fsk-imgoptimize/lib/gifstream.js

图片优化工具

var Writable = require('stream').Writable;
var Readable = require('stream').Readable;
var ImgOptimize = require('./imgOptimize.js');
var util = require('util');
var imgtypes = require('./constants.js');
util.inherits(GIFStream, ImgOptimize);

function GIFStream(options) {
  if (!(this instanceof GIFStream)) {
    return new GIFStream(options);
  }
  ImgOptimize.call(this);
}

GIFStream.prototype.handleSignature = function() {
  this.addHandle(6, this.parseSignature.bind(this));
};

GIFStream.prototype.parseSignature = function(data) {
  var sign = data.toString();
  if (sign === 'GIF89a' || sign === 'GIF79a') {
    this.push(data);
    this.addHandle(7, this.parseLSD.bind(this));
  } else {
    throw new Error('unvalid file type');
  }
};

//LSD===Logical Screen Description 逻辑屏幕描述块
GIFStream.prototype.parseLSD = function(data) {
  this.push(data);
  var packedField = data.readUIntBE(4, 1);
  var g = packedField.toString(2).charAt(1);
  //g === 1时表示有全局颜色表 读取后三位计算全局颜色表的大小
  if (g === '1') {
    var globalColorTableSize = (2 << (packedField & 7 )) * 3;
    this.addHandle(globalColorTableSize, this.parseGCT.bind(this));
  } else {
    this.addHandle(1, this.parseOther.bind(this));
  }
}
//GCT === global color table 全局颜色表
GIFStream.prototype.parseGCT = function(data) {
  this.push(data);
  this.addHandle(1, this.parseOther.bind(this));
};

GIFStream.prototype.parseOther = function(data) {
  var sign = data.readUIntBE(0, data.length);
  switch (sign) {
    case 33 :
      this.addHandle(1, this.parseExtChunk.bind(this));
      break;
    case 44:
      this.push(data);
      this.addHandle(9, this.parseImg.bind(this));
      break;
    case 59:
      this.push(data);
      this.end();
    default:
      break;
  }
};
GIFStream.prototype.parseExtChunk = function(data) {
  var sign = data.readUIntBE(0, data.length);
  switch (sign) {
    case 1:   //扩展文本标识
      this.push(new Buffer([0x21]));
      this.push(data);
      this.addHandle(13, this.parseTextExt.bind(this));
      break;
    case 255:   //应用扩展
      this.push(new Buffer([0x21]));
      this.push(data);
      this.addHandle(12, this.parseAppExt.bind(this));
      break;
    case 249:   //图形控制扩展
      this.push(new Buffer([0x21]));
      this.push(data);
      this.addHandle(6, this.parseGraphicControl.bind(this));
      break;
    case 254:   //注释
      //注释标识之后紧跟着就是 注释子数据块
      //所以可以直接读取自数据块 并且忽略掉注释的数据
      this.handleSubBlock(true);
      break;
    default:
      break;
  }
};
GIFStream.prototype.parseTextExt = function(data) {
  this.push(data);
  this.handleSubBlock(false);
}
GIFStream.prototype.parseAppExt = function(data){
  this.push(data);
  this.handleSubBlock();
}
GIFStream.prototype.handleSubBlock = function(ignore) {
  var self = this;
  this._queue.push({
    length: 1,
    callback: function(data) {
      if (!ignore) {
        self.push(data);
      } else {
        // console.log(data.readUIntBE(0, 1));
      }
      var length = data.readUIntBE(0, 1);
      //如果length为0 则认为数据子块结束 继续读取下一个数据块
      //否则继续调用handleSubBlock读取下一段数据子块
      if(length) {
        self._queue.push({
          length: length,
          callback: function(content) {
            if (!ignore) {
              self.push(content);
            }
            self.handleSubBlock(ignore);
          }
        });
      } else {
        self.addHandle(1, this.parseOther.bind(this));
      }
    }
  });
};

GIFStream.prototype.parseGraphicControl = function(data) {
  this.push(data);
  this.addHandle(1, this.parseOther.bind(this));
}
GIFStream.prototype.parseImg = function(data) {
  this.push(data);
  var packedField = data.readUIntBE(8, 1);
  var l = packedField.toString(2).charAt(1);
  //若标识为1 则说明有局部颜色列表  解析之
  if ( l === '1') {
    var LocalColorTableSize = (2 << (packed & 7)) * 3;
    this.addHandle(LocalColorTableSize, this.parseLCT.bind(this));
  } else {
    //等于说明没有局部颜色列表  直接读取LZW数据
    this.addHandle(1, this.parseLZW.bind(this));
  }
}
//LCT == local color table 局部颜色列表
GIFStream.prototype.parseLCT = function(data) {
  this.push(data);
  //解析完LCT继续读LZW
  this.addHandle(1, this.parseLZW.bind(this));
};
GIFStream.prototype.parseLZW = function(data) {
  this.push(data);
  this.handleSubBlock();
};
module.exports = GIFStream;
// var fs = require('fs');
// fs.createReadStream('a.gif').pipe(new GIFStream()).pipe(fs.createWriteStream('9.gif'));


// // var buf = new Buffer([ 0x47, 0x49, 0x46, 0x38, 0x39, 0x61 ])
// fs.readFile('9.gif', function(err, data) {
//   var pos = 0;
//   while (pos < data.length){
//     console.log(data.slice(pos, pos + 16));
//     pos += 16;
//   }
// });