Week 1: Building Blocks

Platform games, like Manic Miner and Mario Bros, are all about the central figure moving around a world and bumping into things. Some things, like walls and floors, just stop you moving. Some things, like baddies, will kill you. Some things, like keys, you want to collect. Some things are just scenery and don’t affect the game at all.

That means that knowing when you’ve bumped into something (called collision detection) is important. Scratch gives you a few blocks for collision detection: <touching? [sprite]>, <touching color []?>, <color [] is touching []?>, and (distance to [sprite]). However, to make the game, you need to know more than just whether you’re touching something: you need to know which side is touching it. If you walk up to a wall on the left, you can’t go left any more. But you can still go right, or jump, or fall if there’s no floor.

Touching a baddie may hurt you; but if you touch it with your feet, you may hurt the baddie. None of the built-in Scratch blocks tell you about direction of contact. So we’ve got to build our own collision detectors.

Task 1: Collision detectors

We’ll have four sprites that follow Frantic Felix around. Each will detect collisions in one direction and set a variable if it’s detected one. The main Felix sprite will use these variables to control how Felix can move. Each collision detector will have a bar of colour and use the <color [] is touching [] ?> block to detect things. We’ll use black for the floor and green for obstacles. We’ll use red for the collision detectors.

Task 2: Walking, floors and falling

• Next is to make Felix walk. We’ll use the left and right arrow keys to move him left and right. If he’s not supported by a floor, he’ll fall down. (We’ll do jumping next.) If the left arrow key is pressed, we want Felix to point left and move left a bit. But we also want him not to move left if there’s an obstacle there.
• We could use the <when [key] pressed> hats to make Felix move, but that gives a jerky motion. Things move more smoothly if you put <if key [left arrow] pressed?> blocks in forever loops. Collision detection means we need to include the test of the blocked left variable in the if, using an and block so that Felix only moves left if the key is pressed and left blocked is zero. We need to do the same for moving right.
• We could animate Felix’s legs in the same block, but they end up moving too fast. Do the animation in a separate forever block under another when green flag clicked hat.

• The final thing to do is falling. If there’s no solid ground under Felix, we want him to fall. That’s another forever loop under another green flag hat.

  when FLAG clicked //handle moving
      forever
          if <<key [left arrow v] pressed?> and <(blocked left)=(0)>> then
              point in direction (-90 v)
              move (2) steps
          end
  
  
          if <<key [right arrow v] pressed?> and <(blocked left)=(0)>> then
              point in direction (90 v)
              move (2) steps
          end
  
  
  when FLAG clicked //[animate Felix]
      forever
          if <<key [left arrow v] pressed?>or<key [right arrow v] pressed?>> then
              next costume
              wait (0.1) secs
          end
      end
  
  when FLAG clicked //handle falling
      forever
          if <(blocked bottom)=(0)> then
              change  y by (-2)
          end
      end

• We also want the collision detectors to disappear. We can’t just use a hide block, because then they won’t detect any collisions. Instead, put a set [ghost] effect to 100 right under the green flag hat in each collision detector. That makes the sprite invisible without hiding it.

Task 3: Jumping

The final part of making Felix move is jumping. Let’s have the space key make Felix jump. There are a few wrinkles to jumping:

1. We don’t want Felix to fall if he’s going up.
2. We don’t want Felix to be stopped by floors when he’s going up, but we still want him to be stopped by floors when he’s coming down.
3. We don’t want Felix to jump up into the underside of a green obstacle.
4. We want the jump to be animated, so we don’t want Felix to instantly move up a large distance.
5. Felix should only be able to jump if he’s standing on a floor. The game would be too easy if Felix could jump off thin air.

• We’ll control jumping with a new variable, height to jump. If this is greater than zero, Felix is going up. If it’s zero, Felix is falling (or has fallen), as per the previous stage.
• We’ll make Felix jump up 100 pixels, all things being equal. Add another if block inside the forever block that handles key presses. If we press the space key and Felix is standing on a floor (that is blocked below is set to 1), set height to jump to 100.
• We want to change the falling script. Inside the forever, we want an if-else block that detects if Felix is jumping up or not. The condition for this if-else block is whether height to jump is greater than zero. The existing falling if block goes in the else part of the new if-else block.
• When we know Felix is jumping up, we need to check if he’s bumped his head against something. If blocked top is 1, set height to jump to zero. (This stops Felix jumping up into obstacles). Otherwise, move Felix up by ten and reduce height to jump by ten.

• You should end up with this:

  when FLAG clicked //handle falling
      forever
          if <(height to jump)>(0)> then
              if <(blocked top)=(1)> then
                  set [height to jump v] to (0)
              else
                  change y by (10)
                  change [height to jump v] by (-10)
              end
          else
              if <(blocked bottom)=(0)> then
                  change y by (-2)
              end
          end
      end

Task 4: Keys and goals

We’ve got Felix moving around the world. Now to make him finish a level. We’ll put three keys around the cavern. Felix collects a key by touching it. When he’s collected all three, he’ll be able to climb into an escape pod and flee!

• We’ll have a new variable, keys to get, which tracks how many keys there are still to get. A new script on the stage should set this to 3 when the green flag is clicked.
• The keys and the escape pod will each be sprites. (Use frantic-felix/key for the keys and frantic-felix/escape-pod for the pod.)
• Each key needs two scripts: one places the key in the right position, with the size and angle, then has a forever loop to change the colour (that just makes it easy to see on the screen). The other script is a forever if loop that waits until Felix touches the key. As soon as that happens, the script hides the key and decrements keys to get.

• The escape pod has a slightly more complex script. It uses a forever if block to wait for keys to get to become zero. As soon as that happens, the pod starts flashing (to show the player that they can escape). Then we can use another if to detect when Felix touches the flashing escape pod. As soon as he does, the pod broadcasts a win message and says “You win!”. Felix responds to the win message by hiding.

  when FLAG clicked
      go to x:(220) y:(-125)
      forever
          if <(key to get) = (0)> then
              change [color v] effect by (25)
              if <touching [Felix v]?> then
                  broadcast [win v]
                  say [You win!]
              end
          end
      end

Task 5: Baddies and deadly scenery

Now, baddies!

We’ll have two types of dangerous things. One type will be baddies that wander around and hurt Felix if he bumps into them. The other type will be bits of dangerous scenery.

Let’s do the roving baddie first. It will just move along a fixed path.

Activity Checklist

• Create a new sprite, using whatever costume you like. Make sure it’s about the same size as Felix (We used the things/flower-vase costume, edited to be made smaller by four steps). The baddie needs just a single script that both moves it and detects when it’s touching Felix.

• Put three if statements inside a forever loop. The first if checks whether the baddie is touching Felix; if it is, it broadcasts the lose message. The other two if’s check whether the baddie has reached the end of its path; if it has, the baddie turns round. Finally, the baddie takes two steps. (Using move instead of glide blocks makes it easier to control how fast the baddie goes.) We don’t need to use the collision detector sprites here, as we don’t care which side of Felix touches the baddie.

  when FLAG clicked
      go to x:(-50) y:(47)
      point in direction (-90 v)
      forever
          if <touching [Felix v]?> then
              broadcast [lose v]
          end
          if <(x position) < (-200)> then
              point in direction (90 v)
          end
          if <(x position) > (-50)> then
              point in direction (-90 v)
          end
          move (2) steps
      end

• Add scripts to both Felix and the escape pod to respond to the lose message. Felix should just hide in response. The pod should say [You lose!].

Week 2: Designing levels

Last week, you built all the parts of a platform game. This week, you’ll use those pieces to make your own levels.

To recap, you made several things happen:

1. Felix can walk left and right and jump.
2. Felix falls unless a black floor stops him.
3. Felix can’t move through green obstacles.
4. Blue bits of scenery and baddies kill Felix.
5. Baddies move around fixed patrol routes.
6. Baddies kill Felix if he touches them.
7. If Felix moves into a key, he collects it.
8. When Felix has all the keys, an escape pod lights up to take him to safety (or the next level).

Treat these bits as a toolkit. Use them to build your own levels.

You want to create a series of levels for Felix to go though. Next week, you’ll look at how to connect the levels. Levels can have large platforms or small ones, many platforms or few. There can be lots of baddies or none. There could be lots of obstacles or lethal bits of scenery. Don’t make the level too constrained; it should like there are a few routes to success, even if one is easier than others. Think about how hard or easy the level is.

You can change the special colours (black, green, and blue) but you need to update the colour touching blocks in all the scripts, but you need to keep the same colours in all the levels. (You could have different colours in different levels, but that would mean putting lots of or blocks around the colour-touching blocks.)

Test your levels. If you have time, create your levels in Scratch and play them. Check that they’re not too hard and not too easy. If you implement the levels, make sure to save the backgrounds you make and note the starting positions of Felix, they keys, and any baddies. Also make sure you note the directions the baddies move in and how far they go.

If you’ve already come up with a few levels, and put them in Scratch, try these extension tasks:

Extension: Stomping

How about making baddies die if they’re stomped on? Perhaps you could add a script to the baddies that does something if the baddie is touching the bottom collision detector sprite.

Extension: Power pills

You could create a “power pill” that allows Felix to destroy the baddies. When Felix collects the power pill, Felix destroys any baddie he touches. The effects wear off after a while.

It’s up to you to think about how to make that work. You might want to make the baddies look different when Felix can destroy them.

Week 3: Assembling the game

You’ve got some levels. You have enough tools to make each one work. Now all you have to do is put them all together into a whole game! If you’re able to put some levels together quickly, you may want to look at tasks 2 and 3 before playing your games.

Task 2: Play!

You’ve got games. Your fellow Code Clubbers have games. Play all the games! Can you beat theirs? Can they beat yours?

Extension: Multiple lives

Felix will probably need a few lives to try to get through the levels. Create a new sprite with three costumes, showing one, two, and three hearts.

Use the frantic-felix/1-heart, frantic-felix/2-heart, and frantic-felix/3-heart costumes. Make the sprite appear in the corner of the stage. When the green flag is clicked, it should show three hearts. Every time it receives a lose message, it should show one fewer heart. Instead of showing zero hearts, it should hide and send a game over message.

The game over and win messages should be handled by a new sprite, which hides when the green flag is clicked and shows the right banner when the game over and win messages are sent. This sprite should also stop all the scripts when it displays itself.

You’ll also need to modify how the escape pod handles winning and losing, as that is now the responsibility of the lives and finish banner sprites.

You may find that your baddies are a bit too quick on the mark, pouncing on Felix before the level properly restarts. If Felix loses more than one life when he bumps into a baddie. hide the baddie as soon as it sends the lose message. That gives the rest of the game (including Felix) to reset before the baddie detects another collision with Felix.

Extension: Time limits

Put Felix against the clock! Use the Timer Scratch card to add a time limit. If the timer runs out, send the lose message. Remember to reset the timer at the start of each level.