put similar to the the first iteration of the program you will need to change the **FOR** loop and **PLOT** commands to: ``` 10 FOR n= 0 TO 511 20 PLOT n,80*SQR((511-n)/128) ``` This will invert the coordinates to simulate the *Layer 0* display, by drawing inverted, extend the **PLOT** *x coordinate* to 512 pixels and make sure the **PLOT** doesn't get out of bounds (that's why we divide by **128** instead of **64**). In reality, you do not need to check if you **PLOT** out of bounds for layers other than *Layer 0*, as graphics commands for these accept locations outside the screen's pixel boundaries, however it's good practice to do so if you want your program to work across layers. ### DRAW and CIRCLE To help you with your pictures, the computer will draw straight lines, circles and parts of circles for you, using the **DRAW** and **CIRCLE** statements. The statement **DRAW** to draw a straight line takes the form: **DRAW** *x_coordinate*, *y_coordinate* The starting place of the line is the pixel where the last **PLOT**, **DRAW** or **CIRCLE** statement left off (this is called the **PLOT** position; **RUN**, **CLEAR**, **CLS** and **NEW** reset it to the *coordinate 0* of the selected Layer (bottom left hand corner, at (0,0) for *Layer 0*, top left hand corner for all other layers), and the finishing place is **x** pixels to the *RIGHT* of that and **y** pixels *UP* or *DOWN* depending on which layer you're on. This would be *UP* for *Layer 0* and *DOWN* for all other layers. The **DRAW** statement on its own determines the length and direction of the line, but not its starting point. Experiment with a few **PLOT** and **DRAW** commands, for instance: ``` PLOT 0,100: DRAW 80,-35 PLOT 90,150: DRAW 80,-35 ``` Notice that the numbers in a **DRAW** statement can be negative, although those in a **PLOT** statement can't. Remember *always*, that the display direction of the **DRAW** statement changes according to the coordinate system used, ergo which layer you choose is very important. You can also plot and draw in colour, although you have to bear in mind all that were discussed in *Chapter 15*. Depending on the chosen layer, colours may cover the whole of an attribute position instead of individual pixels. Only *LoRe*s and *Layer 2* modes offer full individual colour pixel control whereas other layers rely on the attribute used. The following program demonstrates this: ``` 10 LAYER 2,0: REM disable Layer 2 20 FOR m=0 TO 5 30 PROC LayChange (m) 40 BORDER 0:PAPER 0:INK 7:CLS: REM black out screen 50 x1,y1=0:REM line start 60 c=1: REM ink, starts with blue 70 FOR r = 0 TO 9:REM 10 repetitions ```