The first picture

Four lines and a rectangle appears on the display. Without the last of them nothing appears at all

easyYou need: Links and getlink, The text buffer

A display is a separate block, and like a message block it has to be reached with a link. The program draws not on it but in the processor’s buffer, and only drawflush carries what has piled up onto the screen.

steptick 0

Draw0

clear

rgb

Draw1

color

rgb

a

Draw2

rect

xy

widthheight

Draw Flush3

Stop4

@counter0number

Press “run”. An orange rectangle appears on a grey background — four lines, and each one does exactly one thing.

Line by line

draw clear 40 40 50
draw color 255 180 60 255
draw rect 20 20 60 40
drawflush display1

Line	What it does
clear	fills the whole display with a colour: three fields — red, green, blue
color	chooses the colour for whatever is drawn next; the fourth field is opacity
rect	fills a rectangle: corner x, y, then width and height
drawflush	hands what has piled up to the display

The colour holds until the next color: it is not attached to a shape, it simply becomes the current one. So two shapes of one colour are one color and two drawing commands.

Coordinates count from the bottom left corner

A large display’s side is 176 dots, so 0 0 is the bottom left corner and 175 175 the top right. The Y axis grows upwards.

draw rect 20 20 60 40

That is a rectangle 60 wide and 40 tall whose bottom left corner sits at 20 20. Not the centre — the corner.

Note

A display’s side is better asked for than remembered: sensor width display1 @displayWidth gives 80 on an ordinary one and 176 on a large one. Then the picture survives a change of block.

Without drawflush nothing appears

steptick 0

Draw0

clear

rgb

Draw1

color

rgb

a

Draw2

rect

xy

widthheight

Stop3

@counter0number

The same program without the last line. Press “run” — the display stays dark, although all three commands run perfectly well: they sit in the processor’s buffer waiting to be sent.

Silent failure

A forgotten drawflush produces no error and no empty screen with a message — the display simply stays the way it was. If a picture did not appear, the first thing to check is not the coordinates but whether there is a flush line and whether it is addressed to the right block.

The picture stays until it is wiped

A display remembers what was drawn. It has no notion of a “frame”: draw nothing new and the old stays. So a program drawing a moving arrow starts its iteration with clear — otherwise the previous arrows stay where they were.

Remember

• draw piles commands up in the processor’s buffer; drawflush shows them.
• clear fills the whole display, color sets the colour for the shapes that follow.
• Coordinates count from the bottom left corner, the Y axis grows upwards.
• rect takes a corner, a width and a height — not two corners.
• What is drawn stays on the display until a new clear wipes it.

Task

In the first example change draw rect 20 20 60 40 to draw rect 20 20 200 200 and run it: the rectangle covers the whole display. Anything that does not fit is simply cut off by the edge — that is not considered an error.

Then put the sizes back and remove the line draw clear 40 40 50. The rectangle stays and the background turns black: a display does not fill itself, and black is its initial state.