public Iterator<Pixel> sample(int initX, int initY, int dx, int dy) {
if (dx < 0 || dy < 0) {
throw new IllegalArgumentException("Dx or Dy cannot be negative");
} else if (initX < 0 || initX > width) {
throw new IllegalArgumentException("initX must be within the picture");
} else if (initY < 0 || initY > height) {
throw new IllegalArgumentException("initY must be within the picture");
}

Iterator<Pixel> iter = new SampleIterator(this, initX, initY, dx, dy);
return iter;
}

public Iterator<SubPicture> window(int windowWidth, int windowHeight) {
if (windowWidth < 0 || windowHeight < 0) {
throw new IllegalArgumentException("windowWidth or windowHeight cannot be negative");
} else if (windowWidth > width || windowHeight > height) {
throw new IllegalArgumentException("windowWidth and windowHeight must be smaller or equal to picture width and height");
}

Iterator<SubPicture> iter = new WindowIterator(this, windowWidth, windowHeight);
return iter;
}

public Iterator<SubPicture> tile(int tileWidth, int tileHeight) {
if (tileWidth < 0 || tileHeight < 0) {
throw new IllegalArgumentException("tileWidth or tileHeight cannot be negative");
} else if (tileWidth > width || tileHeight > height) {
throw new IllegalArgumentException("tileWidth and tileHeight must be smaller or equal to picture width and height");
}

Iterator<SubPicture> iter = new TileIterator(this, tileWidth, tileHeight);
return iter;
}

public Iterator<Pixel> zigzag() {
Iterator<Pixel> iter = new ZigzagIterator(this);
return iter;
}