Drawing With Light

Under the character grid is a finer one: pixels, 256 across and 176 tall. Three keywords draw on it — PLOT a point, DRAW a line, CIRCLE a circle. There’s one catch to learn first, and it’s about which way is up.

Milestone 1 — a point and a line

  10 BORDER 0
  20 PAPER 0
  30 INK 7
  40 CLS
  50 PLOT 20,20
  60 DRAW 200,140

PLOT 20, 20 lights a pixel near the bottom-left — because the pixel grid counts from the bottom-left, with y going up. DRAW 200, 140 then draws a line relative to that point: 200 pixels right and 140 up. So the line climbs as it goes right.

That’s the catch: PLOT and DRAW measure from the bottom-left, y upward, while PRINT AT measures rows from the top. Two grids, two origins — mixing them up is the most common drawing mistake.

Milestone 2 — a box and a circle

DRAW runs from wherever the last point was, so four DRAWs make a closed box. CIRCLE takes a centre and a radius:

  10 BORDER 0
  20 PAPER 0
  30 CLS
  40 INK 5
  50 PLOT 40,40
  60 DRAW 80,0
  70 DRAW 0,80
  80 DRAW -80,0
  90 DRAW 0,-80
 100 INK 6
 110 CIRCLE 200,80,35

The four DRAWs trace a square — right, up, left, down, back to the start. CIRCLE 200, 80, 35 draws a circle centred at (200, 80) with a radius of 35.

Milestone 3 — frame the title card

Now both grids at once. The title card’s text is placed with PRINT AT (rows from the top); the frame around it is drawn with PLOT and DRAW (pixels from the bottom). Line them up and the title card gets its border:

  10 BORDER 1
  20 PAPER 1
  30 INK 6
  40 CLS
  50 PRINT AT 8,11;"MEET BASIC"
  60 PRINT AT 10,7;"a Spectrum primer"
  70 PLOT 48,70
  80 DRAW 160,0
  90 DRAW 0,50
 100 DRAW -160,0
 110 DRAW 0,-50

When it doesn’t work

• The drawing came out upside down. You used PRINT AT thinking, top-down. PLOT/ DRAW count up from the bottom — a bigger y is higher on screen.
• Integer out of range. A point off the grid. Pixels run 0–255 across and 0–175 up.
• The box didn’t close. DRAW is relative — each one starts where the last ended. To close a box, the four moves must sum back to the start.

Before and after

You started able to place text and finished drawing shapes — a line, a box, a circle, and a frame around the title card. The idea underneath: PLOT lights a pixel, DRAW runs a line from it, CIRCLE draws a circle — on a 256-by-176 grid measured from the bottom-left, y upward.

Try this

• A triangle. Three DRAWs from a PLOT, back to the start.
• A target. Stack three CIRCLEs at the same centre with growing radii.
• Resize the frame. Widen the title-card box by changing the DRAW lengths — and watch which way “up” moves the top edge.

What you’ve learnt

• The pixel grid is 256 across, 176 up, measured from the bottom-left.
• PLOT x, y lights a pixel; DRAW dx, dy runs a line relative to the last point; CIRCLE x, y, r draws a circle.
• PLOT/DRAW count y upward — the opposite of PRINT AT’s rows-from-the-top.
• Text and graphics share the screen: place with PRINT AT, draw with PLOT/DRAW.

What’s next

The title card has everything but life. In Unit 13 we make it move — and discover, in the flicker, exactly why the Spectrum’s legends reached for assembly.