code: function prepareOpts(opts) {
    opts = opts || {};
    var size = opts.size || '128x128',
    text = opts.text || size,
    bgcolor = opts.bgcolor || randomColor(), //other placeholder default bgcolor is '#ccc',
    color = opts.color || randomColor(), //other placeholder default color is '#969696',
    //font参数
    fstyle = opts.fstyle || 'normal', //normal / italic / oblique
    fweight = opts.fweight || 'bold', //normal / bold / bolder / lighter
    fsize = opts.fsize || '', //auto calculate the font size to response to the image size
    ffamily = opts.ffamily || 'consolas', //规定字号和行高，以像素计。
    new_opts = {};
  
    size = size.split('x'); 
    if (size.length !== 2) {
        size = [128, 128];
    }
    //减少键值，压缩之后可以减少包大小
    new_opts.a = size;
    new_opts.b = text;
    new_opts.c = bgcolor;
    new_opts.d = color;
    new_opts.e = fstyle;
    new_opts.f = fweight;
    new_opts.g = fsize;
    new_opts.h = ffamily;
    opts = null;
    return new_opts;
  }
docs: 预处理配置信息，补全默认项