Creating a trail of moving object in Processing
--D. Thiebaut 08:41, 17 June 2012 (EDT)
This tutorial page explores various techniques for marking the path of animated objects in Processing. Many of the ideas and techniques presented here are taken from the Processing.org site, which remains one of the best resource for Processing. Don't hesitate to browse its pages for good ideas!
Contents
Unlimited Trail
Here we just start with the basic idea which we already played with in [Processing Skeleton Project and Simple Exercises | the previous tutorial in this section].
package tutorial1;
import processing.core.*;
public class Main4 extends PApplet {
public void setup() {
// define the window size, make graphics softer, and make
// the background white
size(600, 600);
smooth();
background( 0xeeeeff );
}
public void draw() {
// change color of circle paint depending on mouse button
if (mousePressed) {
stroke( 255 );
fill(0);
}
else {
stroke( 0 );
fill(255);
}
// draw a circle where the mouse is located
ellipse(mouseX, mouseY, 80, 80);
}
}
- Try it out!
- Let's play some with this simple example.
Growing and shrinking circles
- One way to make the circle grow and shrink as the animation goes on is to control its radii with a sine function whose angle is simply the number of frames since the beginning of the animation.
- The trick here relies on knowing that Processing counts the number of frames it displays and keeps the count in a variable named frameCount. The counter starts with 0, and every time draw() is called by Processing, frameCount is incremented by 1.
- Modify the program above, and add a new variable inside the draw() function:
float radius = 50 + 30 * sin( frameCount * 0.05f );
- Because the sin() function oscillates between -1 and 1 as the angle increases, radius will oscillate between 50 - 30, and 50 + 30, or 20 and 80.
- Change the call to ellipse() and replace the constant 80 by the variable radius.
// draw a circle where the mouse is located
ellipse(mouseX, mouseY, radius, radius);
- Try your new code!
Exercise #1 |
- Make the color change with the growing and shrinking of the circle. You may find the information in the processing page on color very useful!
- Keep one of the radius values constant, and the other one equal to the variable radius.
- Use two different variables, radius1 and radius2 which are initialized the same way radius is, but use the sin() function for one, and the cos() function for the other one. Use radius1 and radius2 as the two radii value in the ellipse() function...
Fading Trail
One way to make the trail disappear is to make whatever is drawn in the applet become more and more transparent as it ages. In this case, objects that have been drawn in the current frame are fully opaque, while objects that have been drawn in earlier frames become more and more transparent, to the point where they disappear.
One way to accomplish this is to paint a transparent rectangle over the whole applet before we draw new objects in the window. In this case we use these three lines:
noStroke();
fill( 0xee, 0xee, 0xff, 50);
rect(0, 0, width, height);
inside the draw() function, as the first thing we do. The magic number here is 50 which is the amount of transparency we give the whole applet before we draw a new circle.
The new draw() function looks something like this:
public void draw() {
//Fade everything which is drawn
noStroke();
fill( 0xee, 0xee, 0xff, 50);
rect(0, 0, width, height);
float radius = 50 + 30 * sin( frameCount * 0.05f );
// change color of circle paint depending on mouse button
if (mousePressed) {
stroke( 255, 255, 255 );
fill( 0,0,0 );
}
else {
stroke( 0, 0, 0 );
fill( 255, 255, 255 );
}
// draw a circle where the mouse is located
ellipse(mouseX, mouseY, radius, radius);
}
- Try it!
Exercise #2 |
- Change the value of the transparency (50) and try different values to see how they affect the applet and the trail.
- Try adding a transparency value to the circles, to see how they are affected. Example of a transparency of 100:
if (mousePressed) {
stroke( 255, 255, 255, 100 );
fill( 0,0,0, 100 );
}
(For reference, a transparency of 255 is fully transparent, while 0 is fully opaque.)
- Change the background color to white, or black, and, again, play with different values of transparencies to better understand how the applet is affected.
Limited Trail
In the limited trail case, we only draw a finite number of circles in the trail. Say 10. So, when we draw a new circle, we also draw the last 9 circles we drew last, and none of the others. So we need a data structure that will hold circles in a First In, First Out way. Processing supports ArrayList (as does Java). So we need to figure out what we need to remember for each circles. In our case, it's the x and y of the center of the ellipse, as well as its radii. The best way to do this is to create a circle class that will hold these values.
First, an Applet with A Circle Class
Study the code below and see how the new class encapsulate the circle information.
package tutorial1;
import processing.core.*;
/**
* CircleClass: a simple container to hold the location of
* the center of the circle, and the frameCount when it
* appears on the applet, as the frameCount is used to
* define the rate at which the circles "breathes".
*/
class CircleClass {
public float x;
public float y;
public int frameCount;
public void CircleClass( float xx, float yy, int fc ) {
x = xx;
y = yy;
frameCount = fc;
}
public void draw( PApplet p ) {
float radius1 = 50 + 30 * p.sin( frameCount * 0.05f );
float radius2 = 50 + 30 * p.sin( frameCount * 0.05f );
p.ellipse( x, y, radius1, radius2 );
}
}
/**
* Main3: the main Processing applet.
*
*/
public class Main3 extends PApplet {
CircleClass c;
/**
* setup(). Sets the applet up.
*/
public void setup(){
size(800,600); // Size of Background/panel
background( 0xeeeeff ); //Color of Background, Black
frameRate(30); //Frame rate set at 30
c = new CircleClass( );
}
/**
* draw(): called to draw every frame.
* draws a circle where ever the mouse is.
*/
public void draw() {
//Draw ellipses
stroke( 0 );
fill( 255 );
c.x = mouseX;
c.y = mouseY;
c.frameCount = frameCount;
c.draw( this );
}
}
- Try out the code, and play with it!
A Trail of the Last n circles
All we need to do now is to remember the last n (say 10) circles drawn, and every call to draw we erase the applet, and draw these n circles. To remember the circles, we store them in a queue (FIFO) implemented for right now with an ArrayList.
Paste the code below in a class called Main. Study the code. See the new import statements that introduce the ArrayList and the Iterator in the applet.
package tutorial1;
import java.util.ArrayList; // needed to
import java.util.Iterator;
import processing.core.*;
/**
* CircleClass: a simple container to hold the location of
* the center of the circle, and the frameCount when it
* appears on the applet, as the frameCount is used to
* define the rate at which the circles "breathes".
*/
class CircleClass {
public float x;
public float y;
public int frameCount;
CircleClass( float xx, float yy, int fc ) {
x = xx;
y = yy;
frameCount = fc;
}
public void draw( PApplet p ) {
float radius1 = 50 + 30 * p.sin( frameCount * 0.05f );
float radius2 = 50 + 30 * p.sin( frameCount * 0.05f );
p.ellipse( x, y, radius1, radius2 );
}
}
/**
* Main3: the main Processing applet.
*
*/
public class Main extends PApplet {
ArrayList circles;
/**
* setup(). Sets the applet up.
*/
public void setup(){
size(800,600); // Size of Background/panel
background( 0xeeeeff ); //Color of Background, Black
frameRate(30); //Frame rate set at 30
circles = new ArrayList();
}
/**
* draw(): called to draw every frame.
* draws a circle where ever the mouse is.
*/
public void draw() {
// erase the applet
background( 0xeeeeff );
// add a new circle to the end of the list
circles.add( new CircleClass( mouseX, mouseY, frameCount ) );
// remove oldest (first) circle if more than 10
if ( circles.size() > 10 )
circles.remove( 0 );
// display the circles
stroke( 0 ); // black outline
fill( 255 ); // white interior
for ( Iterator< CircleClass > it = circles.iterator(); it.hasNext(); ) {
CircleClass c = it.next();
c.draw( this );
}
}
}
- Run the code and see how we now have a trail of 10 circles.
Exercise #3 |
- Change the number of circles kept in the trail. Try large numbers, such as 100, or 500.
- Modify the program so that a transparency value is added to the color of each circle (fill and stroke), and make the transparency of the most recent (last) circle fully opaque (0), and the transparency of the oldest one (at Index 0), 255. You may find that looping through the circles in reverse order makes this simpler:
for ( int i = circles.size()-1; i >= 0; i = i - 1 ) {
CircleClass c = circles.get( i );
...
}
Exercise #4 |
- Instead of having the leading circle follow the mouse, make it move randomly, in a manner similar to what we explored in
Exercise #4 |
- Are you ready for a bit of challenge? Try keeping track of 2 circles, each with a trail of 10 circles...
- When you're done, try keeping track of 20 circles, each with a trail of 10 circles...
A More Efficient Trail
The previous example is fine for short trail, but ArrayList are not efficient data structures when we remove their 0th element: they shift all the remaining elements by one, which is an O(N) operation. If the trail is long, say 10,000 element, then removing the 0th one every call to the draw() function will take 10,000 steps.
The queue, which is an interface to the LinkedList collection in Java is a better suited data-structure, with an O(1) add() and remove() operation (remove() is called poll() for queues).
Below is the same example as above, implemented with queues:
package tutorial1;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Queue;
import processing.core.*;
/**
* Main3: the main Processing applet.
*
*/
public class Main5 extends PApplet {
/**
* CircleClass: a simple container to hold the location of the center of the
* circle, and the frameCount when it appears on the applet, as the
* frameCount is used to define the rate at which the circles "breathes".
*/
class CircleClass {
public float x;
public float y;
public int frameCount;
CircleClass(float xx, float yy, int fc) {
x = xx;
y = yy;
frameCount = fc;
}
public void draw(PApplet p) {
float radius1 = 50 + 30 * p.sin(frameCount * 0.05f);
float radius2 = 50 + 30 * p.sin(frameCount * 0.05f);
p.ellipse(x, y, radius1, radius2);
}
}
// member variables
Queue circles;
int maxTrail = 10; // number of circles in the trail
/**
* setup(). Sets the applet up.
*/
public void setup() {
size(800, 600); // Size of Background/panel
background(0xeeeeff); // Color of Background, Black
frameRate(30); // Frame rate set at 30
circles = new LinkedList();
}
/**
* draw(): called to draw every frame. draws a circle where ever the mouse
* is.
*/
public void draw() {
// erase the applet
background(0xeeeeff);
// add a new circle to the end of the list
circles.add(new CircleClass(mouseX, mouseY, frameCount));
// remove oldest (first) circle if more than 10
if (circles.size() > maxTrail)
circles.poll();
// display the circles
stroke(0); // black outline
fill(255); // white interior
for (Iterator<CircleClass> it = circles.iterator(); it.hasNext();) {
CircleClass c = it.next();
c.draw(this);
}
}
}
- Note that I have made the CircleClass local to the Main class. This way CircleClass does not conflict in Eclipse with other CircleClass definitions that may exist in different versions of my projects.
Exercise #5 |
- Repeat Exercise #4, but with a queue data-structure to keep track of the trail. You may still want to use an ArrayList to keep track of the several circles with trails...
