Sketch Pad IV

Review — Sketch Pad III 5 min

• change pen (color v) by (2) makes a rainbow line.
• An eraser is a white pen. A stamp prints the costume.
• A tool variable holds a word, and the loop asks it before drawing.

Quick-fire

A pencil is at x: 80, y: 20. Where is the point that matches it on the other side of the Stage, the same distance from the middle?

Today’s Topic 3 min

• The mirror sum: ((0) - (mouse x))
• A second pencil that draws the mirror image
• Two sprites that hear the same message
• Getting an app ready for someone else to use

Learning outcome

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

1. Work out a mirrored position with one subtraction.
2. Make two sprites draw at once from one mouse.
3. Finish an app so that a new user knows what to do.

Learnt — two pencils, one hand 28 min

1 · The mirror sum

The middle of the Stage is x: 0. A point at x: −100 is 100 to the left. Its mirror is 100 to the right, at x: 100. The y does not change.

So the mirror of any x is ((0) - (x position)). Try it: 0 − (−100) is 100. 0 − 60 is −60.

2 · A Mirror pencil

1. Right-click the Pencil and choose duplicate. Rename the copy Mirror.
2. Delete the Mirror’s key scripts. Keep its green-flag script.
3. Swap its go to [mouse-pointer v] for go to x: ((0) - (mouse x)) y: (mouse y).
4. Give it set [ghost v] effect to (50), so you can tell the two apart.

when flag clicked
go to [back v] layer
set pen color to [#4C4CFF]
set pen size to (4)
set [ghost v] effect to (50)
forever
  go to x: ((0) - (mouse x)) y: (mouse y)
  if <<mouse down?> and <(mouse y) < (130)>> then
    pen down
  else
    pen up
  end
end

when I receive [new colour v]
set pen color to (colour)
            

3 · Ready to share

An app you made makes sense to you. A new user has never seen it. Three things help them:

• A line of help when it starts. Run it as its own green-flag script, so it does not hold up the pencil.
• A clear key, so nobody has to press the green flag to start again.
• Instructions, and Notes and Credits, written on the project page. Say which keys do what, and name anything you borrowed.

Basic — the finished Sketch Pad 25 min

Open your sketch pad from last lesson.

Step 1 — the Mirror

1. Make the Mirror from the Learnt.
2. Test it with the pencil tool: both halves should match.

Step 2 — the Mirror changes tools

1. Give the Mirror key 1, 2 and 3 scripts that set its own pen size and colour, like the Pencil’s.
2. In the Mirror’s loop, add the same if <(tool) = [rainbow]> then before pen down.
3. Test the eraser on both sides.

Step 3 — ready to share

1. Add a second green-flag script to the Pencil: say [Drag to draw. Keys 1, 2, 3 change the tool. C clears.] for (3) seconds.
2. Add when [c v] key pressed with erase all.
3. Click See Project Page. Write the Instructions and the Notes and Credits.

when flag clicked
say [Drag to draw. Keys 1, 2, 3 change the tool. C clears.] for (3) seconds

when [c v] key pressed
erase all
            

A mirror switch

• A variable called mirror, shown on the Stage. It starts at 1.
• Press M and it becomes 0: the Mirror hides and stops drawing.
• Press M again and it comes back.

when [m v] key pressed
set [mirror v] to ((1) - (mirror))
            

Four ways

Add two more pencils, so one drag makes four lines.

• Mirror left and right, as now.
• Also mirror top and bottom of the paper.
• The paper runs from y: −180 to y: 130, so its middle is y: −25.
• No pencil ever draws inside the toolbar.

go to x: (mouse x) y: ((-50) - (mouse y))
            

A start card

A three-second message is easy to miss. Make the user read it first.

• On the green flag, the pencil shows how to use the app, and stays still.
• Nothing can be drawn yet.
• A click anywhere starts the app, and the message goes away.

say [Sketch Pad! Drag to draw. Click a ball for a colour. Click to start.]
wait until <mouse down?>
wait until <not <mouse down?>>
say []
            

A kaleidoscope

Eight pencils from one hand. Not eight sprites — one hidden sprite and seven clones.

• The pattern turns round the middle of the paper, (0, −25).
• Your Pencil is one of the eight. Seven clones are the rest.
• Each clone knows which of the seven it is, and works out its own place.

when flag clicked
hide
set [slot v] to (0)
repeat (7)
  change [slot v] by (1)
  create clone of [myself v]
end

when I start as a clone
forever
  set [dx v] to (mouse x)
  set [dy v] to ((mouse y) - (-25))
  if <(slot) = [1]> then
    go to x: ((0) - (dx)) y: ((-25) + (dy))
  end
  if <(slot) = [4]> then
    go to x: (dy) y: ((-25) + (dx))
  end
  if <<mouse down?> and <(y position) < (130)>> then
    pen down
  else
    pen up
  end
end
            

Summary 5 min

• The mirror of x is ((0) - (mouse x)). The y does not change.
• A second pencil can draw from the same mouse, at a worked-out place.
• One broadcast reaches every sprite, so each pencil can change its own pen.
• An app is ready to share when a new user can use it from the screen alone.

Mirror image

The same shape, flipped, as if seen in a mirror.

Symmetry

When one half of a picture matches the other half.

Instructions

The box on the project page that tells a user what to do.

Credits

Naming the people and things your project borrowed from.

Hand in

Save your Sketch Pad — the arc is finished. Next lesson the pen stops following your hand and starts following rules: shapes drawn by the code itself.