void setPixel(int x, int y, int level) {
		if (x < 0 || x >= width) {
			return;
		}
		if (y < 0 || y >= height) {
			return;
		}
		if (level < 0 || level >= grayLevel) {
			throw new IllegalArgumentException("level=" + level + " grayLevel=" + grayLevel);
		}
		pixels[y][x] = level;
	}