Space Race III

Review — where we got to 5 min

The race has a countdown nobody can beat. Two ideas from much earlier come back today.

• when I start as a clone and delete this clone, from Topic 1.10.
• A falling clone from a random x along the top — the catching game’s whole shape.

Quick-fire

Your race is won by whoever holds their key hardest. Why is that not much of a game?

Today’s Topic 3 min

• Three keys per player: forward, up and down
• Asteroids as clones drifting down across both lanes
• Why the touching check belongs on the rocket, not on the asteroid
• A star that pays instead of costing

Learning outcome

By the end of this lesson you will be able to:

1. Give a player two-way steering as well as forward.
2. Fill a Stage with clones that appear and clean themselves up.
3. Say why a collision check belongs on one sprite rather than the other.

Learnt — steering, rocks, stars 28 min

1 · Two more keys

A lane was one key. To dodge, a player needs to move up and down as well, which is two more ifs in the same forever loop.

1. Open rocket 1’s controller.
2. Right-click the key-if and choose Duplicate. Twice.
3. Change one copy’s key to w and its block to change y by (4).
4. Change the other to s and change y by (-4).
5. On rocket 2, use the up and down arrows to match its right arrow.

forever
  if <key [d v] pressed?> then
    change x by (4)
  end
  if <key [w v] pressed?> then
    change y by (4)
  end
  if <key [s v] pressed?> then
    change y by (-4)
  end
end
            

2 · Asteroids

One sprite makes all of them. It hides itself, and every second it makes a clone that starts somewhere random along the top and drifts down.

when flag clicked
hide
forever
  wait (1) seconds
  create clone of [myself v]
end
            

when I start as a clone
set size to (70) %
go to x: (pick random (-120) to (170)) y: (170)
show
repeat (36)
  change y by (-10)
end
delete this clone
            

3 · Who asks the question?

You could put the touching check on the asteroid or on the rocket. On the rocket is right, and it is worth knowing why.

1. An asteroid touching “a rocket” does not know WHICH rocket, so it cannot knock the right one back.
2. One asteroid can touch both rockets at once. Two separate checks handle that; one shared check does not.
3. The rocket is the thing being affected, and the sprite that changes is usually the sprite that should ask.

when flag clicked
forever
  if <touching [Asteroid v]?> then
    start sound [Bonk v]
    change x by (-40)
    wait (0.4) seconds
  end
end
            

4 · Something worth chasing

An obstacle course is all cost and no reward. The star is the same clone shape with the sign flipped, plus one extra job: disappear when it is caught.

when I start as a clone
go to x: (pick random (-120) to (160)) y: (170)
show
repeat (36)
  change y by (-10)
  if <touching [Rocket 1 v]?> then
    start sound [Collect v]
    delete this clone
  end
end
delete this clone
            

And on the rocket, one more if in the collision stack, paying out instead of charging.

if <touching [Star v]?> then
  change x by (60)
end
            

Basic — build the obstacle race 25 min

Four passes. Test after every one, on your own, before you play it against anybody.

Step 1 — steering

1. Add the up and down ifs to rocket 1.
2. Test flying in every direction, including diagonally.
3. Do the same on rocket 2 with the arrow keys.

Step 2 — the asteroids

1. Add a rock sprite and name it Asteroid.
2. Build the maker stack. Remember the hide.
3. Build the clone stack, ending in delete this clone.
4. Run it and watch for a minute. The field should stay steady.

Step 3 — the bump

1. Add the collision stack to rocket 1.
2. Test by flying into rocks on purpose.
3. Duplicate the stack onto rocket 2.

Step 4 — the star

1. Add a star sprite with its own maker and clone stacks.
2. Make it rarer than the asteroids.
3. Add the catch check inside its repeat loop, one per rocket.
4. Add the boost if to both rockets.

Step 5 — race it

1. Race yourself five times, driving both rockets.
2. Count how often a race is decided by a star.
3. If it is every single time, the star is worth too much.

A field you can actually fly through

Too many rocks and the race is luck. Too few and nobody notices them. Find the number in between.

• You have tried at least three different spawn speeds.
• A good player can cross without being hit.
• A player who is not looking gets hit.
• You can say which speed you kept.

A bump you can feel

Right now a hit is a small jump backwards. Make it something a player notices without looking at their own rocket.

• A sound.
• Something visible: a colour, a spin, a shrink, a bubble.
• It goes back to normal afterwards.
• Two hits in a row do not leave the rocket stuck looking hurt.

A star worth the detour

A star should be a decision: go for it, or stay on the fast line? Tune it until the answer is not obvious.

• The boost is big enough to be tempting.
• Going for one out of your way can lose you the race.
• Stars are rare enough that a race can pass without one.
• You can say what you set the boost to and why.

An asteroid that hunts

Add one asteroid that does not drift straight down. It heads for whoever is in front, so being in the lead costs something.

• One asteroid moves towards a rocket rather than straight down.
• It picks whichever rocket is further ahead.
• It still deletes itself once it leaves the Stage.
• The ordinary drifting asteroids still work.

when I start as a clone
go to x: (pick random (-120) to (170)) y: (170)
show
if <([x position v] of [Rocket 1 v]) > ([x position v] of [Rocket 2 v])> then
  point towards [Rocket 1 v]
else
  point towards [Rocket 2 v]
end
repeat (60)
  move (6) steps
end
delete this clone
            

Summary 4 min

The race stopped being about holding a key and started being about where to fly.

1. Three ifs give a player forward, up and down.
2. One hidden sprite makes every asteroid, and every clone deletes itself.
3. The sprite that changes is usually the sprite that should ask the question.
4. A cost and a reward together make a decision. Either one alone does not.

Clone

A copy of a sprite made while the project runs. It must delete itself when done.

Obstacle

Something that costs the player. Here, forty steps backwards.

Boost

Something that pays the player. Worth a detour, but not always.

Spawn rate

How often new clones are made. The difference between a field and a wall.

Next lesson: the last lesson of the arc — three races, a scoreboard, and a champion.