Andrew Kesterson
1fb808f480
Chapters 17 and 18 read as a code review of a finished listing: they explained why each decision had been made, walked through the project's own history, and led with what had once been broken. A reader who wanted to build the game got the reasoning and had to reconstruct the program. Both are now step-by-step. Each opens with a picture of the finished game and a bullet list of the steps, each bullet is a section, and each section states its goal, shows the code, and says how to check it. Chapter 17 is sixteen steps and Chapter 18 is thirteen, and the last of each is the assembly: the order of the file, the full declaration block, and the routines the earlier steps referred to. Project history is gone -- it belongs in Chapter 14 and in git -- and where a listing has to do something awkward, the tutorial shows how first and names the `TODO.md` item that will make it unnecessary second. **Three defects had no entry anywhere**, which the rewrite found by trying to state each rule as a rule. §6 item 35: `a - b + c` computes `a - (b + c)`, because `subtraction()` sits above `addition()` as its own precedence level and the inner loop eats the `+`. Item 36: only one unparenthesised `AND` or `OR` is matched, which is item 12's `if`-where-`while` on the one operator pair item 12 did not reach. Item 37: a `GOTO` out of a `FOR` or a `DO` leaks the loop's scope, so a main loop written that way stops on the thirty-second lost life -- which is why both games are built out of `LABEL` and `GOTO`, and it is a workaround rather than a preference. Chapter 3 gains the identifier rule the third trial ran into: there is no underscore in a name, and the error says `UNKNOWN TOKEN _`. **`tools/screenshot.c` learned to draw the text layer**, behind a new `text=1` fence attribute, because Chapter 17's game is characters in the grid and a figure without that layer is two sprites on black. It opens the bundled font at the size the standalone frontend uses, so a figure's cell size is the reader's cell size, and it uses the akgl sink alone rather than a tee so the program's output lands in the picture instead of on the stdout the caller reads to decide a figure failed. Both new figures -- `breakout-game.png` and `breakout-game-artwork.png` -- are generated from listings in the chapters like every other one. Verified by handing each chapter, alone, to an agent on a much smaller model and telling it to build the game from the tutorial text with the `examples/` tree off limits. The first pass scored 3.5 and 3 out of 10 and named what was missing: routines referred to but never shown, the third level layout, the sprite `DATA`, the font table, edits to earlier routines that were never marked as edits. Those are now in. The second pass built a 658-line Chapter 17 game that plays itself for ninety seconds with the score at 1890 and no error line, and the third built a 1053-line Chapter 18 game with 61 labels, no invented routines, no gaps found, and forty seconds clean. 9/10 and 8/10. One real bug in the new prose, caught in review: Chapter 18's `HITBAR` did not set the `HIT#` that `BALLPADDLE` reads to decide whether the paddle already caught the ball. Both suites green in both configurations, `docs_examples` and `docs_screenshots --check` pass. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL
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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. 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, 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:
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:
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 string array works: SSHAPE SH$(2), ... writes into element 2 and GSHAPE SH$(2)
stamps what element 2 names, so a program can keep a set of shapes in one array.
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.
A redraw also has to fit inside one batch. The host runs a fixed number of source
lines and then presents, and presenting throws the drawing buffer away — so a run of
drawing verbs longer than one batch is torn rather than merely transient, and an
SSHAPE at the end of it captures only the part issued since the present. Watching
TI# change is how a program finds the boundary; Chapter 13 has the measured numbers and
Chapter 18 has a
routine that uses them.