function inRange(i, width, height) {
    return ((i>=0) && (i < width*height*4));
}

function averageNeighbors(imageData, width, height, i) {
    var v = imageData[i];

    // główne kierunki
    var north = inRange(i-width*4, width, height) ? imageData[i-width*4] : v;
    var south = inRange(i+width*4, width, height) ? imageData[i+width*4] : v;
    var west = inRange(i-4, width, height) ? imageData[i-4] : v;
    var east = inRange(i+4, width, height) ? imageData[i+4] : v;

    // sąsiedzi po skosie
    var ne = inRange(i-width*4+4, width, height) ? imageData[i-width*4+4] : v;
    var nw = inRange(i-width*4-4, width, height) ? imageData[i-width*4-4] : v;
    var se = inRange(i+width*4+4, width, height) ? imageData[i+width*4+4] : v;
    var sw = inRange(i+width*4-4, width, height) ? imageData[i+width*4-4] : v;

    // średnia
    var newVal = Math.floor((north + south + east + west + se + sw + ne + nw + v)/9);

    if (isNaN(newVal)) {
        sendStatus("zła wartość " + i + " dla wysokości " + height);
        throw new Error("NaN");
    }
    return newVal;
}

function boxBlur(imageData, width, height) {
    var data = [];
    var val = 0;
    for (var i=0; i<width*height*4; i++) {
        val = averageNeighbors(imageData, width, height, i);
        data[i] = val;
    }

    return data;
}
