# Animating sine waves

Discussion in 'Java' started by Albert, Jan 18, 2008.

1. ### AlbertGuest

across this example from http://www.javaworld.com/jw-03-1996/animation/Example4Applet.html

I'm having a lot of trouble understanding what is going on (that is,
the maths behind it all). Now I know sine as sin in trigonometry
problems and I know that the applet does indeed display the sine wave
as would be plotted on a graph but what's with multiplying by such
tiny numbers and adding 1, adding or subtracting the frame to the
iterator?

More related to java, what's a dimension and how does it take the
size() method?

Albert, Jan 18, 2008

2. ### Thomas FritschGuest

A Dimension is has a width and a height. In your applet example the
size() method gives the dimension of the entire applet, i.e. width=500
and height=50. You should look into the HTML code of Example4Applet.html
to see how it comes to exactly these width and heigth.

You can look into the Java documentation at
<http://java.sun.com/j2se/1.5.0/docs/api/>,
especially the links given there to "Dimension", "Math", and "Graphics".
From there you will find the explanations of every single function,
like "sin" (on the "Math" page) and "size" (on the "Dimension" page).

Thomas Fritsch, Jan 18, 2008

3. ### Thomas FritschGuest

--little correction--:
.... "width" and "height" (on the "Dimension" page), "size" (on the
"Applet" page).

Thomas Fritsch, Jan 18, 2008
4. ### AlbertGuest

I understand what the function is, I can look that up in any maths
book but I don't understand how the developer is using it.

Albert, Jan 18, 2008
5. ### Thomas FritschGuest

[I don't know your current knowledge-level of Java or ComputerScience in
general. Therefore some of the hints below may or may not seem trivial
to you.]

Some facts to know beforehand:
(*) In Java the origin (x=0,y=0) is in top-left corner.
x runs to the right, y runs to the bottom.
(*) In Java Math.sin(...) expects the angle in radians, not in degrees.

For convenience I quote the Java code from
<http://www.javaworld.com/jw-03-1996/animation/Example4Applet.html> here:
public void paint(Graphics g) {
Dimension d = size();
int h = d.height / 2;
for (int x = 0 ; x < d.width ; x++) {
int y1 = (int)((1.0 + Math.sin((x - frame) * 0.05)) * h);
int y2 = (int)((1.0 + Math.sin((x + frame) * 0.07)) * h);
g.drawLine(x, y1, x, y2);
}
}
and the HTML code
<applet code=Example4Applet.class width=500 height=50>
<param name=fps value=10>
</applet>

A strategy for understanding an algorithm is stepping through it with
pencil and paper (and a desk-calculator for the sine) as if you were the
computer.

(1) In the HTML code above note the applet parameter: fps=10 (by the
way: "fps" probably means "frames per second").

(2) Now look into the complete Example4Applet.java (into method
"init()"). There the "fps" applet-parameter is used to calculate a
variable "delay". You get delay = 20 (Repeat that calculation fo yourself!)

(3) Now look into method "run()" of Example4Applet.java.
There the "delay" variable is used to increment the variable "frame"
(which initially had the value 0) by 1 every 20 milliseconds.

(4) Proceed to method "paint(Graphics)". Now your paper-work begins:
Step through the paint method line by line; do the the calculation of
write down the calculated value of the variables.
Your sheet of paper might look like this:
d = Dimension(width=500,height=50)
h = 50/2 = 25
frame = 0
/* 1st iteration of the for-loop */
x = 0
y1 = (int)((1.0 + Math.sin((0 - 0) * 0.05)) * 25) = 1
y2 = (int)((1.0 + Math.sin((0 + 0) * 0.07)) * 25) = 1
//draw a black line from (x|y1) to (x|y2)
/* 2nd iteration of the for-loop */
x = 1
y1 = (int)((1.0 + Math.sin((1 - 0) * 0.05)) * 25) = (int) 2.25 = 2
y2 = (int)((1.0 + Math.sin((1 + 0) * 0.07)) * 25) = (int) 2.75 = 2
//draw a black line from (x|y1) to (x|y2)
/* 3rd iteration of the for-loop */
.....

Thomas Fritsch, Jan 20, 2008