Write a program that plots a Sierpinski triangle, as illustrated below. Then develop a program that plots a recursive patterns of your own design.

Part 1.   The Sierpinski triangle is another example of a fractal pattern like the H-tree from Section 2.3. The pattern was described by Polish mathematician Waclaw Sierpinski in 1915, but has appeared in Italian art since the 13th century. Though the Sierpinski triangle looks complex, it can be generated with a short recursive program. Your task is to write a program Sierpinski.java with a recursive function sierpinski() and a main() function that calls the recursive function once, and plots the results using standard draw. Think recursively: your function should draw one black triangle (pointed downwards) and then call itself recursively 3 times (with an appropriate stopping condition). Also, when writing your program, exercise good design: include a (non-recursive) function triangle() that draws an equilateral triangle of a specified size at a specified location.

Your program will read in a command line parameter N to control the depth of the recursion. First, make sure that your program draws a single black triangle when N is set to 1. Then, check that it draws four black triangles when N is set to 2. Your program will be nearly (or completely) debugged when you get to this point. Below are the target Sierpinski triangles with N set to 1, 2, 3, 4, 5 and 6, respectively.

A diversion: fractal dimension.   In grade school, we learn that the dimension of a line segment is one, the dimension of a square is two, and the dimension of a cube is three. But we rarely learn what we mean by dimension. How can we express what it means mathematically or computationally? Formally, we can define the Hausdorff dimension or similarity dimension of a self-similar figure by partitioning the figure into a number of self-similar pieces, of smaller size. We define the dimension to be the log (# self similar pieces) / log (scaling factor in each spatial dimension). For example, we can decompose the unit square into 4 smaller squares, each of side length 1/2; or we can decompose it into 25 squares, each of side length 1/5. Here, the number of self-similar pieces is 4 (or 25) and the scaling factor is 2 (or 5). Thus, the dimension of a square is 2 since log (4) / log(2) = log (25) / log (5) = 2. We can decompose the unit cube into 8 cubes, each of side length 1/2; or we can decompose it into 125 cubes, each of side length 1/5. Therefore, the dimension of a cube is log(8) / log (2) = 3.

We can also apply this definition directly to the (set of white points in) Sierpinski triangle. We can decompose the unit Sierpinski triangle into 3 Sierpinski triangles, each of side length 1/2. Thus, the dimension of a Sierpinski triangle is log (3) / log (2) = 1.585. Its dimension is fractional - more than a line segment, but less than a square! With Euclidean geometry, the dimension is always an integer; with fractal geometry, it can be a fraction. Fractals are widely applicable for describing physical objects like the coastline of Britain.

Part 2.   In this part you will design and create your own recursive fractal. Your job is to write a program Art.java that takes a command line parameter N that controls the depth of recursion (typical values between 0 and 7) and produces a recursive pattern of your own choosing. You are free to choose a geometric pattern (like Htree or Sierpinski) or a random construction (like Brownian). Originality and creativity in the design will be a factor in your grade.

Submission.   Submit the files: Sierpinski.java, Art.java, and readme.txt file.