Go初接触之libjpeg-turbo

　　libjpeg-turbo是一个效率很高的图片处理的C库，所以严格来说放在Go这里不太妥当，但是之后我们会把它封装成Go可以调用的类库。据说libjpeg-turbo在处理jpeg格式图片时比ImageMagick库效率高了三倍，参照示例代码写了一个缩略图的功能：

//
//  main.cpp
//  libjpegturbo
//
//  Created by 张皓然 on 2018/4/17.
//  Copyright © 2018年 张皓然. All rights reserved.
//

#include <iostream>
#include "/opt/libjpeg-turbo/include/jpeglib.h"

using namespace std;

int compress_JPEG_file(string strImageName, string strDstImageName, int quality, int scale_num, int scale_denom) {
    //创建必要变量，后缀in是源图片信息，out是目标图片信息，buffer是缓冲区
    struct jpeg_decompress_struct cinfo_in;
    struct jpeg_compress_struct cinfo_out;
    struct jpeg_error_mgr jerr_in, jerr_out;
    FILE *infile, *outfile;
    JSAMPARRAY buffer;
    //表示每行的宽度，为width*components
    int row_stride;
    
    //初始化对象
    jpeg_create_decompress(&cinfo_in);
    jpeg_create_compress(&cinfo_out);
    //绑定错误结构
    cinfo_in.err = jpeg_std_error(&jerr_in);
    cinfo_out.err = jpeg_std_error(&jerr_out);
    //C库方式打开图片文件，并填入cinfo对象
    if((infile = fopen(strImageName.c_str(), "rb")) == NULL) {
        fprintf(stderr, "can't open %s
", strImageName.c_str());
        return -1;
    }
    if((outfile = fopen(strDstImageName.c_str(), "wb")) == NULL) {
        fprintf(stderr, "can't open %s
", strDstImageName.c_str());
        return -1;
    }
    jpeg_stdio_src(&cinfo_in, infile);
    jpeg_stdio_dest(&cinfo_out, outfile);
    //读取文件信息
    jpeg_read_header(&cinfo_in, TRUE);
    
    //设定文件压缩比例，M/N
    cinfo_in.scale_num = scale_num;
    cinfo_in.scale_denom = scale_denom;
    
    //开始解压缩
    jpeg_start_decompress(&cinfo_in);
    
    //将压缩后的信息存入cinfo_out中作为新图片的参数
    cinfo_out.image_height = cinfo_in.output_height;
    cinfo_out.image_width = cinfo_in.output_width;
    cinfo_out.input_components = 3;
    cinfo_out.in_color_space = JCS_RGB;
    jpeg_set_defaults(&cinfo_out);
    //设置图片质量
    jpeg_set_quality(&cinfo_out, quality, TRUE);
    //开始压缩
    jpeg_start_compress(&cinfo_out, TRUE);
    
    //设定每行的宽度准备写入，并分配缓冲区空间
    row_stride = cinfo_in.output_width * cinfo_in.output_components;
    buffer = (*cinfo_in.mem -> alloc_sarray)((j_common_ptr)&cinfo_in, JPOOL_IMAGE, row_stride, 1);
    
    while(cinfo_in.output_scanline < cinfo_in.output_height) {
        //从in中读取数据并写入out中
        jpeg_read_scanlines(&cinfo_in, buffer, 1);
        jpeg_write_scanlines(&cinfo_out, buffer, 1);
    }
    
    //结束，释放变量
    jpeg_finish_decompress(&cinfo_in);
    jpeg_destroy_decompress(&cinfo_in);
    
    jpeg_finish_compress(&cinfo_out);
    jpeg_destroy_compress(&cinfo_out);
    
    fclose(infile);
    fclose(outfile);
    
    return 0;
}

int main() {
    clock_t start, finish;
    start = clock();
    for(int i = 0; i < 100; i ++) compress_JPEG_file("/Users/zhanghaoran/Desktop/abc.jpg", "/Users/zhanghaoran/Desktop/abc_compress.jpg", 80, 1, 2);
    finish = clock();
    cout << "Total : " << (double)(finish - start) / CLOCKS_PER_SEC << endl;
}