Capture and save image with robotjs - node.js

Question

I tried to capture and save an image from screen with robotjs (http://robotjs.io/) but when I open the file bitmap the image is not in a valid format. This is my code:
var robot = require("robotjs");
var fs = require("fs");
var size = 10;
var img = robot.screen.capture(0, 0, size, size);
fs.writeFileSync('img.bmp',img.image);

Answer 1

Jimp supports converting Raw Pixel Buffer into PNG out-of-the-box and works a lot faster.
let robot = require("robotjs");
let Jimp = require('jimp');
const img = robot.screen.capture(0, 0, width, height).image;
new Jimp({data: img, width, height}, (err, image) => {
image.write(fileName);
});
The image will be saved with the wrong colors. To fix it, you can use the following code:
function screenCaptureToFile2(robotScreenPic, path) {
return new Promise((resolve, reject) => {
try {
const image = new Jimp(robotScreenPic.width, robotScreenPic.height);
let pos = 0;
image.scan(0, 0, image.bitmap.width, image.bitmap.height, (x, y, idx) => {
image.bitmap.data[idx + 2] = robotScreenPic.image.readUInt8(pos++);
image.bitmap.data[idx + 1] = robotScreenPic.image.readUInt8(pos++);
image.bitmap.data[idx + 0] = robotScreenPic.image.readUInt8(pos++);
image.bitmap.data[idx + 3] = robotScreenPic.image.readUInt8(pos++);
});
image.write(path, resolve);
} catch (e) {
console.error(e);
reject(e);
}
});
}
var pic = robot.screen.capture();
screenCaptureToFile2(pic)

Answer 2

Note that your img.image Buffer from Robotjs is a raw buffer with pixels; not a BMP or PNG or any other format.
You should do some data conversion and probably save it using a library that supports writing to file (I do not see that in Robotjs).
Please look at this other question, which also uses robot.screen.capture and saves file to a PNG file using Jimp library. That code answers your question too.