_scale);

                    if (i % y_v_param == 0 && j % y_h_param == 0)
                        yuv[yPos][i % ROW][j % COL] = y;
                    if (i % cb_v_param == 0 && j % cb_h_param == 0)
                        yuv[cbPos][i / cb_v_param][j / cb_h_param] = cb;
                    if (i % cr_v_param == 0 && j % cr_h_param == 0)
                        yuv[crPos][i / cr_v_param][j / cr_h_param] = cr;
                    // cout<
                }
                // cout<
            }
            // cout<
            int pos = 0;
            for (int i = 0; i < 3; i++) {
                int huffmanID = i == 0 ? 0 : 1;
                for (int j = 0; j < YUV[i]; j++, pos++) {
                    DCT(yuv[pos]);
                    double *temp = ZigZag(yuv[pos]);
                    //encode DC
                    temp[0] = round(temp[0] / (i == 0 ? YQualityTable[0] : CQualityTable[0])); //quality
                    int dcDiff = temp[0] - preDCValue[i];// 进行差分编码
                    int lenDC=getBitLength((int)abs(dcDiff));
                    preDCValue[i] = temp[0];
                    if (dcDiff < 0) dcDiff = pow(2, lenDC) + dcDiff - 1;
                    writeBit(file, lenDC, dcDiff, en_dc_huffman[huffmanID].table[lenDC]);
                    //encode AC
                    int endPos = 63;
                    while(endPos>0&&temp[endPos]==0) endPos--;
                    for(int k=1;k<=endPos;k++){
                        int zeroCount = 0;
                        temp[k] = round(temp[k] / (i == 0 ? YQualityTable[k] : CQualityTable[k]));//quality
                        while (k < endPos && temp[k] == 0) {
                            temp[k + 1] = round(temp[k + 1] / (i == 0 ? YQualityTable[k + 1] : CQualityTable[k + 1]));
                            zeroCount++;
                            k++;
                        }
                        int lenAC = getBitLength((int)abs(temp[k]));
                        if (temp[k] < 0) temp[k] = pow(2, lenAC) + temp[k] - 1;
                        while(zeroCount>=16){
                            writeBitToFile(file,en_ac_huffman[huffmanID].table[0xf0].second, en_ac_huffman[huffmanID].table[0xf0].first);
                            zeroCount -= 16;
                        }
                        writeBit(file, lenAC, temp[k], en_ac_huffman[huffmanID].table[(zeroCount << 4) | lenAC]);
                        zeroCount = 0;
                    }
                    if(endPos!=63) writeBitToFile(file, en_ac_huffman[huffmanID].table[0x00].second, en_ac_huffman[huffmanID].table[0x00].first);
                }
            }
            FREE_LP_3(yuv, YUV[0] + YUV[1] + YUV[2], ROW)
        }
        // cout<
        if (bitCurPos != 0) writeByte(file, (uint8_t)curBitValue);
        bitCurPos = curBitValue = 0;
        preDCValue[0] = preDCValue[1] = preDCValue[2] = 0;
        writeTwoByte(file, (uint16_t)((FLAG << 8) + DriFLAG++));
        if (DriFLAG > 0xD7)
            DriFLAG = 0xD0;
    }
}
void JPEGQuality::write(fstream &file){
    writeByte(file, FLAG);
    writeByte(file, DQT);
    writeTwoByte(file, (uint16_t)0x0043);
    writeByte(file, (uint8_t)0x00);
    for(int i=0;i<64;i++){
        writeByte(file, YQualityTable[i]);
    }
    writeByte(file, FLAG);
    writeByte(file, DQT);
    writeTwoByte(file, (uint16_t)0x0043);
    writeByte(file, (uint8_t)0x01);
    for(int i=0;i<64;i++){
        writeByte(file, CQualityTable[i]);
    }
}
void JPEGHuffmanCode::write(fstream &file){
    // Y dc
    writeByte(file, (uint8_t)FLAG);
    writeByte(file, (uint8_t)DHT);
    writeTwoByte(file, (uint16_t)0x001F);
    writeByte(file, (uint8_t)0x00);
    for(int i=0;i<16;i++) writeByte(file, bits_dc_luminance[i]);
    for(int i=0;i<12;i++) writeByte(file, val_dc_luminance[i]);
    // Y ac
    writeByte(file, (uint8_t)FLAG);
    writeByte(file, (uint8_t)DHT);
    writeTwoByte(file, (uint16_t)0x00B5);
    writeByte(file, (uint8_t)0x10);
    for(int i=0;i<16;i++) writeByte(file, bits_ac_luminance[i]);
    for(int i=0;i<162;i++) writeByte(file, val_ac_luminance[i]);
    // UV dc
    writeByte(file, (uint8_t)FLAG);
    writeByte(file, (uint8_t)DHT);
    writeTwoByte(file, (uint16_t)0x001F);
    writeByte(file, (uint8_t)0x01);
    for(int i=0;i<16;i++) writeByte(file, bits_dc_chrominance[i]);
    for(int i=0;i<12;i++) writeByte(file, val_dc_chrominance[i]);
    // UV ac
    writeByte(file, (uint8_t)FLAG);
    writeByte(file, (uint8_t)DHT);
    writeTwoByte(file, (uint16_t)0x00B5);
    writeByte(file, (uint8_t)0x11);
    for(int i=0;i<16;i++) writeByte(file, bits_ac_chrominance[i]);
    for(int i=0;i<162;i++) writeByte(file, val_ac_chrominance[i]);
}
static int bitCurPos = 0;   // 当前字节在哪个bit位
static int curBitValue = 0;    // 当前值是多少
void writeBitToFile(fstream& file,int len,int realData){
    while (len > (8 - bitCurPos))
    {
        int rightMoveBit = len + bitCurPos - 8;
        int bitValue = realData >> rightMoveBit;
        curBitValue |= bitValue;
        writeByte(file, (uint8_t)curBitValue);
        if (curBitValue == 0xFF)
            writeByte(file, (uint8_t)0);
        realData -= bitValue << rightMoveBit;
        len -= 8 - bitCurPos;
        curBitValue = bitCurPos = 0;
    }
    curBitValue |= realData << (8 - bitCurPos - len);
    bitCurPos += len;
    if (bitCurPos >= 8)
    {
        writeByte(file, (uint8_t)curBitValue);
        if (curBitValue == 0xFF)
            writeByte(file, (uint8_t)0);
        curBitValue = bitCurPos = 0;
    }
}
void writeBit(fstream& file,int len, int realData,pair<uint16_t,uint8_t>& huffmanCode){

    int codeLen=huffmanCode.second,code=huffmanCode.first;
    writeBitToFile(file, codeLen, code);

    writeBitToFile(file, len, realData);
}

double* ZigZag(double** originArray){
    double* res=new double[ROW*COL];
    // for(int i=0;i<64;i++){
    //     res[Zig[i]]=originArray[i/ROW][i%COL];
    // }
    int cur=0,x=0,y=0;
    bool flag = true;//true是右上 false是左下
    while (cur < 64) {
        res[cur++] = round(originArray[y][x]);
        if (flag) { x++; y--; }
        else { x--; y++; }
        if (x < 0 || y < 0 || x>7 || y>7) flag = !flag;
        if (x < 0 && y>7) { x = 1; y = 7; }
        if (x < 0) x = 0;
        else if (x > 7) { x = 7; y += 2; }
        if (y < 0) y = 0;
        else if (y > 7) { y = 7; x += 2; }
    }
    return res;
}

void writeByte(fstream& file,uint8_t val){
    file.write((char*)&val, 1);
    // cout<
}

void writeTwoByte(fstream& file,uint16_t val){
    writeByte(file, val>>8);
    writeByte(file, val&0xFF);
}
int getBitLength(int num){
    int res=0;
    while(num){
        res+=1;
        num>>=1;
    }
    return res;
}
double YCbCrValueLimit(double input){
    if(input<-128) return -128;
    else if(input>128) return 128;
    return input;
}
//段类型
enum JPEGPType{
    SOF0    = 0xC0,     //帧开始
    SOF1    = 0xC1,     //帧开始
    SOF2    = 0xC2,     //帧开始
    DHT     = 0xC4,     //哈夫曼表
    SOI     = 0xD8,     //文件头
    EOI     = 0xD9,     //文件尾
    SOS     = 0xDA,     //扫描行开始
    DQT     = 0xDB,     //定义量化表
    DRI     = 0xDD,     //定义重新开始间隔
    APP0    = 0xE0,     //定义交换格式和图像识别信息
    APP1    = 0xE1,     //定义交换格式和图像识别信息
    APP2    = 0xE2,     //定义交换格式和图像识别信息
    COM     = 0xFE,     //注释
	FLAG	= 0xFF		//开始
};

如果有问题可以去这里看源码这里的代码是好使的，里面的图片解析器只是实现了精度为1字节的量化表的解析