Configurable delay between two scans

[JauernigIT]

I added a scanDelay property to the Quagga config, so there is a configurable delay between two scans. It's used with a timeout in the onmessage function. I needed this functionality in my scenario, where the processor load had to be lowered (and where we don't need realtime processing).

[serratus]

Hi Matthias, thanks for the proposal. Having the timeout directly in the onmessage solves only half of the problems. I propose totally different approach:

1. Instead of scanDelay I propose an option called frequency which defines the desired scan-frequency in scans per second.
2. The scheduler should be placed outside of the worker, directly in the frame function where it is currently called via requestAnimFrame. In my opinion, this is a much better place for scheduling, because all the other work like capturing the video-frame is also delayed.

I just tried something like this and it worked like a charm:

function startContinousUpdate() {
    var next = null,
        delay = 1000 / (_config.frequency || 60);

    _stopped = false;
    (function frame(timestamp) {
        next = next || timestamp;
        if (!_stopped) {
            if (timestamp >= next) {
                next += delay;
                update();
            }
            window.requestAnimFrame(frame);
        }
    }(performance.now()));
}

function start() {
    if (_onUIThread && _config.inputStream.type === "LiveStream") {
        startContinousUpdate();
    } else {
        update();
    }
}

[JauernigIT]

That's brilliant! I've made the first proposal, but already thought that the location was not the best. You're more into the whole library, so you know where to put it. I've pushed the updated branch, works fine for me.