Pipe image through graphics magic only half complete - node.js

Question

I am trying to set up a transform stream to pipe an image through with GM https://github.com/aheckmann/gm. So I can do something like:
readStream.pipe(resize()).pipe(writeStream);
I have used through2 along with gm to try and achieve this. It works but only parses half the image, leaving a large portion just grey.
'use strict';
var fs = require('fs')
, gm = require('gm').subClass({imageMagick: true})
, through2 = require('through2');
let readStream = fs.createReadStream('landscape.jpg');
let writeStream = fs.createWriteStream('landscape-out.jpg');
let resize = function(width, height) {
return through2(function(chunk, enc, callback){
gm(chunk)
.resize(800)
.gravity('Center')
.crop(800, 500, 0, 0)
.stream((err, stdout, stderr) => {
stdout.on('data', chunk => {
this.push(chunk);
});
stdout.on('end', () => {
callback();
});
});
});
}
readStream.pipe(resize()).pipe(writeStream);

Answer 1

In
through2(function(chunk, enc, callback){
chunk is only a small part of the image.
So the behavior you got seems normal, you are resizing the chunks of the image, not the image itself.
This said, in the doc,
// GOTCHA:
// when working with input streams and any 'identify'
// operation (size, format, etc), you must pass "{bufferStream: true}" if
// you also need to convert (write() or stream()) the image afterwards
// NOTE: this buffers the readStream in memory!
var readStream = fs.createReadStream('/path/to/my/img.jpg');
gm(readStream)
.size({bufferStream: true}, function(err, size) {
this.resize(size.width / 2, size.height / 2)
this.write('/path/to/resized.jpg', function (err) {
if (!err) console.log('done');
});
});
but it s going to buffer the picture in memory, so it s not optimum.
As you are using imagemagick, you shall just let it manage all that part of the processing. And later fs.readStream, the output.
From the doc
var writeStream = fs.createWriteStream('/path/to/my/reformatted.png');
gm('/path/to/my/img.jpg')
.stream('png')
.pipe(writeStream);