Image artifacting even just with readimage -> writegif no-op

[jasonrhodes]

It's hard for me to narrow down if this is a problem with writegif or with readimage, but I've noticed that while unaltered meatspace GIFs come out of readimage/writegif perfectly, a lot of other GIFs come through with strange pixel artifacting.

Simplest reproduction:

function makeDataUri(type, buffer) {
  return 'data:' + type + ';base64,' + buffer.toString('base64');
}

function imgTag(image) {
  return "<img src='" + makeDataUri("image/gif", image) + "' />";
}

// readimage/writegif no-op
function writegiftest(buffer, cb) {
  readimage(buffer, function (err, buf) { 
    writegif(buf, cb); 
  });
}

// Express.js route
app.get("/writegiftest", function (req, res) {
  fs.readFile("./gifs/example.gif", function (err, buffer) {

    if (err) {
      res.send(err);
      return;
    }

    writegiftest(buffer, function (err, image) {
      res.send(imgTag(image));
    });

});

You can see two examples below, one is a Photoshop-resized meatspace GIF, the other is just a GIF of Homer Simpson from GIS.

[brycebaril]

I'm pretty sure this is due to gif optimization, where I think optimized gifs have no data set for the pixels that don't change.

However, since data isn't cleared from Node buffers by default, I think what happens is the "empty" data gets occupied with random noise.