Per-Level Backdrops — Scratch L4

Where we are now: three backdrops named with their level number, switched by the join trick, each triggering sprite resets via when backdrop switches to hats. One backdrop swap cascades into a full world rebuild.

Learning Goals 3 min

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

Paint three backdrops on the Stage — Forest, Cave, Sky — and name them level1, level2, level3 so a script can choose them by number.
Use switch backdrop to (join [level] (level)) on the Stage so the backdrop auto-matches the value of the (level) variable.
Use when backdrop switches to [level1 v] hats on each sprite to reset that sprite for the new level — player to start, coins re-spawned, enemies in position.

Warm-Up — three levels, one boring view 7 min

Last lesson you added a (level) variable that climbs from 1 to 3 every time the player reaches the goal. The number changes, but the game still looks the same forest from start to finish. Boring!


when flag clicked
set [level v] to (1)
forever
  if <touching [Goal v] ?> then
    change [level v] by (1)
    broadcast (level-up v)
  end
end

From L04-18 — the level number bumps up, but nothing visible changes.

Predict — if you painted a second backdrop called cave and added switch backdrop to [cave v] right after change [level v] by (1), would it cope when level reaches 3?

Reveal the answer

Nope. You'd be stuck on the cave forever, because the script only knows the word "cave". To handle level 1, 2, 3, … n you need a script that picks the backdrop based on the level number, not by name. That's today's lesson.

Today: paint three backdrops, name them with the level number on the end, and let one tiny join trick pick the right one automatically.

New Concept — naming backdrops so scripts can find them 15 min

Scratch lets you switch a backdrop two ways: by dropdown (you pick a name) or by computed name (a script builds the name from text + a number). The second way is what makes a multi-level platformer possible.

Step A — paint three backdrops on the Stage

Click the Stage thumbnail (bottom-right, right of all the sprites), then the Backdrops tab at the top. Paint three:

Forest — brown ground strip across the bottom, green trees behind.
Cave — grey ground strip, dark walls, a few stalactites.
Sky — white clouds, no ground (the player will jump on platform sprites).

Rename each one in the list on the left: level1, level2, level3. The names matter — that's how a script will find them.

Step B — the join trick

If (level) is 2, then join [level] (level) reports the text level2. Drop that into the Stage's backdrop block:


when I receive (level-up v)
switch backdrop to (join [level] (level))

On the Stage. When level changes to 2, switch backdrop to level2. Works for any level number that has a matching backdrop name.

Step C — sprites listen to the backdrop change

Now we need each sprite to react when the world changes. The Player needs to go back to the start. The Coin needs to re-appear. The Enemy needs to take its level-1 position again. Scratch gives every sprite a special hat for this:


when backdrop switches to [level1 v]
go to x: (-200) y: (-100)

On the Player sprite. As soon as the Stage flips to level1, the Player snaps back to the left edge.

Each sprite gets three such hats — one for level1, one for level2, one for level3. The hats only fire on a switch, so they're perfect for "set up this level".

Common mistakes

Forgetting to rename the backdrops. If you leave them as backdrop1, backdrop2, backdrop3, the join needs to be join [backdrop] (level). Pick one naming style and stick with it.
Putting the switch backdrop on a sprite. Any sprite can switch the Stage's backdrop, but it's cleanest to let the Stage own its own backdrops. The Player sprite owns the Player. The Stage owns the world.
Capital-L vs small-l. Scratch backdrop names are case-sensitive in the join path. Level1 and level1 are different backdrops to a script.
Two scripts fighting over the Player's position. If both when flag clicked → go to x: (−200) y: (−100) AND when backdrop switches to [level1 v] run, that's fine — they say the same thing. But don't put a different start position in the flag-click; let the backdrop hat own it.

Worked Example — wiring three Malaysian biomes 12 min

We'll build the spine of a three-biome platformer set somewhere in Malaysia: Taman Negara (forest), Gua Niah (cave), Genting clouds (sky). One Player, one Coin, one Enemy — and three backdrops.

Stage plan. Forest biome (level1). The cat (✏️ badge) snaps to x:−200, y:−100 via when backdrop switches to [level1 v]. Same hat pattern on every sprite, three times each.

Step 1 — Paint three backdrops

On the Stage, Backdrops tab, paint Forest (brown ground), Cave (grey ground), Sky (white clouds). Rename to level1, level2, level3.

Step 2 — Make the level variable

Variables → Make a Variable → level, For all sprites. Tick the box so it shows on the Stage.

Step 3 — Stage script: start at level 1


when flag clicked
set [level v] to (1)
switch backdrop to [level1 v]

On the Stage. Every new game starts at Taman Negara.

Step 4 — Stage script: auto-switch on level-up


when I receive (level-up v)
switch backdrop to (join [level] (level))

On the Stage. The Player or Goal sprite broadcasts level-up after change [level v] by (1).

Step 5 — Player: three start positions


when backdrop switches to [level1 v]
go to x: (-200) y: (-100)

Forest — start on the left side of the brown ground.


when backdrop switches to [level2 v]
go to x: (-220) y: (-80)

Cave — slightly higher because the cave floor sits higher.


when backdrop switches to [level3 v]
go to x: (-210) y: (-60)

Sky — start on the leftmost cloud platform.

Step 6 — Coin: re-show on every level change


when backdrop switches to [level1 v]
go to x: (150) y: (-60)
show

The Coin disappears when collected; this hat puts it back for the next biome. One similar hat per level.

Step 7 — Enemy: position per level


when backdrop switches to [level1 v]
go to x: (50) y: (-100)
show

Forest enemy patrols the middle of the brown ground.

Step 8 — Test

Click the flag → forest, Player at x: −200. On the Stage, right-click the (level) watcher and pick change variable value, set to 2, then broadcast level-up from a temporary test stack. The backdrop should flip to cave and every sprite should jump to its level-2 position.

The Stage's full assembled stack


when flag clicked
set [level v] to (1)
switch backdrop to [level1 v]

when I receive (level-up v)
switch backdrop to (join [level] (level))

Five blocks on the Stage. Two scripts. That's the entire level-switching engine.

What you just built: a world that changes shape based on a single number. Add a fourth biome? Paint one more backdrop called level4 and add three when backdrop switches to [level4 v] hats. Zero changes to the Stage.

Try It Yourself — three biome drills 15 min

🟢 Easy

Goal: Add a fourth backdrop called level4 — paint a Sabah-beach scene with yellow sand. Wire just the Player so it starts at x: −230, y: −110 when level 4 begins. Don't touch the Stage scripts — the join trick handles it for free.


when backdrop switches to [level4 v]
go to x: (-230) y: (-110)

Think: You added one backdrop and one hat — zero edits to the Stage's level-switching script. That's why "name them with the number" is so powerful.

🟡 Medium

Goal: Play a short different sound on every backdrop switch. Forest = Crickets, Cave = Drip, Sky = Wind. Put the sound on the Stage so the Stage owns its own audio mood.


when backdrop switches to [level2 v]
play sound (Drip v) until done

One hat per backdrop. Three tiny scripts on the Stage.

Think: You could also use start sound instead of play sound until done. Start sound doesn't pause the script — the cave drip plays under the gameplay, not before it.

🔴 Stretch

Goal: Make a small "Loading next level…" message appear for one second between backdrops. Put a sprite called LoadingBanner in the centre that shows on when I receive (level-up v), says Loading… for 1 sec, then hides.


when I receive (level-up v)
show
say [Loading next level…] for (1) seconds
hide

Think: The banner fires before the Stage finishes switching the backdrop (broadcasts run in parallel). For a polished version, use broadcast (level-up v) and wait from whatever sprite raised it.

Mini-Challenge — the cave that never shows up 5 min

"Nadia's stuck forest"

Nadia painted three backdrops — forest, cave, sky — and wrote the Stage script below. She clicks the flag, plays level 1, reaches the goal, and… nothing changes. The Player teleports back to x: −200 but the forest is still there.


when flag clicked
set [level v] to (1)
switch backdrop to [forest v]

when I receive (level-up v)
switch backdrop to (join [level] (level))

Nadia's Stage scripts. Forest shows up fine. Cave never does. Why?

What's wrong, and what's the smallest fix?

Reveal one valid solution

Nadia named her backdrops forest, cave, sky — words. But the join script computes the name level2, level3, … Scratch can't find a backdrop called level2, so it doesn't switch. The script doesn't error — it just does nothing.

Two valid fixes. Pick one:

Fix the names. Rename the backdrops to level1, level2, level3. Update the flag-click to switch backdrop to [level1 v]. Done.
Fix the join. Build a list of names instead — item (level) of [biomes v] where biomes is a list containing forest, cave, sky. (You met lists in L03-06.)

Lesson: when a script builds a name from text, the things it's looking for have to be named to match. Naming is a contract between two scripts.

Recap 3 min

You turned a single (level) number into three different worlds. The trick is naming — backdrops called level1, level2, level3 let the Stage build the right backdrop name with join [level] (level). Every sprite then listens with when backdrop switches to [level1 v] to set itself up for the new biome — start positions, visibility, spawn locations. Add a level? Paint one backdrop, add three hats. That's it.

Backdrop: An image on the Stage (not on a sprite). The Stage can have many backdrops, switched with switch backdrop to. Backdrops have names you can edit.
Computed name: A backdrop name that a script builds out of text plus a value — e.g. join [level] (level) reports level1 when level is 1. Lets one block handle any level number.
when backdrop switches to: An events hat that fires the instant the Stage's backdrop changes to the chosen one. Used on every sprite that needs to reset, hide, or move when the world changes.
Biome: A themed environment in a game — forest, cave, sky, beach. In this lesson, one backdrop = one biome = one level.
level-up broadcast: A broadcast sent by whatever script increments the (level) variable. The Stage listens and switches the backdrop; sprites listen and re-position.

Homework 2 min

The Three-Biome Skeleton. Open a fresh project. Build a working level-switcher with no gameplay yet — just the world flipping.

Paint three backdrops on the Stage: level1 (forest, brown ground), level2 (cave, grey ground), level3 (sky, white clouds). Optional fourth: level4 for any Malaysian place you like.
Make a variable level, For all sprites. Show its watcher on the Stage.
On the Stage: the two scripts from the worked example — flag-click sets level to 1 and shows level1; when I receive (level-up v) switches by join.
On the Player sprite: one when backdrop switches to hat per level, moving the cat to a sensible start position for that biome.
Make a tester button — a sprite called NextLevelButton with: when this sprite clicked → change [level v] by (1) → broadcast (level-up v). Click it three times in a row to walk through all three biomes.

Save as HW-L4-19-Three-Biomes.sb3. Test by clicking the NextLevelButton three times — backdrop should change forest → cave → sky and the cat should jump to a new start each time.

Bring back next class:

The .sb3 file.
Your answer to: "What happens if you click the button a fourth time when there are only three backdrops? Try it and tell me what the Stage shows."

Heads up for next class: SCR-L04-20 turns that enemy sprite into three different enemies — slow patrols in the forest, fast jumpers in the cave, chasers in the sky — all on the same sprite, switched by the (level) variable.

Learning Goals 3 min

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

Paint three backdrops on the Stage — Forest, Cave, Sky — and name them level1, level2, level3 so a script can choose them by number.
Use switch backdrop to (join [level] (level)) on the Stage so the backdrop auto-matches the value of the (level) variable.
Use when backdrop switches to [level1 v] hats on each sprite to reset that sprite for the new level — player to start, coins re-spawned, enemies in position.

Warm-Up — three levels, one boring view 7 min

Last lesson you added a (level) variable that climbs from 1 to 3 every time the player reaches the goal. The number changes, but the game still looks the same forest from start to finish. Boring!


when flag clicked
set [level v] to (1)
forever
  if <touching [Goal v] ?> then
    change [level v] by (1)
    broadcast (level-up v)
  end
end

From L04-18 — the level number bumps up, but nothing visible changes.

Predict — if you painted a second backdrop called cave and added switch backdrop to [cave v] right after change [level v] by (1), would it cope when level reaches 3?

Reveal the answer

Today: paint three backdrops, name them with the level number on the end, and let one tiny join trick pick the right one automatically.

New Concept — naming backdrops so scripts can find them 15 min

Step A — paint three backdrops on the Stage

Click the Stage thumbnail (bottom-right, right of all the sprites), then the Backdrops tab at the top. Paint three:

Forest — brown ground strip across the bottom, green trees behind.
Cave — grey ground strip, dark walls, a few stalactites.
Sky — white clouds, no ground (the player will jump on platform sprites).

Rename each one in the list on the left: level1, level2, level3. The names matter — that's how a script will find them.

Step B — the join trick

If (level) is 2, then join [level] (level) reports the text level2. Drop that into the Stage's backdrop block:


when I receive (level-up v)
switch backdrop to (join [level] (level))

On the Stage. When level changes to 2, switch backdrop to level2. Works for any level number that has a matching backdrop name.

Step C — sprites listen to the backdrop change


when backdrop switches to [level1 v]
go to x: (-200) y: (-100)

On the Player sprite. As soon as the Stage flips to level1, the Player snaps back to the left edge.

Each sprite gets three such hats — one for level1, one for level2, one for level3. The hats only fire on a switch, so they're perfect for "set up this level".

Common mistakes

Forgetting to rename the backdrops. If you leave them as backdrop1, backdrop2, backdrop3, the join needs to be join [backdrop] (level). Pick one naming style and stick with it.
Putting the switch backdrop on a sprite. Any sprite can switch the Stage's backdrop, but it's cleanest to let the Stage own its own backdrops. The Player sprite owns the Player. The Stage owns the world.
Capital-L vs small-l. Scratch backdrop names are case-sensitive in the join path. Level1 and level1 are different backdrops to a script.
Two scripts fighting over the Player's position. If both when flag clicked → go to x: (−200) y: (−100) AND when backdrop switches to [level1 v] run, that's fine — they say the same thing. But don't put a different start position in the flag-click; let the backdrop hat own it.

Worked Example — wiring three Malaysian biomes 12 min

Stage plan. Forest biome (level1). The cat (✏️ badge) snaps to x:−200, y:−100 via when backdrop switches to [level1 v]. Same hat pattern on every sprite, three times each.

Step 1 — Paint three backdrops

On the Stage, Backdrops tab, paint Forest (brown ground), Cave (grey ground), Sky (white clouds). Rename to level1, level2, level3.

Step 2 — Make the level variable

Variables → Make a Variable → level, For all sprites. Tick the box so it shows on the Stage.

Step 3 — Stage script: start at level 1


when flag clicked
set [level v] to (1)
switch backdrop to [level1 v]

On the Stage. Every new game starts at Taman Negara.

Step 4 — Stage script: auto-switch on level-up


when I receive (level-up v)
switch backdrop to (join [level] (level))

On the Stage. The Player or Goal sprite broadcasts level-up after change [level v] by (1).

Step 5 — Player: three start positions


when backdrop switches to [level1 v]
go to x: (-200) y: (-100)

Forest — start on the left side of the brown ground.


when backdrop switches to [level2 v]
go to x: (-220) y: (-80)

Cave — slightly higher because the cave floor sits higher.


when backdrop switches to [level3 v]
go to x: (-210) y: (-60)

Sky — start on the leftmost cloud platform.

Step 6 — Coin: re-show on every level change


when backdrop switches to [level1 v]
go to x: (150) y: (-60)
show

The Coin disappears when collected; this hat puts it back for the next biome. One similar hat per level.

Step 7 — Enemy: position per level


when backdrop switches to [level1 v]
go to x: (50) y: (-100)
show

Forest enemy patrols the middle of the brown ground.

Step 8 — Test

The Stage's full assembled stack


when flag clicked
set [level v] to (1)
switch backdrop to [level1 v]

when I receive (level-up v)
switch backdrop to (join [level] (level))

Five blocks on the Stage. Two scripts. That's the entire level-switching engine.

Try It Yourself — three biome drills 15 min

🟢 Easy


when backdrop switches to [level4 v]
go to x: (-230) y: (-110)

Think: You added one backdrop and one hat — zero edits to the Stage's level-switching script. That's why "name them with the number" is so powerful.

🟡 Medium

Goal: Play a short different sound on every backdrop switch. Forest = Crickets, Cave = Drip, Sky = Wind. Put the sound on the Stage so the Stage owns its own audio mood.


when backdrop switches to [level2 v]
play sound (Drip v) until done

One hat per backdrop. Three tiny scripts on the Stage.

Think: You could also use start sound instead of play sound until done. Start sound doesn't pause the script — the cave drip plays under the gameplay, not before it.

🔴 Stretch


when I receive (level-up v)
show
say [Loading next level…] for (1) seconds
hide

Mini-Challenge — the cave that never shows up 5 min

"Nadia's stuck forest"


when flag clicked
set [level v] to (1)
switch backdrop to [forest v]

when I receive (level-up v)
switch backdrop to (join [level] (level))

Nadia's Stage scripts. Forest shows up fine. Cave never does. Why?

What's wrong, and what's the smallest fix?

Reveal one valid solution

Two valid fixes. Pick one:

Fix the names. Rename the backdrops to level1, level2, level3. Update the flag-click to switch backdrop to [level1 v]. Done.
Fix the join. Build a list of names instead — item (level) of [biomes v] where biomes is a list containing forest, cave, sky. (You met lists in L03-06.)

Lesson: when a script builds a name from text, the things it's looking for have to be named to match. Naming is a contract between two scripts.

Recap 3 min

Backdrop: An image on the Stage (not on a sprite). The Stage can have many backdrops, switched with switch backdrop to. Backdrops have names you can edit.
Computed name: A backdrop name that a script builds out of text plus a value — e.g. join [level] (level) reports level1 when level is 1. Lets one block handle any level number.
when backdrop switches to: An events hat that fires the instant the Stage's backdrop changes to the chosen one. Used on every sprite that needs to reset, hide, or move when the world changes.
Biome: A themed environment in a game — forest, cave, sky, beach. In this lesson, one backdrop = one biome = one level.
level-up broadcast: A broadcast sent by whatever script increments the (level) variable. The Stage listens and switches the backdrop; sprites listen and re-position.

Homework 2 min

The Three-Biome Skeleton. Open a fresh project. Build a working level-switcher with no gameplay yet — just the world flipping.

Paint three backdrops on the Stage: level1 (forest, brown ground), level2 (cave, grey ground), level3 (sky, white clouds). Optional fourth: level4 for any Malaysian place you like.
Make a variable level, For all sprites. Show its watcher on the Stage.
On the Stage: the two scripts from the worked example — flag-click sets level to 1 and shows level1; when I receive (level-up v) switches by join.
On the Player sprite: one when backdrop switches to hat per level, moving the cat to a sensible start position for that biome.
Make a tester button — a sprite called NextLevelButton with: when this sprite clicked → change [level v] by (1) → broadcast (level-up v). Click it three times in a row to walk through all three biomes.

Save as HW-L4-19-Three-Biomes.sb3. Test by clicking the NextLevelButton three times — backdrop should change forest → cave → sky and the cat should jump to a new start each time.

Bring back next class:

The .sb3 file.
Your answer to: "What happens if you click the button a fourth time when there are only three backdrops? Try it and tell me what the Stage shows."