_emitLightFromCell(x, y, color, litCells) {
        let key = x + "," + y;
        let fov;
        if (key in this._fovCache) {
            fov = this._fovCache[key];
        }
        else {
            fov = this._updateFOV(x, y);
        }
        for (let fovKey in fov) {
            let formFactor = fov[fovKey];
            let result;
            if (fovKey in litCells) { /* already lit */
                result = litCells[fovKey];
            }
            else { /* newly lit */
                result = [0, 0, 0];
                litCells[fovKey] = result;
            }
            for (let i = 0; i < 3; i++) {
                result[i] += Math.round(color[i] * formFactor);
            } /* add light color */
        }
        return this;
    }
Compute one iteration from one cell