Firework Show III — Scratch L3 Lesson 15

Learnt28 min · why Scratch is slow, and the tickbox
Basic24 min · both versions, side by side
Challenge 1–327 min · when it is wrong, 200 sparks, the rule
Extrabonus · a show that uses both

WHAT IT SHOULD LOOK LIKE

Sixty sparks made one at a time is sixty redraws — about two seconds of them dribbling out. One tickbox and the whole burst happens inside a single frame.

refresh ON, part way through

refresh OFF, the same moment

panels run left to right

Review — where we got to 5 min

define launch (x) (height) (sparks) takes inputs.
The ovals in the hat are dragged into the body.
An input only exists inside its own definition.

Quick-fire

You change repeat (14) to repeat (200) in your launch block. What do you expect to see?

Reveal the answer

Most people say “a huge firework”. What you actually get is a slow trickle of sparks lasting several seconds. Today is about why, and the one tickbox that fixes it.

Today’s Topic 3 min

What Scratch does between two blocks
Run without screen refresh — the tickbox in the Make a Block window
When to tick it, and when ticking it ruins everything
A show that uses both

Learning outcome

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

Explain why a loop of a hundred blocks takes a noticeable time.
Turn screen refresh off for a block, and say what changed.
Decide which of your blocks should have it on and which should not.

Learnt — what happens between two blocks 28 min

1 · Scratch redraws, constantly

Scratch runs your blocks and repaints the Stage about thirty times a second. Between one block and the next, it may stop to redraw everything.

That is what makes glide (1) secs to x: () y: () look smooth — you see every step. It is also why a loop that makes sixty clones takes about two seconds: sixty clones, sixty redraws.


define big burst
show
go to x: (0) y: (-170)
glide (0.4) secs to x: (0) y: (90)
hide
repeat (60)
  create clone of [myself v]
end

Sixty clones. Nothing in here says “wait”, and it still takes two seconds.

WHAT IT SHOULD LOOK LIKE

Scratch redraws the Stage between blocks. Every clone gets its own frame, so the burst arrives in a queue you can count.

a queue of sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the sparks arrive. Count them if you can — they come one at a time.

Sixty sparks, refresh on. It is a queue, not a burst.

2 · The tickbox

Right-click your define hat and choose Edit.
At the bottom of the window there is a tickbox: Run without screen refresh.
Tick it and click OK.
Run the same project again.

Nothing in the blocks changed. The whole definition now runs to the end before Scratch redraws the Stage even once — so the sixty clones all appear in the same frame.

WHAT IT SHOULD LOOK LIKE

Tick “Run without screen refresh” and the whole definition finishes before the Stage is redrawn. Same sixty clones, one frame.

all sixty at once

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Same sixty sparks as the demo above — but look how they arrive.

One frame. That is the entire difference, and the blocks are identical.

3 · When you should NOT tick it

Anything the audience is meant to watch happen needs the redraws. Tick the box on a countdown and it counts 3, 2, 1 inside one frame — which looks like nothing at all:


define count down
set [count v] to (3)
repeat (3)
  say (count) for (0.7) seconds
  change [count v] by (-1)
end

Refresh ON. You see 3, then 2, then 1.


define count down
set [count v] to (3)
repeat (3)
  say (count) for (0.7) seconds
  change [count v] by (-1)
end

Refresh OFF. Identical blocks — and you see nothing at all.

Here is the rule, and it is short:

The block…	Refresh
builds something that should appear finished	OFF
performs something you are meant to watch	ON

Basic — both, side by side 24 min

You cannot show a difference with one of something. Build both, in one project, and run them one after the other.

Step 1 — the slow one

Make a block called slow burst. Leave the tickbox alone.
Inside: go to the left of the Stage, hide, and repeat (40) making clones.

Step 2 — the fast one

Make a second block called fast burst, identical, but on the right.
This time tick Run without screen refresh.

Step 3 — count them

Make a variable sparks made and show it.
Bump it inside both repeats, so you can see the number climbing or jumping.

Step 4 — run them in turn

Under the green flag: say which is coming, call the slow one, wait.
Reset the counter, say which is coming, call the fast one.

Step 5 — watch the counter, not the sparks

Run it and watch only the number.
On the left it counts up. On the right it jumps to 40.


when flag clicked
set [sparks made v] to (0)
say [On the left: refresh on.] for (1.5) seconds
slow burst :: custom
wait (2) seconds
set [sparks made v] to (0)
say [On the right: refresh off.] for (1.5) seconds
fast burst :: custom

The two definitions are the same blocks. Only the tickbox differs.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the sparks-made box. Left: it climbs. Right: it jumps.

Forty sparks each. The counter is the clearest evidence — watch that rather than the Stage.

Both bursts look the same.

The tickbox did not take. Right-click the define hat, choose Edit, and check it really is ticked for fast burst and not for slow burst.

Scratch freezes when I run it.

There is a forever loop or a very long repeat inside a refresh-off block. That block never gives the Stage a chance to redraw.

The fast one appears but the slow one does not.

The wait between them is too short, so the slow one is still going when the fast one starts. Give it two seconds.

The sparks all pile up in the corner.

The clone script’s point in direction is missing, so every spark flies the same way.

CHALLENGE 1 · L3-15-M1🪙 30 Adcoins8 min

Tick the wrong box

Find out what refresh-off does to something that is meant to be watched.

A countdown block with the tickbox ON. Run it.
The same countdown with it OFF. Run it.
Write down, in your own words, what the tickbox actually did.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the tickbox in the wrong place. A countdown that finishes inside one frame is a countdown nobody sees.

refresh OFF: nothing

refresh ON: a countdown

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag. The first countdown is refresh-off — watch carefully, you will miss it. The second is refresh-on.

The same three says. One version shows nothing; the other counts you down.

Done when: you can say in one sentence why the first countdown was invisible, without using the word “fast”.

Teacher note

Push them off “it was too fast”. Nothing was faster — the says still took their 0.7 seconds. What happened is that the Stage was never redrawn during the block, so there was no moment at which a 3 was on screen to be seen. That distinction is the whole lesson.

CHALLENGE 2 · L3-15-M2🪙 50 Adcoins9 min

Two hundred sparks

Make a firework that would be impossible with refresh on.

A burst of two hundred clones.
It appears in one frame.
The project does not slow to a crawl afterwards.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — two hundred sparks. Unwatchable with refresh on; instant without it.

one frame, 200 sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Two hundred sparks at once — try to count the moment they arrive.

With refresh on this would take seven seconds. Off, it is a bang.

Done when: two hundred sparks appear together, and the sparks still fade and delete themselves so the project keeps running smoothly.

Teacher note

Scratch caps clones at 300, so this is close to the limit — a good moment to mention that limits exist. If a pupil asks for a thousand, let them try: the clones simply stop being created, silently.

CHALLENGE 3 · L3-15-M3🪙 70 Adcoins10 min

One project, both settings

Use the rule properly: a journey you watch, and a burst you do not.

A walk the rocket block that moves it across the Stage, refresh ON.
A burst block, refresh OFF.
Both in the same show, and both look right.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the rule in one project. The flight is watched, so refresh stays on; the burst is not, so it comes off.

walk, then burst

Runs here in the page. Nothing to install.

How to use this: Click the green flag: the rocket travels across so you can follow it, then bursts all at once.

Two blocks, two settings, one show. The travel is the performance; the burst is the result.

Done when: you can watch the rocket move the whole way, and the burst still arrives in one piece.

Teacher reveal — the test to apply

Ask of each block: would a half-finished version of this make sense to look at? A rocket half way across: yes, that is the point. A burst with 30 of its 60 sparks: no, that is just a broken firework. Half-finished-makes-sense means refresh on.

EXTRA MISSION · L3-15-MX🪙 100 Adcoinsbonus

A show that uses both

Four fireworks, each one a flight you can watch followed by a burst you cannot. Split your launch block in two to do it.

fly to (x) (height) — refresh ON.
burst (sparks) — refresh OFF.
Four flights and four bursts, over two hundred sparks, all smooth.

WHAT IT SHOULD LOOK LIKE

Extra — four flights you can watch and four bursts you cannot, in one show.

220 sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch four rockets fly and burst. Follow one up with your eyes.

220 sparks across four fireworks. Flights you watch, bursts you do not.

Teacher reveal


define fly to (x) (height)
show
go to x: (x) y: (-170)
glide (0.35) secs to x: (x) y: (height)
hide

define burst (sparks)
repeat (sparks)
  create clone of [myself v]
end

Two blocks, and only the second one is ticked. The interesting question to put to the class: why can these not be one block with the tickbox half on? Because the setting belongs to the whole definition — which is a real constraint, and the reason the split is forced rather than chosen.

Summary 5 min

Scratch redraws the Stage between blocks, which is what makes movement smooth.
Run without screen refresh makes a whole definition finish before the next redraw.
It does not do less work. It just does not let you watch.
Tick it for things that should appear finished; leave it for things to watch.
A very long block with it ticked will freeze the project until it ends.

Screen refresh: Scratch repainting the Stage, about thirty times a second.
Frame: One of those repaints.
Run without screen refresh: A My Block setting: finish the whole definition before repainting.
Freeze: What happens when a refresh-off block runs too long.

Hand in

Save your show. Next lesson is the last of the arc: taking a long script that already works and giving its parts names — without the audience noticing anything at all.

Learnt28 min · why Scratch is slow, and the tickbox
Basic24 min · both versions, side by side
Challenge 1–327 min · when it is wrong, 200 sparks, the rule
Extrabonus · a show that uses both

WHAT IT SHOULD LOOK LIKE

Sixty sparks made one at a time is sixty redraws — about two seconds of them dribbling out. One tickbox and the whole burst happens inside a single frame.

refresh ON, part way through

refresh OFF, the same moment

panels run left to right

Review — where we got to 5 min

define launch (x) (height) (sparks) takes inputs.
The ovals in the hat are dragged into the body.
An input only exists inside its own definition.

Quick-fire

You change repeat (14) to repeat (200) in your launch block. What do you expect to see?

Reveal the answer

Most people say “a huge firework”. What you actually get is a slow trickle of sparks lasting several seconds. Today is about why, and the one tickbox that fixes it.

Today’s Topic 3 min

What Scratch does between two blocks
Run without screen refresh — the tickbox in the Make a Block window
When to tick it, and when ticking it ruins everything
A show that uses both

Learning outcome

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

Explain why a loop of a hundred blocks takes a noticeable time.
Turn screen refresh off for a block, and say what changed.
Decide which of your blocks should have it on and which should not.

Learnt — what happens between two blocks 28 min

1 · Scratch redraws, constantly

Scratch runs your blocks and repaints the Stage about thirty times a second. Between one block and the next, it may stop to redraw everything.

That is what makes glide (1) secs to x: () y: () look smooth — you see every step. It is also why a loop that makes sixty clones takes about two seconds: sixty clones, sixty redraws.


define big burst
show
go to x: (0) y: (-170)
glide (0.4) secs to x: (0) y: (90)
hide
repeat (60)
  create clone of [myself v]
end

Sixty clones. Nothing in here says “wait”, and it still takes two seconds.

WHAT IT SHOULD LOOK LIKE

Scratch redraws the Stage between blocks. Every clone gets its own frame, so the burst arrives in a queue you can count.

a queue of sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the sparks arrive. Count them if you can — they come one at a time.

Sixty sparks, refresh on. It is a queue, not a burst.

2 · The tickbox

Right-click your define hat and choose Edit.
At the bottom of the window there is a tickbox: Run without screen refresh.
Tick it and click OK.
Run the same project again.

Nothing in the blocks changed. The whole definition now runs to the end before Scratch redraws the Stage even once — so the sixty clones all appear in the same frame.

WHAT IT SHOULD LOOK LIKE

Tick “Run without screen refresh” and the whole definition finishes before the Stage is redrawn. Same sixty clones, one frame.

all sixty at once

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Same sixty sparks as the demo above — but look how they arrive.

One frame. That is the entire difference, and the blocks are identical.

3 · When you should NOT tick it

Anything the audience is meant to watch happen needs the redraws. Tick the box on a countdown and it counts 3, 2, 1 inside one frame — which looks like nothing at all:


define count down
set [count v] to (3)
repeat (3)
  say (count) for (0.7) seconds
  change [count v] by (-1)
end

Refresh ON. You see 3, then 2, then 1.


define count down
set [count v] to (3)
repeat (3)
  say (count) for (0.7) seconds
  change [count v] by (-1)
end

Refresh OFF. Identical blocks — and you see nothing at all.

Here is the rule, and it is short:

The block…	Refresh
builds something that should appear finished	OFF
performs something you are meant to watch	ON

Basic — both, side by side 24 min

You cannot show a difference with one of something. Build both, in one project, and run them one after the other.

Step 1 — the slow one

Make a block called slow burst. Leave the tickbox alone.
Inside: go to the left of the Stage, hide, and repeat (40) making clones.

Step 2 — the fast one

Make a second block called fast burst, identical, but on the right.
This time tick Run without screen refresh.

Step 3 — count them

Make a variable sparks made and show it.
Bump it inside both repeats, so you can see the number climbing or jumping.

Step 4 — run them in turn

Under the green flag: say which is coming, call the slow one, wait.
Reset the counter, say which is coming, call the fast one.

Step 5 — watch the counter, not the sparks

Run it and watch only the number.
On the left it counts up. On the right it jumps to 40.


when flag clicked
set [sparks made v] to (0)
say [On the left: refresh on.] for (1.5) seconds
slow burst :: custom
wait (2) seconds
set [sparks made v] to (0)
say [On the right: refresh off.] for (1.5) seconds
fast burst :: custom

The two definitions are the same blocks. Only the tickbox differs.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the sparks-made box. Left: it climbs. Right: it jumps.

Forty sparks each. The counter is the clearest evidence — watch that rather than the Stage.

Both bursts look the same.

The tickbox did not take. Right-click the define hat, choose Edit, and check it really is ticked for fast burst and not for slow burst.

Scratch freezes when I run it.

There is a forever loop or a very long repeat inside a refresh-off block. That block never gives the Stage a chance to redraw.

The fast one appears but the slow one does not.

The wait between them is too short, so the slow one is still going when the fast one starts. Give it two seconds.

The sparks all pile up in the corner.

The clone script’s point in direction is missing, so every spark flies the same way.

CHALLENGE 1 · L3-15-M1🪙 30 Adcoins8 min

Tick the wrong box

Find out what refresh-off does to something that is meant to be watched.

A countdown block with the tickbox ON. Run it.
The same countdown with it OFF. Run it.
Write down, in your own words, what the tickbox actually did.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the tickbox in the wrong place. A countdown that finishes inside one frame is a countdown nobody sees.

refresh OFF: nothing

refresh ON: a countdown

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag. The first countdown is refresh-off — watch carefully, you will miss it. The second is refresh-on.

The same three says. One version shows nothing; the other counts you down.

Done when: you can say in one sentence why the first countdown was invisible, without using the word “fast”.

Teacher note

CHALLENGE 2 · L3-15-M2🪙 50 Adcoins9 min

Two hundred sparks

Make a firework that would be impossible with refresh on.

A burst of two hundred clones.
It appears in one frame.
The project does not slow to a crawl afterwards.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — two hundred sparks. Unwatchable with refresh on; instant without it.

one frame, 200 sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Two hundred sparks at once — try to count the moment they arrive.

With refresh on this would take seven seconds. Off, it is a bang.

Done when: two hundred sparks appear together, and the sparks still fade and delete themselves so the project keeps running smoothly.

Teacher note

Scratch caps clones at 300, so this is close to the limit — a good moment to mention that limits exist. If a pupil asks for a thousand, let them try: the clones simply stop being created, silently.

CHALLENGE 3 · L3-15-M3🪙 70 Adcoins10 min

One project, both settings

Use the rule properly: a journey you watch, and a burst you do not.

A walk the rocket block that moves it across the Stage, refresh ON.
A burst block, refresh OFF.
Both in the same show, and both look right.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the rule in one project. The flight is watched, so refresh stays on; the burst is not, so it comes off.

walk, then burst

Runs here in the page. Nothing to install.

How to use this: Click the green flag: the rocket travels across so you can follow it, then bursts all at once.

Two blocks, two settings, one show. The travel is the performance; the burst is the result.

Done when: you can watch the rocket move the whole way, and the burst still arrives in one piece.

Teacher reveal — the test to apply

EXTRA MISSION · L3-15-MX🪙 100 Adcoinsbonus

A show that uses both

Four fireworks, each one a flight you can watch followed by a burst you cannot. Split your launch block in two to do it.

fly to (x) (height) — refresh ON.
burst (sparks) — refresh OFF.
Four flights and four bursts, over two hundred sparks, all smooth.

WHAT IT SHOULD LOOK LIKE

Extra — four flights you can watch and four bursts you cannot, in one show.

220 sparks

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch four rockets fly and burst. Follow one up with your eyes.

220 sparks across four fireworks. Flights you watch, bursts you do not.

Teacher reveal


define fly to (x) (height)
show
go to x: (x) y: (-170)
glide (0.35) secs to x: (x) y: (height)
hide

define burst (sparks)
repeat (sparks)
  create clone of [myself v]
end

Summary 5 min

Scratch redraws the Stage between blocks, which is what makes movement smooth.
Run without screen refresh makes a whole definition finish before the next redraw.
It does not do less work. It just does not let you watch.
Tick it for things that should appear finished; leave it for things to watch.
A very long block with it ticked will freeze the project until it ends.

Screen refresh: Scratch repainting the Stage, about thirty times a second.
Frame: One of those repaints.
Run without screen refresh: A My Block setting: finish the whole definition before repainting.
Freeze: What happens when a refresh-off block runs too long.

Hand in

Save your show. Next lesson is the last of the arc: taking a long script that already works and giving its parts names — without the audience noticing anything at all.