This assignment is due before 11:00PM on Friday, November 3, 2017. There are two parts to this homework assignment:
You’ll need to submit your solutions to both parts of the homework before the deadline.
Collaboration policy.
Seam-carving is a content-aware image resizing technique where the image is reduced in size by one pixel of height (or width) at a time. A vertical seam in an image is a path of pixels connected from the top to the bottom with one pixel in each row; a horizontal seam is a path of pixels connected from the left to the right with one pixel in each column. Below left is the original 505-by-287 pixel image; below right is the result after removing 150 vertical seams, resulting in a 30% narrower image. Unlike standard content-agnostic resizing techniques (such as cropping and scaling), seam carving preserves the most interest features (aspect ratio, set of objects present, etc.) of the image.
Although the underlying algorithm is simple and elegant, it was not discovered until 2007. Now, it is now a core feature in Adobe Photoshop and other computer graphics applications.
In this assignment, you will create a data type that resizes a W-by-H image using the seam-carving technique. Finding and removing a seam involves three parts and a tiny bit of notation:
In image processing, pixel (x, y) refers to the pixel in column x and row y, with pixel (0, 0) at the upper-left corner and pixel (W − 1, H − 1) at the lower-right corner. This is consistent with the Picture data type in algs4.jar
.
a 3-by-4 image
(0, 0) (1, 0) (2, 0)
(0, 1) (1, 1) (2, 1)
(0, 2) (1, 2) (2, 2)
(0, 3) (1, 3) (2, 3)
Warning: this is the opposite of the standard mathematical notation used in linear algebra, where (i, j) refers to row i and column j and (0, 0) is at the lower-left corner.
We also assume that the color of each pixel is represented in RGB space, using three integers between 0 and 255. This is consistent with the java.awt.Color data type.
The first step is to calculate the energy of a pixel, which is a measure of its importance—the higher the energy, the less likely that the pixel will be included as part of a seam (as you will see in the next step). In this assignment, you will use the dual-gradient energy function, which is described below. Here is the dual-gradient energy function of the surfing image above:
The energy is high (white) for pixels in the image where there is a rapid color gradient (such as the boundary between the sea and sky and the boundary between the surfing Josh Hug on the left and the ocean behind him). The seam-carving technique avoids removing such high-energy pixels.
The next step is to find a vertical seam of minimum total energy. (Finding a horizontal seam is analogous.) This is similar to the classic shortest path problem in an edge-weighted digraph, but there are three important differences:
Seams cannot wrap around the image (e.g., a vertical seam cannot cross from the leftmost column of the image to the rightmost column).
The final step is remove from the image all of the pixels along the vertical or horizontal seam. The SeamCarver API. Your task is to implement the following mutable data type:
Your code must throw an exception when a constructor or method is called with an invalid argument, as documented below:
java.lang.IllegalArgumentException
if the constructor, removeVerticalSeam()
, or removeHorizontalSeam()
is called with a null argument.java.lang.IllegalArgumentException
if either removeVerticalSeam()
or removeHorizontalSeam()
is called with an array of the wrong length or if the array is not a valid seam (either an entry is outside the height/width bounds or two adjacent entries differ by more than 1).
Throw a java.lang.IllegalArgumentException
if either removeVerticalSeam()
or removeHorizontalSeam()
is called when the width or height of the current picture is 1, respectively.The data type may not mutate the Picture argument to the constructor.
You will use the dual-gradient energy function: The energy of pixel is , where the square of the x-gradient , and where the central differences are the differences in the red, green, and blue components between pixel (x + 1, y) and pixel (x − 1, y), respectively. The square of the y-gradient is defined in an analogous manner. To handle pixels on the borders of the image, calculate energy by defining the leftmost and rightmost columns as adjacent and the topmost and bottommost rows as adjacent. For example, to compute the energy of a pixel (0, y) in the leftmost column, use its right neighbor (1, y) and its “left” neighbor (W − 1, y).
As an example, consider the 3-by-4 image (supplied as 3x4.png) with RGB values—each component is an integer between 0 and 255—as shown in the table below:
RGB values and dual-gradient energies of a 3-by-4 image
yielding ; and pixels (1, 1) and (1, 3) for the y-gradient
yielding
Thus, the energy of pixel (1, 2) is Similarly, the energy of pixel (1, 1) is .
yielding and pixels (1, 3) and (1, 1) for the y-gradient
yielding Thus, the energy of pixel (1, 0) is .
The findVerticalSeam()
method returns an array of length H such that entry y is the column number of the pixel to be removed from row y of the image. For example, the dual-gradient energies of a 6-by-5 image (supplied as 6x5.png) are shown in the table below.
The minimum energy vertical seam is highlighted in blue. In this case, the method findVerticalSeam()
returns the array { 3, 4, 3, 2, 2 } because the pixels in the minimum energy vertical seam are (3, 0), (4, 1), (3, 2), (2, 3), and (2, 4). Remember that seams cannot wrap around the image. When there are multiple vertical seams with minimal total energy, your method can return any such seam.
Finding a horizontal seam. The behavior of findHorizontalSeam()
is analogous to that of findVerticalSeam()
except that it returns an array of length W such that entry x is the row number of the pixel to be removed from column x of the image. For the 6-by-5 image, the method findHorizontalSeam() returns the array { 2, 2, 1, 2, 1, 2 } because the pixels in the minimum energy horizontal seam are (0, 2), (1, 2), (2, 1), (3, 2), (4, 1), and (5, 2).
The width()
, height()
, and energy()
methods must take constant time in the worst case. All other methods must run in time proportional to W H (or better) in the worst case.
Submit SeamCarver.java
and SeamCarverTest.java
. You may not call any library functions other than those in java.lang, java.util, java.awt.Color, and algs4.jar.
The goals of this assignment are to test your understanding of the material covered in sections 4.1-4.4 of the textbook, and the lecture and recitation materials. You should read the textbook chapters before doing this part of the assignment.
Written homeworks must be typeset in LaTeX and submitted in PDF format. Please insert a page break between each question, so that your answer to each question starts on a new page in your PDF document.
Analysis of running time. Estimate empirically the running times (in seconds) to remove one row and one column from a W-by-H image as a function of W, and H. Use tilde notation to simplify your answer. Both should be functions of W and H. Removal should involve exactly one call to the appropriate find method and one call to the appropriate remove method.
Show your data. Justify your answer to part 1 with sufficient data using large enough W and H values. To dampen system effects, you may wish to perform many trials for a given value of W and H and average the results.
A former senator from Pennsylvania is really interested in a particular kind of graph. He wants to color each node in a graph as either pink or blue. He is very concerned that the edges in the graph should only connect pink nodes to blue nodes, and that no edges connect a blue node to another blue node, or a pink node to another pink node. He’s fine if a blue node connects to multiple pink nodes. He knows one blue node that has connected with at least three pink nodes, and there have been reports that it has connected to many other pink nodes. In a sign of surprising progressiveness, he’s also OK with a pink connecting with multiple blues. Write an algorithm to check a graph for the former senator, so that it tells him whether it conforms to his constraints or not.
Prove that your algorithm is correct and give its running time. Senators love efficiency.
Give a proof or a counterexample for the following assertion: An edge-weighted graph has a unique MST only if its edge weights are distinct.
Given the graph above, show what the correct shortest path tree should look like, starting at vertex 0. Would Dijkstra’s algorithm produce the correct shortest path tree for the graph? Why or why not?
You could convert a graph with negative edge weights to one that has only positive weights by adding the value of the most negative edge weight to all edges in the graph. Would this cause Dijkstra’s algorithm to find the correct shortest path tree? Prove your answer.
Explain how the Bellman-Ford algorithm is able to find the shortest path even when there are negative weights. When does Bellman-Ford fail to output the shortest path tree?
You can topologically sort a DAG by repeatedly finding a node that doesn’t have any incoming edges, and then removing it and all of its outgoing edges from the graph.
Create an algorithm using this idea with running time O(V + E).
Would your algorithm work if the graph was cyclic? Why or why not?