sub tick {
    my ($self,$texture_id) = @_;

    # save current frame from input fbo
    glBindTexture(GL_TEXTURE_2D, $texture_id);
    my $frame;
    for my $i (reverse (0..$self->height-1)) {
        glReadPixels_s(0,$i,$self->width,1,GL_RGB,GL_UNSIGNED_BYTE,my $line);
        $frame .= $line;
    };
    syswrite $self->stream, $frame, $self->width * $self->height * $self->depth
        or die "Write failure";
};