Scroll World II

Review — move the world 5 min

• scroll x says how far along the world the Stage is looking.
• Everything sets set x to ((my x) - (scroll x)).
• Far-away sprites hide, or they wait at the edge of the Stage.

Quick-fire

Last lesson the arrow keys changed scroll x instead of Pico’s x. What goes wrong if you try that with Jump Quest’s Pico, which has walls?

Today’s Topic 3 min

• Walk for real, then pull the world back
• Screen x and world x: ((scroll x) + (x position))
• Respawn points, enemies and checkpoints that live in the world

Learning outcome

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

1. Put a Jump Quest character into a scrolling level without rewriting its physics.
2. Tell a screen position from a world position, and use the right one.
3. Make a respawn, an enemy and a checkpoint work in a world bigger than the Stage.

Learnt — a camera that follows 28 min

1 · Walk for real, then pull the world back

Keep every block of Pico’s Jump Quest loop: walk, gravity, land, the jump. They move Pico across the Stage as they always did, and the walls and floors work because the world is standing still while they do. Then, at the very end of the tick, add one My Block:

define scroll the world
change [scroll x v] by (x position)
set x to (0)
if <(scroll x) < (0)> then
  set x to (scroll x)
  set [scroll x v] to (0)
end
if <(scroll x) > (1440)> then
  set x to ((scroll x) - (1440))
  set [scroll x v] to (1440)
end
broadcast [scroll v]
            

2 · Two kinds of x

Pico’s x position is now nearly always 0, so it tells you nothing about where Pico is in the level. That is its screen x. Where Pico really is — its world x — is the scroll plus that:

set [world x v] to ((scroll x) + (x position))
            

Basic — the scroller 24 min

Combine your road from last lesson with your Pico from Jump Quest.

Step 1 — a level worth scrolling

1. Repaint the four Ground costumes as a level: gaps with water in them, steps 50 up, a crate. Name them L1 screen 1 to L1 screen 4.
2. Keep the ground and steps one green, and the water one blue, exactly as in Jump Quest.
3. Change the costume join to (join [L1 screen ] (screen)).

Step 2 — bring Pico across

1. Drag your Jump Quest Pico onto this project’s sprite list, or export and import it.
2. Its land, walk and hitbox come with it. Change nothing.
3. Delete last lesson’s road-walking Pico.

Step 3 — pull the world back

1. Make the scroll the world block from Learnt part 1.
2. Put it as the last block inside Pico’s forever loop.
3. Start Pico at x −180 with scroll x at 0.

Step 4 — water and the flag

1. Touching the water: go to x −180, set y velocity to 0, set scroll x to 0, and broadcast scroll.
2. Put the finish flag at world x 1600, placed the same way as the screens.

when flag clicked
go to x: (-180) y: (-104)
set [scroll x v] to (0)
set [y velocity v] to (0)
forever
  if <key (right arrow v) pressed?> then
    point in direction (90)
    walk (4) :: custom
  end
  if <key (left arrow v) pressed?> then
    point in direction (-90)
    walk (-4) :: custom
  end
  change [y velocity v] by (-1)
  change y by (y velocity)
  land :: custom
  if <<key (space v) pressed?> and <(on ground) = (1)>> then
    set [y velocity v] to (12)
  end
  scroll the world :: custom
  if <touching color [#2F80ED]?> then
    go to x: (-180) y: (-104)
    set [y velocity v] to (0)
    set [scroll x v] to (0)
    broadcast [scroll v]
  end
  if <touching (Flag v)?> then
    say [Level complete!]
  end
end
            

The respawn that forgot the camera

Fall into the first pit twice — once with a respawn that only moves Pico, once with one that resets scroll x too. Report Pico’s world x each time.

• Both respawns go to x −180.
• Only one sets scroll x to 0.
• Work out Pico’s world x after each.

A crab in the world

Add a crab that patrols a stretch of ground — in the world, not on the Stage.

• The crab keeps its own my x and changes that to patrol.
• It places itself with set x to ((my x) - (scroll x)), and hides when it is off the Stage.
• Try a second crab that just changes its x, and watch the difference.

forever
  repeat (20)
    change [my x v] by (-2)
    set x to ((my x) - (scroll x))
  end
  repeat (20)
    change [my x v] by (2)
    set x to ((my x) - (scroll x))
  end
end
            

A checkpoint in the world

Put a checkpoint flag at world x 540. Once Pico has touched it, falling in the water brings Pico back there — the camera as well.

• A respawn x variable, −180 at the start, set to 540 by the checkpoint.
• The water sets scroll x to respawn x, puts Pico at x 0, and pulls the world.

A camera that looks ahead

In most side-scrollers the hero is not in the middle: they are a little to the left, so you can see more of what is coming. Keep Pico at x −100 instead of 0.

• Change scroll the world so it aims Pico at −100.
• The ends of the level still work.
• Play a level both ways and decide which is fairer.

Summary 5 min

• Pico walks for real, then the world is pulled back by the same amount.
• scroll the world goes last in the tick, after every collision.
• Screen x is where on the Stage; world x is ((scroll x) + (x position)).
• Respawns, enemies and checkpoints are all world positions.

Camera

The part of the world the Stage is showing — here, set by scroll x.

Screen x

A position on the Stage, from −240 to 240.

World x

A position in the whole level.

Respawn

Coming back after a fall — which, in a scrolling game, moves the camera too.

Hand in

Save your scroller. Next lesson it gets a second level, with its own screens and its own sky.