Sketch Pad II

Review — Sketch Pad I 5 min

• The Pen blocks come from an extension.
• pen down draws, pen up stops, erase all wipes.
• The pencil follows the mouse and draws while <mouse down?> is true.

Quick-fire

Your pencil draws while the button is held. You click a button at the top of the Stage. What happens to the paper?

Today’s Topic 3 min

• set pen color to [#D400AB] and set pen size to (4)
• Every sprite has its own pen
• A colour button that tells the pencil, with a message
• One variable that carries the colour

Learning outcome

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

1. Change the pen’s colour and size.
2. Explain why a button cannot change the pencil’s pen by itself.
3. Build a palette of colour buttons that all work the same way.

Learnt — colour, size, and whose pen 28 min

1 · Two blocks: a colour and a size

set pen color to [#D400AB]

set pen size to (8)

1. Drag set pen color to [#D400AB] into the code area.
2. Click its coloured box. Three sliders open: Color, Saturation and Brightness.
3. Drag the Color slider to choose a colour.
4. Or click the eyedropper at the bottom, then click any colour on the Stage to copy it.

2 · Paint a toolbar

The buttons need somewhere to live. Give the backdrop a strip across the top.

1. Click the Stage, then the Backdrops tab.
2. Pick the rectangle tool and a pale colour.
3. Draw a strip across the very top, about 50 pixels deep. Its bottom edge is at y: 130.

3 · A colour button — and a surprise

Add the Ball sprite. Pick its pink costume, set its size to 60, and put it in the strip. Then give it this script:

when this sprite clicked
set pen color to [#D400AB]
            

4 · The ball tells the pencil

Only the pencil can change the pencil’s pen. So the ball sends a message, and the pencil does the work.

when this sprite clicked
broadcast [pink v]
              

when I receive [pink v]
set pen color to [#D400AB]
              

5 · Five buttons, one message

Five colours would need five messages and five scripts on the pencil. There is a shorter way. Each button puts its colour in a variable, then sends one message:

when this sprite clicked
set [colour v] to [#D400AB]
broadcast [new colour v]
              

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

The colour code is a # and six letters or digits, like #D400AB. set pen color to (colour) understands it.

Basic — a palette 25 min

Open your sketch pad from last lesson.

Step 1 — the pencil’s new start

1. Make a variable called colour, for all sprites.
2. In the pencil’s green-flag script, after erase all, add go to [back v] layer.
3. Add set [colour v] to [#4C4CFF] and set pen color to (colour).
4. Check the if says <<mouse down?> and <(mouse y) < (130)>>.

Step 2 — the pencil listens

1. Add when I receive [new colour v] to the pencil.
2. Under it, set pen color to (colour).

Step 3 — the first ball

1. Add the Ball sprite. Size 60. Put it at the left of the strip.
2. Give it when this sprite clicked, set [colour v] to [#FF8000] and broadcast [new colour v].
3. Test it: click the ball, then draw. The line should be orange.

Step 4 — four more

1. Right-click the ball and choose duplicate. Do it four times.
2. Give each copy a different costume and drag it along the strip.
3. Change each copy’s colour code: blue #0077FF, pink #D400AB, green #00A010, purple #5A00CC.

when flag clicked
go to [back v] layer
erase all
set [colour v] to [#4C4CFF]
set pen color to (colour)
set pen size to (4)
forever
  go to [mouse-pointer v]
  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)
            

The colour you are holding

You cannot see the pencil’s colour until you draw. Show it.

• A big ball sits at the right end of the strip.
• It changes to the colour of the ball you clicked.
• It starts blue, like the pencil.

when I receive [new colour v]
switch costume to (held)
            

Three brushes

• Key 1 gives a thin brush, size 2.
• Key 2 gives a middle brush, size 8.
• Key 3 gives a fat brush, size 20.
• Changing the brush does not change the colour.

when [1 v] key pressed
set pen size to (2)
            

A brush that stops

Swap the three keys for two arrows, and show the size on the Stage.

• A variable called brush, shown on the Stage. It starts at 4.
• ↑ makes the brush 2 bigger. ↓ makes it 2 smaller.
• It never goes below 2 or above 30, however long you hold the key.
• The pen size always matches the number on the Stage.

when [up arrow v] key pressed
if <(brush) < (30)> then
  change [brush v] by (2)
end
set pen size to (brush)
            

A surprise colour

Add a star to the end of the palette. It gives the pencil a colour nobody picked.

• Click the star and the pencil gets a random colour.
• Every click gives a different one.
• The five balls still work afterwards.

when this sprite clicked
broadcast [surprise v]

when I receive [surprise v]
set pen (color v) to (pick random (0) to (100))
            

Summary 5 min

• set pen color to [#D400AB] and set pen size to (8) change the next line you draw.
• Every sprite has its own pen. A button cannot change the pencil’s pen.
• The button sends a message. The pencil hears it and changes its own pen.
• A variable can carry the colour, so one message works for every button.

Pen colour

The colour of the next line a sprite draws.

Colour code

A colour written as # and six letters or digits.

Swatch

A small sample of a colour you click to choose it.

Toolbar

The strip of buttons along the top of an app.

Hand in

Save your sketch pad. Next lesson it gets tools: a rainbow brush, an eraser and a stamp.