# 6. Graphics Everything in this chapter needs the SDL build and a graphics device. Without one each verb refuses by name: ```basic requires=noakgl 10 DRAW 1, 0, 0 TO 100, 100 ``` ```output ? 10 : RUNTIME ERROR DRAW needs a graphics device and this runtime has none ``` ## The coordinate space **A drawing coordinate is a pixel of the host's window**, with (0, 0) at the top left. On the standalone interpreter's 800 by 600 window, `DRAW 1, 799, 599` lands on the bottom-right pixel and everything in between is reachable. A game embedding the interpreter gets whatever size its own renderer is. `RGR` is how a program finds out: ```basic requires=akgl 10 PRINT "THE SCREEN IS" 20 PRINT RGR(1) 30 PRINT "BY" 40 PRINT RGR(2) 50 DRAW 1, 0, 0 TO RGR(1) - 1, RGR(2) - 1 ``` Subtracting one is not a wart, it is the last pixel: a window `RGR(1)` wide has columns 0 through `RGR(1) - 1`. **A C128 listing assumes 320 by 200 and will draw in the top-left corner.** Give it the whole window by naming the space it was written for: ```basic requires=akgl 10 SCALE 1, 319, 199 20 BOX 1, 0, 0, 319, 199 ``` That box is now the border of the window whatever size the window is. When no device answers the size question at all, 320 by 200 is what the interpreter assumes — the space a C128 listing was written for is the right thing to fall back to. ## Colour `COLOR` binds a *source* to a palette index, and the drawing verbs name the source rather than the colour: ```basic requires=akgl 10 COLOR 1, 3 20 DRAW 1, 10, 20 ``` Sources are numbered 0 to 6; palette indices are 1 to 16, as on a C128. That indirection is BASIC 7.0's, and it is why every drawing verb's first argument is a small number that is not a colour. ## The verbs ### GRAPHIC `GRAPHIC mode` chooses a screen mode; `GRAPHIC CLR` clears it. Mode 0 is text and refuses to draw. Every picture in this chapter is generated by running the listing above it; see `MAINTENANCE.md` if you are editing one. ### DRAW ```basic requires=akgl screenshot=draw 10 COLOR 1, 8 20 DRAW 1, 20, 180 TO 90, 40 TO 160, 150 TO 230, 20 TO 300, 120 30 COLOR 2, 6 40 DRAW 2, 20, 190 TO 300, 190 50 LOCATE 160, 100 60 COLOR 3, 3 70 DRAW 3 ``` ![](images/draw.png) One coordinate pair plots a point. Two or more, separated by `TO`, draw a polyline. A bare `DRAW 3` — the last line above, and the single red pixel in the middle of the picture — plots wherever `LOCATE` left the pixel cursor. ### BOX ```basic requires=akgl screenshot=box 10 COLOR 1, 8 20 BOX 1, 20, 30, 130, 140 30 COLOR 2, 6 40 BOX 2, 180, 30, 290, 140, 30 50 COLOR 3, 3 60 LOCATE 300, 190 70 BOX 3, 20, 160 ``` ![](images/box.png) Corners, and an optional rotation angle — the green box is the same box turned 30 degrees about its own centre. Two coordinates instead of four take the other corner from the pixel cursor, which is the red box. **`BOX` always outlines; it cannot fill.** BASIC 7.0 selects fill with a seventh argument and that is not implemented here — see Chapter 13. `PAINT` is the fill you have. ### CIRCLE ```basic requires=akgl screenshot=circle 10 COLOR 1, 8 20 CIRCLE 1, 80, 70, 60, 60 30 COLOR 2, 6 40 CIRCLE 2, 230, 70, 75, 45 50 COLOR 3, 3 60 CIRCLE 3, 160, 140, 130, 50, 90, 270 ``` ![](images/circle.png) Source, centre, then the two radii — equal radii give a circle and unequal ones an ellipse. Two further arguments are a start and an end angle, which is what makes the red arc: 90 to 270 is the bottom half, because angles here are degrees clockwise from straight up, the same convention `MOVSPR` uses. Beyond those come a rotation and the degree increment, and a large increment is what turns a circle into a polygon. ### PAINT ```basic requires=akgl screenshot=paint 10 COLOR 1, 8 20 CIRCLE 1, 100, 100, 70, 70 30 BOX 1, 180, 50, 290, 150 40 COLOR 2, 6 50 PAINT 2, 100, 100 60 COLOR 3, 3 70 PAINT 3, 230, 100 ``` ![](images/paint.png) Flood-fills the region containing a point, stopping at whatever is already drawn — so the outline you fill inside can come from any verb. If the region is too large for the fill's own working space it stops and reports rather than leaving a half-painted screen with no explanation. ### LOCATE Moves the pixel cursor, which is where a bare `DRAW` plots and where a `BOX` with two coordinates finishes. ### SCALE ```basic requires=akgl screenshot=scale size=640x400 10 COLOR 1, 3 20 BOX 1, 0, 0, 319, 199 30 SCALE 1, 319, 199 40 COLOR 2, 6 50 BOX 2, 0, 0, 319, 199 60 DRAW 2, 0, 0 TO 319, 199 ``` ![](images/scale.png) That picture is 640 by 400, and both boxes name the same four numbers. The red one is drawn with `SCALE` off, so its coordinates are pixels and it covers exactly the top-left 320 by 200 of the window — which is what a C128 listing does here. The green one is drawn after `SCALE 1, 319, 199` and fills the window, and the diagonal confirms that 319, 199 reaches the last pixel rather than stopping one short of it. Turns on user coordinates and gives their maxima. With it on, your coordinates are mapped onto the drawing surface: 0 is the first pixel and the maximum you gave is the *last* one, so `DRAW 1, 1023, 1023` above reaches the bottom-right corner rather than missing it by a pixel. `SCALE 1` on its own uses 7.0's 1023 by 1023. `SCALE 0` turns it off and coordinates go back to being window pixels. ### RGR `RGR(0)` is the current `GRAPHIC` mode. `RGR(1)` and `RGR(2)` are the drawing surface's width and height in pixels — those two are ours rather than 7.0's, and they are what a program needs to use a window whose size it did not choose. They refuse when there is no graphics device, unlike `RGR(0)`, which is a mode this interpreter recorded rather than a screen it has to go and measure. **`RGR(3)` and `RGR(4)` are a character cell's width and height**, also ours. They come from the *text* device rather than the graphics one — a character grid belongs to the sink — so they refuse by naming that instead. Between them and [`RWINDOW`](12-function-reference.md), which gives the current text window in columns and rows, a program can place a character and a sprite at the same spot without hardcoding a number measured against whatever font the host loaded: ```basic requires=akgl 10 CW# = RGR(3) 20 CH# = RGR(4) 30 COL# = 12 40 ROW# = 3 50 CHAR 1, COL#, ROW#, "X" 60 MOVSPR 1, COL# * CW#, ROW# * CH# ``` `RWINDOW` follows a `WINDOW` call, because it reports the window. `RGR(3)` and `RGR(4)` do not, because windowing does not change how big a character is. ### WIDTH `WIDTH 1` or `WIDTH 2` sets how thick a drawn line is. A thick line is drawn as parallel passes; see Chapter 13. ## Saving and stamping regions `SSHAPE` copies a rectangle off the screen and `GSHAPE` stamps it back: ```basic requires=akgl screenshot=shapes 10 COLOR 1, 8 20 CIRCLE 1, 40, 40, 30, 30 30 COLOR 2, 6 40 PAINT 2, 40, 40 50 SSHAPE A$, 8, 8, 72, 72 60 GSHAPE A$, 120, 20 70 GSHAPE A$, 200, 60 80 GSHAPE A$, 120, 120 ``` ![](images/shapes.png) One disc drawn, captured, and stamped three times. **`A$` holds a handle, not the pixels.** On a C128 the string holds the bitmap, so a program could save it to disk or take its `LEN`. Here a string is a fixed 255 bytes and the region is a device surface, so what goes in the string is a reference to it — `SHAPE:0`. You can pass it to `GSHAPE` and to `SPRSAV`, which is everything BASIC ever does with one, but you cannot store it or measure it. ## What is not here `FILTER` parses and then refuses: there is no filter stage to configure. See Chapter 7. The graphics verbs draw straight to the renderer rather than into a display list, so anything drawn is overwritten by the text layer on the next frame. A program that wants its drawing to persist has to redraw it. This is recorded as a defect rather than a design; see Chapter 13.