nginx 利用image_fillter 模块可以实现实时的图片略缩和裁剪功能。但是根据之前的网上的配置，对于前端来说请求过于复杂。所以这里限定几种略缩图的大小供前端调用。

在此之前的配置请看:https://blog.wangjunfeng.com/archives/669

location ~ (.+)\.(jpg|gif|png|jpeg)@(large|middle|small)$ {
    # 限制referer，防盗链
    valid_referers *.example.com;
    if ($invalid_referer) {return 403;}
    set $size $3;
    set $image_path  $1.$2; #真实图片地址
    set $cache_path  $1_resize_$3.$2;  #临时文件地址
  
    set $image_uri  /image-resize/$image_path?w=$size;
    if (-f $document_root$cache_path) {
        rewrite (.+)\.(jpg|gif|png|jpeg)@(large|middle|small)$ $1_resize_$3.$2;
        break;
    }
    if (!-f $document_root$cache_path) {
        proxy_pass http://$server_name$image_uri;
        break;
    }
    proxy_store $document_root$cache_path;
    proxy_store_access user:rw group:rw all:r;
    proxy_set_header Host $host;
    expires  10d; # 设置图片过期时间10天
}

location ~ /image-resize(.+)\.(jpg|gif|png|jpeg) {
    if ($arg_w = large){
    set $w 750;
    set $h 750;
    }
    if ($arg_w = middle){
    set $w 400;
    set $h 400;
    }
    if ($arg_w = small){
    set $w 100;
    set $h 100;
    }
    rewrite /image-resize(.+)\.(jpg|gif|png|jpeg)$ $1.$2 break;
    image_filter resize $w $h;
    image_filter_jpeg_quality 100;
    image_filter_buffer 10M;
    try_files $1.$2 /notfound.jpg;
}