1. Week 8 - Monday
CS 121

2. Last time
What did we talk about last time?
StdAudio

3. Questions?

4. Project 3

5. StdDraw

6. StdDraw
StdDraw is a library of Java code developed by Robert Sedgewick and Kevin Wayne
StdDraw allows you to draw output on the screen easily
You can draw points, lines, and polygons in various colors
You can clear and resize the drawing area and even save the results
StdDraw is not standard Java that everyone uses, but it’s a nice tool for graphics

7. Lines and Points

8. Lines and points (0,1) (1,1) (0,0) (1,0)
The simplest things you can draw with StdDraw are lines and points
The first thing you should be aware of is that the canvas is drawn like Quadrant I of a Cartesian plane
(0,1)
(1,1)
(0,0)
(1,0)

9. Line and point methods
The following methods can be used to draw lines and points
Method
Use
void line(double x0, double y0, double x1, double y1)
Draw a line from (x0,y0) to (x1,y1)
void point(double x, double y)
Draw a point at (x,y)

10. Line example
Let’s draw a box then divide it into two halves, like so:

11. Circles and Polygons

12. Who wants to take all that time to make a square?
There are built in commands for drawing:
Circles
Squares
Arbitrary polygons
Filled versions of each one of these
We won’t bother with the arbitrary polygons
It is also possible to set the color

13. Shape methods
Here are some methods for drawing circles and squares and setting the color for doing so:
Method
Use
void circle(double x, double y, double r)
Draw a circle centered at (x,y) with radius r
void filledCircle(double x, double y, double r)
Draw a filled circle centered at (x,y) with radius r
void square(double x, double y, double r)
Draw a square centered at (x,y) with edges 2r
void filledSquare(double x, double y, double r)
Draw a filled square centered at (x,y) with edges 2r
void setPenColor(Color c)
Start drawing with color c

14. Colors Eventually you will be able to define your own colors
For now you are limited to 13 presets
For example, to make something magenta, you would use the value StdDraw.MAGENTA
BLACK
BLUE
CYAN
DARK_GRAY
GRAY
GREEN
LIGHT_GRAY
MAGENTA
ORANGE
PINK
RED
WHITE
YELLOW

15. Screen saver?
Let’s write some code for making 100 circles at random locations with random sizes and random colors
Location is easy
Size is easy, we just decide on the range of sizes we want and do some math
Color is more painful
We need a switch statement with 13 choices

16. What about a chessboard?
We just want to make a pattern of black and white squares on the screen
Hint: We need two loops

17. Controlling the Draw Area

18. Controls
A number of methods are given to give us more control over the display
Method
Use
void setXscale(double x0, double x1)
Set the x scale
void setYscale(double y0, double y1)
Set the y scale
void setPenRadius(double r)
Set the pen radius
void setCanvasSize(int w, int h)
Set canvas size
void clear()
Clear canvas to white
void clear(Color c)
Clear canvas to color c
void pause(int delay)
Delay for delay ms

19. Scales
As you have seen, the default scale of the canvas is in the range [0,1] for both x and y
We can use the setXscale() method to set the minimum and maximum x values
We can use the setYscale() method to set the minimum and maximum y values
Useful for plotting functions

20. Canvas size
Note that changing the scale doesn’t change the size of the window, just what is shown in it
If you want to change the size of the window, use the setCanvasSize() method to set the width and the height of the canvas in terms of screen pixels

21. What is this pause() thing?
The pause() method lets you specify a delay in milliseconds before things are drawn on the screen
You can use it to slow down or speed up animations

22. Applications

23. Cannon simulator Can we simulate a cannon being fired?
Let the user enter an initial velocity in m/s
Let the user an angle between 0° and 90°
Assume each iteration takes 1/10 of a second
Assume an initial height of 20 m
We draw the path of the cannon ball as it flies through the air
Let’s also set the x and y scales to both be [0,100]

24. Plotting functions Plotting functions is really useful
Getting smooth curves is hard
Instead, we just pick a whole bunch of x points and figure out the function value
We can just draw dots to plot those values
We can connect them with lines for a more connected look
Let’s write some code to draw cubic polynomials

25. Function plotting algorithm
Ask the user for the coefficients of the four terms (ax3 + bx2 + cx + d)
Ask the user for an x range
Run through the function and find the minimum and maximum y values hit
Rescale the drawing area to show everything
Plot the function

26. Upcoming

27. Next time…
Static methods

28. Reminders Read Chapter 8 of the textbook
Get an early start on Project 3
It's harder than the previous two!