Make it Walk

So far your sprites have been a single frozen picture sliding around. In this chapter they start to move their legs.

Animation sounds complicated and is not: it is two pictures swapped over and over. Once you have seen that, the rest of the chapter is about letting Scratch for Java do the swapping for you.

Nothing here needs a downloaded file.

Step 1: Two costumes

A sprite can hold as many costumes as you like. Give it two, and it starts with the first one.

import org.openpatch.scratch.*;
 
public class Walker extends Sprite {
  public Walker() {
    this.addCostume("alienGreen_walk1");
    this.addCostume("alienGreen_walk2");
    this.setSize(60);
  }
}

Put it on a stage the usual way and you will see the alien standing still — it is wearing costume one and never changes.

Step 2: Swap them by hand

nextCostume() moves to the following costume and wraps back to the first at the end. It is the Scratch block of the same name:

next costume

If you called it in run() it would swap sixty times a second, which is far too fast to see. So it needs a timer: everyMillis(200) is true once every fifth of a second and false the rest of the time.

  public void run() {
    if (this.getTimer().everyMillis(200)) {
      this.nextCostume();
    }
  }

The alien is walking on the spot. That is all animation is. Try changing 200 to 50 and to 600 and watch what happens.

Step 3: Let the library do it

Two costumes and one timer is fine. Six costumes, plus a jump, plus a standing pose, and your run() fills up with counting.

AnimatedSprite is a Sprite that already knows how to do this. You give the animation a name and tell it which costumes belong to it:

this.addAnimation("walk", "alienGreen_walk%d", 2);

The %d is a placeholder for a number. This one line means "the animation called walk is made of alienGreen_walk1 and alienGreen_walk2" — the library fills the number in for you, which is why the built-in costumes are named the way they are.

Step 4: Walk when you walk

Now put movement and animation together. The alien should move its legs only while it is actually going somewhere, and stand still otherwise.

import org.openpatch.scratch.*;
 
public class Walker extends AnimatedSprite {
  public Walker() {
    this.addCostume("alienGreen_stand");
    this.addAnimation("walk", "alienGreen_walk%d", 2);
    this.setAnimationInterval(150);
    this.setSize(60);
    this.setRotationStyle(RotationStyle.LEFT_RIGHT);
    this.setY(-50);
  }
 
  public void run() {
    boolean walking = false;
 
    if (this.isKeyPressed(KeyCode.RIGHT)) {
      this.setDirection(90);
      this.move(3);
      walking = true;
    }
    if (this.isKeyPressed(KeyCode.LEFT)) {
      this.setDirection(-90);
      this.move(3);
      walking = true;
    }
 
    if (walking) {
      this.playAnimation("walk");
    } else {
      this.switchCostume("alienGreen_stand");
    }
 
    this.ifOnEdgeBounce();
  }
}

And a stage to put it on:

import org.openpatch.scratch.*;
 
public class WalkStage extends Stage {
  public WalkStage() {
    super(500, 260);
    this.addBackdrop("background");
    this.add(new Walker());
  }
 
  public static void main(String[] args) {
    new WalkStage();
  }
}

Three things are new here:

• extends AnimatedSprite instead of extends Sprite. It can still do everything a sprite can — move, setDirection, ifOnEdgeBounce are all there — it simply knows about animations as well.
• boolean walking remembers whether either key was held this frame, so the decision about which costume to wear is made once, after both checks.
• setAnimationInterval(150) is the same idea as the 200 in step 2: how long each picture stays on screen.

The finished project

If something will not run, you can compare against the finished version. It needs no assets, only the library itself.

Things to try

• Give the alien a third state. alienGreen_jump is a costume; show it while the alien is off the ground.
• Swap the alien for a different character. The built-in sprites page has walk cycles for bunny1_walk%d, alienBlue_walk%d and several others — the pattern is always the same.
• Make the alien walk faster and animate faster when you hold shift.