使用javascript将图像文件转换为base64字符串

Question

I want to upload image file into couchdb using javascript. For this i am using inline attachment concept. while uploading file i have to use Base64 encode(). This method has string argument only. How can i convert image file to base64 String using javascript. Please can anybody share me the sample snippet.

Thanks

Answer 1

You can use canvas in browsers that support it, as long as the image is loaded from the same domain.

function encodeImage(src, callback) {
    var canvas = document.createElement('canvas'),
        ctx = canvas.getContext('2d'),
        img = new Image();

    img.onload = function() {
        canvas.width = img.width;
        canvas.height = img.height;
        ctx.drawImage(img, 0, 0, img.width, img.height);
        callback(canvas.toDataURL());
    }
    img.src = src;
}

---

See an example here .

Answer 2

Here is what I've done:

function addAttachment(inputId, inputDb) {
    var db = $.couch.db(inputDb);
    var file = document.getElementById("attachment").files[0];
    var reader  = new FileReader();

    reader.onloadend = function () {
       //That's CouchDB's internal Base64.
       var base64 = Base64.encode( reader.result );
           db.openDoc(inputId, {
            success:function (result) {
                db.saveDoc({
                    "_id": inputId,
                    "_rev":result._rev,
                    "_attachments": {
                        "yourImageHere.jpg":
                        {
                            "content_type":"image\/jpeg",
                            "data": base64
                        }
                    }
                },  {
                    success : function () {
                        location.reload();
                    },
                    error : function (a, b, c) {
                        alert('Cannot save');
                    }
                });
            },
            error:function (a, b, c) {
                alert('Error: ' + c);
            }
        });
    }

    if (file) {
      reader.readAsBinaryString(file);
    }
}

And my HTML:

<form id="upload" method="post" enctype="multipart/form-data" >
    <input type="file" id="attachment" name="_attachments"/>
    <br/>
    <input type="button" value="Submit"/>
</form>

It's important you read as binary (readAsBinaryString) and convert in Base64 using the CouchDB internal library:

<script src="/_utils/script/base64.js"></script>

Answer 3

I have used the following javascript base64 encoder/decoder successfully: http://hellerim.net/base64_src.php

I did make some changes to these routines that you may find useful:

///
///
// This file implements base64 encoding and decoding.
// Encoding is done by the function base64Encode(), decoding
// by base64Decode(). The naming mimics closely the corresponding
// library functions found in PHP. However, this implementation allows
// for a more flexible use.
//
// This implementation follows RFC 3548 (http://www.faqs.org/rfcs/rfc3548.node),
// so the copyright formulated therein applies.
//
// Dr.Heller Information Management, 2005 (http://www.hellerim.de).
//
///

var base64 = function () { };

// provide for class information
base64.classID = function () {
  return 'system.utility.base64';
};

//disallow subclassing
base64.isFinal = function () {
  return true;
};

// original base64 encoding
base64.encString = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
// URL and file name safe encoding
base64.encStringS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';

/// BEGIN_DOC(base64).METHOD(encode)
///
// method String base64.encode(INPUTTYPE inp [, bool uc [, bool safe]])
//
// Encode input data into a base64 character string.
//
// Function arguments:
//     INPUTTYPE inp:        data to be encoded. INPUTTYPE may be String or Array.
//                           Any other INPUTTYPE results in an output value of null.
//                           If INPUTTYPE is String each character is converted into 
//                           two bytes each of which is encoded separately.
//     bool uc               Optional. If this parameter has a value of 'true' which is
//                           the default, code of each character is treated as a 16-bit
//                           entity (UniCode), i.e. as two bytes. Otherwise, the codes
//                           are truncated to one byte (8-bit character set) which
//                           may result in information loss. If INPUTTYPE is Array,
//                           the value of this parameter has no effect.
//     bool safe:            Optioanal. If this parameter is set to true, the standard base64 
//                           character set is replaced with a modified version where
//                           the characters '+' and '/' are replace with '-' and '_',
//                           repectively, in order to avoid problems with file system
//                           namings which otherwise could occur on some systems.
//                           By default, the value of this argument is assumed to be
//                           false.
// Return value:             The function returns a character string consisting of
//                           the base64 representaion of the input. Its length is a
//                           multiple of 4. If the encoding yields less than this
//                           the string is stuffed with the '=' character. In each case, 
//                           the string maybe empty but not null if no error occurred.
// Errors:                   Whenever an error occurs, null is returned. Parameter values
//                           not defined above are considered errors.
// Remarks:                  If the input array contains something different from
//                           a byte at some position the first 8 bits only of this entity are
//                           processed silently without returning an error, which probably
//                           results in garbage converted to base64.
//
/// END_DOC
base64.encode = function (inp, uc, safe) {
  // do some argument checking
  if (arguments.length < 1) return null;
  var readBuf = [];    // read buffer
  if (arguments.length >= 3 && safe != true && safe != false) return null;
  var enc = (arguments.length >= 3 && safe) ? this.encStringS : this.encString; // character set used
  var b = (typeof inp == "string"); // how input is to be processed
  if (!b && (typeof inp != "object") && !(inp instanceof Array)) return null; // bad input
  if (arguments.length < 2) {
    uc = true;                  // set default
  } // otherwise its value is passed from the caller
  if (uc != true && uc != false) return null;
  var n = (!b || !uc) ? 1 : 2;  // length of read buffer
  //  var out = '';                 // output string
  var out = [];                 // output string
  var c = 0;                    // holds character code (maybe 16 bit or 8 bit)
  var j = 1;                    // sextett counter
  var l = 0;                    // work buffer
  var s = 0;                    // holds sextett

  // convert  
  for (var i = 0; i < inp.length; i++) {  // read input
    c = (b) ? inp.charCodeAt(i) : inp[i]; // fill read buffer
    for (var k = n - 1; k >= 0; k--) {
      readBuf[k] = c & 0xff;
      c >>= 8;
    }
    for (var m = 0; m < n; m++) {         // run through read buffer
      // process bytes from read buffer
      l = ((l << 8) & 0xff00) | readBuf[m];   // shift remaining bits one byte to the left and append next byte
      s = (0x3f << (2 * j)) & l;              // extract sextett from buffer
      l -= s;                              // remove those bits from buffer;
      out.push(enc.charAt(s >> (2 * j)));        // convert leftmost sextett and append it to output
      j++;
      if (j == 4) {                         // another sextett is complete
        out.push(enc.charAt(l & 0x3f));        // convert and append it
        j = 1;
      }
    }
  }
  switch (j) {                            // handle left-over sextetts
    case 2:
      s = 0x3f & (16 * l);                // extract sextett from buffer
      out.push(enc.charAt(s));               // convert leftmost sextett and append it to output
      out.push('==');                        // stuff
      break;
    case 3:
      s = 0x3f & (4 * l);                 // extract sextett from buffer
      out.push(enc.charAt(s));               // convert leftmost sextett and append it to output
      out.push('=');                         // stuff
      break;
    default:
      break;
  }

  return out.join('');

}

/// BEGIN_DOC(base64).METHOD(decode)
///
// method RETURNTYPE base64.decode(String inp [, enum outType [, bool safe [, bool lax]]])
//
// Encode input data into a base64 character string.
//
// Function arguments:
//     String inp:           base64 encoded data string to be decoded.
//     enum outType          Optional. This parameter specifies the type of the output and determines
//                           how the input data is to be interpreted.:
//                             0  - binary data; create a byte array (default)
//                             1  - 8-bit character string, assuming 1-byte characters encoded in inp
//                             2  - 16-bit (UniCode) character string, assuming 2-byte 
//                                  characters encoded in inp
//                           If 2 is passed to the function, but the number of base64 characters
//                           is odd, a value of null is returned.
//     bool safe             Optional. If this parameter is set to true, the standard base64 
//                           character set is replaced with a modified version where
//                           the characters '+' and '/' are replaced with '-' and '_',
//                           repectively, in order to avoid problems with file system
//                           namings which otherwise could occur on some systems.
//                           By default, the value of this argument is assumed to be
//                           false.
//     bool lax              Optional. If set to true, the function skips all input characters which
//                           cannot be processed. This includes the character '=', too, if
//                           it is followed by at least one different character before the string
//                           ends. However, if skipping infeasible characters amounts to a number
//                           of allowed base64 characters which is not amultiple of 4,
//                           this is considered an error and null is returned.
//                           If lax is set to false (the default), null is returned
//                           whenever an infeasible character is found.
//                           The purpose of this parameter is to give support in cases
//                           where data has been base64 encoded and later on was folded by
//                           some other software, e.g. '\r\n\'s have been inserted in email.
//                           exchange.
// Return value:             The function's processing result value is stored in a string or in
//                           a byte array before it is returned, depending on the value 
//                           assigned to the type parameter. In each case, the value
//                           maybe empty but not null if no error occurred.
// Errors:                   Whenever an error occurs, null is returned. Parameter values
//                           not defined above are considered errors.
//
/// END_DOC

base64.decode = function (inp, outType, safe, lax) {

  // do some argument checking
  if (arguments.length < 1) return null;
  if (arguments.length < 2) outType = 0; // produce character array by default
  if (outType != 0 && outType != 1 && outType != 2) return null;
  if (arguments.length >= 3 && safe != true && safe != false) return null;
  var sEnc = (arguments.length >= 3 && safe) ? this.encStringS : this.encString;  // select encoding character set
  if (arguments.length >= 4 && lax != true && lax != false) return null;
  var aDec = {};                // create an associative array for decoding
  for (var p = 0; p < sEnc.length; p++) { // populate array
    aDec[sEnc.charAt(p)] = p;
  }
  var out = (outType == 0) ? [] : '';
  lax = (arguments.length == 4 && lax); // ignore non-base64 characters
  var l = 0;               // work area
  var i = 0;               // index into input
  var j = 0;               // sextett counter
  var c = 0;               // input buffer
  var k = 0;               // index into work area
  var end = inp.length;    // one position past the last character to be processed
  var C = '';
  // check input
  if (lax) {
    var inpS = '';         // shadow input
    var ignore = false;    // determines wether '=' must be counted
    var cnt = 0;
    for (var p = 1; p <= inp.length; p++) {    // check and cleanup string before trying to decode
      c = inp.charAt(end - p);
      if (c == '=') {
        if (!ignore) {
          if (++cnt > 1) ignore = true;
        } else {
          continue;
        }
      } else if (undefined != aDec[c]) { // the character is base64, hence feasible
        if (!ignore) ignore = true;      // no more '=' allowed
        inpS = c + inpS;                 // prepend c to shadow input
      }
    }
    for (var p = 0; p <= cnt; p++) {     // at most cnt '=''s were garbage, a number in 
      if (p == 2) return null;           // [inpS.length, inpS.length + cnt] must be a
      if ((inpS.length + cnt) % 4 == 0) break;  // multiple of 4
    }
    if (inpS.length % 4 == 1) return null;   // must be 0, 2, or 3 for inpS to contain correctly base64 encoded data
    inp = inpS;                          // inp now contains feasible characters only
    end = inp.length;
  } else {
    if (inp.length % 4 > 0) return null;   // invalid length
    for (var p = 0; p < 2; p++) {        // search for trailing '=''s
      if (inp.charAt(end - 1) == '=') {
        end--;
      } else {
        break;
      }
    }
  }
  // convert
  for (i = 0; i < end; i++) {
    l <<= 6;                             // clear space for next sextett
    if (undefined == (c = aDec[inp.charAt(i)])) return null; // lax must be false at this place!
    l |= (c & 0x3f);    // append it
    if (j == 0) {
      j++;
      continue;                          // work area contains incomplete byte only
    }
    if (outType == 2) {
      if (k == 1) {                      // work area contains complete double byte
        out += String.fromCharCode(l >> (2 * (3 - j)));  // convert leftmost 16 bits and append them to string
        l &= ~(0xffff << (2 * (3 - j)));       // clear the 16 processed bits
      }
      k = ++k % 2;
    } else {                             // work area contains complete byte
      if (outType == 0) {
        out.push(l >> (2 * (3 - j)));          // append byte to array
      } else {
        out += String.fromCharCode(l >> (2 * (3 - j))); // convert leftmost 8 bits and append them to String
      }
      l &= ~(0xff << (2 * (3 - j)));           // clear the 8 processed bits
    }
    j = ++j % 4;                           // increment sextett counter cyclically
  }
  if (outType == 2 && k == 1) return null;  // incomplete double byte in work area

  return out;
}

Answer 4

There is a base64 function included in couchDB. See this recent question & answer.