Andrew Kesterson
16b38c1138
Chapters 6 and 8 described what a verb draws in prose. Eight figures now show it, and each one is produced by running the BASIC listing printed immediately above it -- so a picture cannot drift away from the code beside it, which is the way a screenshot goes wrong and the way nothing notices. tools/screenshot.c is a second SDL host, much smaller than the frontend: dummy video driver, software renderer, run to completion, read the target back, write a PNG. It draws no text layer on purpose, so a READY in the corner is not noise in a figure about BOX and no font has to be resolved. tools/docs_screenshots.sh reads the new screenshot=NAME fence tag straight out of the markdown. size=WxH is the second tag, and SCALE's figure uses it: the point being made is a 320x200 listing filling a larger window, which cannot be made on a 320x200 surface. Two gates, answering different questions. docs_examples fails a tagged block with no image, in both configurations, so a figure cannot be added and forgotten. docs_screenshots -- a CTest, AKGL build only -- re-renders every figure and compares byte for byte, so a listing edited without regenerating fails. Only the second catches a stale picture. The PNGs are checked in because a reader on the forge has no build tree, and docs/images/README.md says loudly that they are generated. Regenerating is never part of a build: the target is run deliberately, so a make cannot put eight binary diffs in front of whoever ran it. Drawing the BOX figure caught a defect in TODO.md itself. Deviation 16 claimed in bold that BOX fills on a negative angle while its own paragraph said the fill was filed rather than implemented. BOX cannot fill, and filled_rect is reached by no verb as a result. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
6.9 KiB
6. Graphics
Everything in this chapter needs the SDL build and a graphics device. Without one each verb refuses by name:
10 DRAW 1, 0, 0 TO 100, 100
? 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:
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:
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:
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
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
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
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
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
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
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
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
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
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
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. All three
refuse when there is no graphics device, except RGR(0), which is a mode this
interpreter recorded rather than a screen it has to go and measure.
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:
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
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.