Re: Animating sine waves

Thomas Fritsch <>
Sun, 20 Jan 2008 17:18:23 +0100
Albert wrote:

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.

[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
<> 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>

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

(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 (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
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
that line (you remembered to switch your desk-calculator to radians),
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 */


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