Decoding an array of integers to PNG (JavaScript, node.js)

Question

I'm trying to convert an array of integers to a PNG-file.

function saveImg(data, dir, name, waveBand) {
    if (!fs.existsSync(dir)){
        logger.info(dir + ' does not exist... creating!');
        fs.mkdirSync(dir);
    }

    logger.info('received data with length ' + data.length);
    try {
        const encoder = new util.TextDecoder("utf-8");
        const decodedData = encoder.decode(new Uint8Array(data));
        logger.info('decoded string: ' + decodedData);
        const imgLocation = path.join(getStoragePublicFolder(), dir, name + '.png');
        logger.info('Writing image to location: ' + imgLocation);

        fs.writeFile(path.resolve(imgLocation), decodedData,
            function(err) {
                if(err) 
                    logger.info(err);
            }
        );

        logger.info('Success! Image saved to ' + imgLocation);
        return imgLocation;
    } 
    catch (err)
    { 
        logger.error('Failed to save image - ' + err); 
    }
    return defaultPath(waveBand);
}

My data looks like this: https://jsfiddle.net/ebaxg3q4/

Unfortunately there is something wrong with the decoding of the data therefore the image is corrupted.

Which kind of encoding is this? What is the error I made?

Thank you in advance!

You're trying to decode the data as UTF-8 text. It's not text but the actual bytes that should be saved to the file. — radulfr
– radulfr, Commented Nov 6, 2019 at 10:42

Terry Lennox · Accepted Answer · 2019-11-06 10:59:19Z

You can create a buffer from your station bitmap data using Buffer.from(Array).

We can then use fs.writeFile to write the buffer to the .png file.

This code works for me (using your sample data):

function saveImg(data, dir, name, waveBand) {
    if (!fs.existsSync(dir)){
        logger.info(dir + ' does not exist... creating!');
        fs.mkdirSync(dir);
    }

    logger.info('received data with length ' + data.length);
    try {
        const decodedData = Buffer.from(data);
        const imgLocation = path.join(getStoragePublicFolder(), dir, name + '.png');
        logger.info('Writing image to location: ' + imgLocation);
        fs.writeFile(path.resolve(imgLocation), decodedData,
            function(err) {if(err) logger.info(err);});
        logger.info('Success! Image saved to ' + imgLocation);
        return imgLocation;
    } catch (err){ logger.error('Failed to save image - ' + err); }
    return defaultPath(waveBand);
}

// Use the first station in the station list.
saveImg(dummyStation.stationList[0].stationIconPicture.data.data, "./test", "station");

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