Decompress a full-screen picture and six frames of video from strings

The megademo gains a scene: an 800x600 vaporwave sunset carried inside
the listing, then six looping delta frames of full motion video -- the
floor grid rolling forward and the sun's slices crawling, about 220
bytes a frame. DATA cannot carry an image (512 items, ~350 owned by the
stroke font), so the picture rides in RLE-encoded string literals
decoded with INSTR: 2 KB of base frame, 1.3 KB of video. Row records
jump the decoder between changed rows, an unreachable colour is the
skip, black draws so a delta can erase, and the rays are struck live
over every frame rather than encoded. The 63-byte bird gliding over it
is the one image DATA does have room for, via SPRSAV's type-in form.

examples/megademo/vaporwave.py composes the image in palette space,
dithers by row so the gradients band instead of shattering the RLE,
proves base-plus-deltas reproduces every frame against a simulation of
the decoder, and owns the generated block between the PICTURE markers.
TODO.md section 4 records the DATA ceiling and the wanted verb.

Co-Authored-By: Claude Code (Fable 5) <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL

examples/megademo/megademo.bas

@@ -57,9 +57,14 @@ DIM WQX#(14)
 DIM WQY#(14)
 DIM WE1#(24)
 DIM WE2#(24)
+DIM PBRD#(63)
 
 ALPHA$ = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 :-.!"
 GC$ = "GAMECUBE"
+REM  The picture decoder's alphabets: a run is two characters, colour
+REM  then length, and INSTR turns each back into a number.
+CPAL$ = "ABCDEFGHIJKLMNOP"
+LTAB$ = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 SEED# = 314159
 QF# = 0
 RB# = 0
@@ -120,6 +125,7 @@ SEED# = SEED# + TI#
 GOSUB LOADTABLES
 GOSUB LOADWIRE
 GOSUB LOADFONT
+GOSUB LOADBIRD
 IF SND# = 1 THEN GOSUB SIDINIT
 GOSUB TUNE
 IF TXT# = 1 THEN CHAR 1, 0, 0, "AKLABS MEGADEMO 85 - PRESS Q TO EXIT"
@@ -429,7 +435,117 @@ LABEL BARDONE
 IF QF# = 1 THEN GOTO BYE
 
 REM =====================================================================
-REM  SCENE 5 : THE SPRITE BANK IS THE SCROLLTEXT, AND THEN IT EXPLODES.
+REM  SCENE 5 : THE PICTURE, AND THEN IT MOVES.  A full 800x600
+REM  vaporwave sunset carried inside the listing, then six delta
+REM  frames of full motion video in a loop. DATA could never hold any
+REM  of it -- the pool is 512 items and the font owns most of them
+REM  (TODO.md section 4) -- so it all rides in string literals:
+REM  run-length encoded, two characters a run, decoded with INSTR and
+REM  painted as 5x5 blocks. A video frame re-encodes only the rows
+REM  that changed ('R' records jump the cursor), 'Q' skips what stayed
+REM  and black draws over what moved, so a frame of grid-scroll and
+REM  sun-shimmer is about two hundred bytes and paints in a few
+REM  presents. The rays are not encoded at all: they are struck live
+REM  over every frame, which is what keeps the floor rows cheap. The
+REM  loop is exact -- the grid advances one geometric step over six
+REM  frames and the slice pattern's period is six. The paint is the
+REM  reveal; the exit is a stride-63 column dissolve. The bird is the
+REM  one thing DATA does hold: a real 24x21 SPRSAV sprite, 63 bytes,
+REM  gliding across its own sunset. vaporwave.py draws the picture,
+REM  proves base plus deltas reproduce every frame, and owns the
+REM  generated block.
+REM =====================================================================
+GRAPHIC 1, 1
+WIDTH 2
+BS% = W# / 160
+IF BS% < 1 THEN BS% = 1
+REM  ---- PICTURE-BEGIN (generated by vaporwave.py; do not
+REM  ---- hand-edit -- rerun the script to change the picture)
+DIM IM$(16)
+DIM VA#(6)
+DIM VB#(6)
+NS# = 9
+VA#(0) = 9
+VB#(0) = 9
+VA#(1) = 10
+VB#(1) = 10
+VA#(2) = 11
+VB#(2) = 12
+VA#(3) = 13
+VB#(3) = 13
+VA#(4) = 14
+VB#(4) = 14
+VA#(5) = 15
+VB#(5) = 15
+IM$(0) = "G9G9G9G9GPGHEHG9GTEHG9GTEHGPGPEHG9GTEHG9GTEHGHGXEHGTBAG8EHG9GTEHG5EHGPEHG5EHGPEHG5EHGAPAG3EHGPEHG5EHGPEHGXGHEHG5EHGPEHG5EHGFBAGIEHGPGPEHGPEHGHEHGPEHGPEHGHEHGPEHGHEHGPEHGPEHGHEHGBPAGMEHGPEHGHEHGPEHGHEHGPEHGPEHGHEHGPEHGPEHGHEHGPEHGHEPGHEHGPEH"
+IM$(1) = "GHEPGHEHGPEHGHEPGHEHGHEGBAEHGHEHGPEHGHEPGHEHGPEHGHEHGPEHGHEPGHEHGPEHGHEPGHEHGPECPAEDGHEPGHEHGHEPGHEPGHEHGHEPGHEPGHEHGHEPGHEHGHEPGHEPGHECBAEDGHEPGHEPGHEHGHEPGHEHGHEPGHEPGHEHGHEPGHEPEPGHEPGHE5GHEPGHE5GHEHEXGHEPGHEEPAEZGHEPGHE5GHE5GHEPGHE5GHEP"
+IM$(2) = "GHE5GHE9ETGHE9ETGHEXEHGHE9ETGHHAE9ESGHEPEPGBBAGEE9EMHOE9ETGHEHE9E7HUE9E6E9E4H0E9E3E9E3H2E9EHBAETEHKHE9ELH6E9ECKHEPEPKHE9EBH9HAE9EIKHEHEXKHE2H9HCE9EPKHE5KHEPKDH9HEKCEPKHE5KHE5KHEKH9HGEBKHEPKHEXEHKHE5KHEBH9HIEIKHEPKHEPEPKHEPKHEHKAH9HKKHEHKHEP"
+IM$(3) = "KHEHKHEPKHEPKHH9HMEGKHEHKHEPKHEHKHEPKHEPH9HMEOKHEHKHEPKHEHKPEHKHEGH9HOKFEPKHEHKPEHKHEHKPEHKGH9HOEFKHEPKHEHKHEPKHEHKPEFH9HQKEEHKHEPKHEHKHEPKHEHKMH9HSKLEHKHEPKHEHKPEHKHEHKEH9HSEDKPEHKPEHKHEHKPEHKHEEH9HSKDEHKPEHKPKPEHKPEHKDH9HUECKHEHKPEHKHKXEH"
+IM$(4) = "KPEDH9HUKCEHK5EHK5EHKLH9HUKKEHK5EHK5EHKCH9HWEBKPEHKXKHEHK8I9IWKZEHKPKPEHK0I9IWK7EHKHKXEHKSI9IWK9KFEHK9KOI9IWK9KNK9KOI9IWK9KNK9KOI9IWK9KNKPIHKZI9IYK6IHKHKXIHKSI9IWK9KFIHK5IHKKI9IWK9KNIHK5IHKCI9IYKPIHKXKHIHK5IHKPIHK5IHKPIHKPKPIHK0I9IWKJIHKPIH"
+IM$(5) = "KHIHKPIHKPI9IZKBIHKHIHKPIHKHIHKPIHKKI9IWKJIHKHIHKPIHKHIHKPIHKDI9IXKPIHKHIHKHKHIHKHIPKHI9IYKCIHKPIHKHIHKPIHKHIPKHIHKPIHKHIPKHIHKPIHKHIHKPIHKHI9I9IHKHIHKPIHKHIPKHIHKHIEC9CSKDIPKHIPKHIHKHIPKHIHKEC9CSIDKHIPKHIPIPKHIPKHIFC9CQKEIHKHIPKHIHIXKHIPKG"
+IM$(6) = "C9COIFKHI5KHI5KHIPKHI5KHIPKHI5KHI5KHIPKHI5KHIPKHIXIHKHI9IDC9CMI4KHIPIPKHI6C9CKI9IDKHIHIXKHIZC9CII9IMKHI9IWC9CGI9IVI9I9I9I9IPI9I9I9I9IPQ9Q9Q9Q9QPE9E9E9E9EPE9E9E9E9EPQ9Q9Q9Q9QPE9E9E9E9EPQ9Q9Q9Q9QPE9E9E9E9EPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9"
+IM$(7) = "Q9QPE9E9E9E9EPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPE9E9E9E9EPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPE9E9E9E9EPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9"
+IM$(8) = "Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QPQ9Q9Q9Q9QP"
+IM$(9) = "RBRQ9QOKMQHK5RBSQ9QTI9IRRBXQ9QTKPQHKHQPKERBYQ9QPI9IHRB3Q9QSKAI5KHIJRB5Q9QTC9CMRB9Q9QWI9IGRCBQ9QYC9CCRCIA9A9A9A9APRCJE9E9E9E9EPRCNA9A9A9A9APRCOE9E9E9E9EPRCUA9A9A9A9APRCVE9E9E9E9EPRC5A9A9A9A9APRC7E9E9E9E9EP"
+IM$(10) = "RBQQ9QOK9KIQHKFRBRQ9QOI9IQRBWQ9QPKLQHKHQHKPRBXQ9QTI9QLIERB2Q9QRIBKHIPKHIPKCRB4Q9QSC9CORB8Q9QVI3KHIGRCAQ9QXC9CERCJA9A9A9A9APRCKE9E9E9E9EPRCOA9A9A9A9APRCPE9E9E9E9EPRCVA9A9A9A9APRCXE9E9E9E9EPRC7A9A9A9A9APRDAE9E9E9E9EP"
+IM$(11) = "RBPQ9QOK9KAQHKNRBQQ9QOI9IWRBVQ9QOKEQHKHQHKPQHKFRBWQ9QPI9IPRB1Q9QQKCIPKHIPKHIDRB3Q9QSC9CORB7Q9QUIWKHIPRB9Q9QWC9CGRCEA9A9A9A9APRCFE9E9E9E9EPRCGA9A9A9A9APRCHE9E9E9E9EPRCPA9A9A9A9APRCQE9E9E9E9EPRCXA9A9A9A9APRCZE9E9E9E9EPRDAA9A9A9A9APRDCE9E9E9E9"
+IM$(12) = "EP"
+IM$(13) = "RBOQ9QNK3QHKWRBPQ9QOI9IWRBUQ9QTKHQHKPQHKNRBVQ9QOI9IWRB0Q9QQIKKHIPKHIHKDRB1Q9QQC9CSRB2Q9QRC9CQRB6Q9QTIPKHIYRB8Q9QVC9CIRCKA9A9A9A9APRCLE9E9E9E9EPRCQA9A9A9A9APRCRE9E9E9E9EPRCZA9A9A9A9APRC1E9E9E9E9EPRDCA9A9A9A9APRDFE9E9E9E9EP"
+IM$(14) = "RBOQ9QNI9IYRBTQ9QOKEQHKPQHKVRBUQ9QTI9IRRBZQ9QTKHQHKPQHKHRB0Q9QQC9CSRB5Q9QTIHKHI5KARB7Q9QUC9CKRCBQ9QYI9ICRCHA9A9A9A9APRCIE9E9E9E9EPRCLA9A9A9A9APRCME9E9E9E9EPRCRA9A9A9A9APRCTE9E9E9E9EPRC1A9A9A9A9APRC3E9E9E9E9EPRDFA9A9A9A9APRDIE9E9E9E9EP"
+IM$(15) = "RBSQ9QTKPQHK3RBTQ9QOI9IWRBYQ9QPKDQHKPQHKHRBZQ9QTI9ILRB4Q9QSIAKHI5KHIBRB6Q9QTC9CMRCAQ9QXI9IERCEE9E9E9E9EPRCFA9A9A9A9APRCGE9E9E9E9EPRCMA9A9A9A9APRCNE9E9E9E9EPRCTA9A9A9A9APRCUE9E9E9E9EPRC3A9A9A9A9APRC5E9E9E9E9EPRDIA9A9A9A9AP"
+REM  ---- PICTURE-END
+SS# = 0
+SE# = NS# - 1
+GOSUB DRAWSTREAM
+GOSUB RAYS
+SPRSAV PBRD#, 1
+SPRITE 1, 1, 1, 0, 1, 1
+MOVSPR 1, 0.16 * W#, 0.14 * H#
+IF HL# = 0 THEN MOVSPR 1, 96 # 1
+COLOR 1, 2
+T$ = "THE PICTURE IS STRINGS - THE BIRD IS DATA"
+SZ% = 0.0038 * W#
+GOSUB CENTERX
+TY% = 0.02 * H#
+GOSUB DRAWTEXT
+IF HL# = 1 THEN GOTO PICDONE
+REM  Six delta frames in a loop: the grid rolls toward you and the
+REM  sun's slices crawl. Each frame repaints only the rows that
+REM  changed, and the rays are struck over the top again.
+VI# = 0
+DL# = TI# + 480
+DO WHILE TI# < DL#
+  SS# = VA#(VI#)
+  SE# = VB#(VI#)
+  GOSUB DRAWSTREAM
+  GOSUB RAYS
+  VI# = VI# + 1
+  IF VI# >= 6 THEN VI# = 0
+  IF TI# > MT# THEN GOSUB TUNE
+  GOSUB POLLKEY
+  IF QF# = 1 THEN EXIT
+LOOP
+REM  The dissolve: black out the 160 columns in stride-63 order.
+COLOR 1, 1
+K2# = 0
+DO WHILE K2# < 160
+  CO# = MOD(K2# * 63, 160)
+  X3% = CO# * BS%
+  DRAW 1, X3%, 0 TO X3%, H# - 1
+  X3% = X3% + 2
+  DRAW 1, X3%, 0 TO X3%, H# - 1
+  X3% = X3% + 2
+  DRAW 1, X3%, 0 TO X3%, H# - 1
+  IF MOD(K2#, 5) = 0 THEN GOSUB VSYNC
+  K2# = K2# + 1
+LOOP
+MOVSPR 1, 0 # 0
+SPRITE 1, 0
+LABEL PICDONE
+IF QF# = 1 THEN GOTO BYE
+
+REM =====================================================================
+REM  SCENE 6 : THE SPRITE BANK IS THE SCROLLTEXT, AND THEN IT EXPLODES.
 REM  Eight letters, eight sprites: draw each with the stroke font,
 REM  SSHAPE it, SPRSAV it, surf it on a sine. Then the letters fall
 REM  into the middle, a supernova throws them at the walls -- SOLID
@@ -573,7 +689,7 @@ LABEL WAVEDONE
 IF QF# = 1 THEN GOTO BYE
 
 REM =====================================================================
-REM  SCENE 6 : THE FINALE CARD.  Everything at once: stars, bars, the
+REM  SCENE 7 : THE FINALE CARD.  Everything at once: stars, bars, the
 REM  logo, the greetz, and the sprite bank surfing a sine.  This is the
 REM  frame a clockless host leaves on the screen.
 REM =====================================================================
@@ -869,6 +985,7 @@ REM  A ball of light: concentric circles three pixels apart, solid at
 REM  WIDTH 2. DCX%/DCY% centre it, DR% sizes it, DCI# colours it --
 REM  DCI# = 1 is the eraser.
 LABEL DISC
+IF DR% < 1 THEN RETURN
 COLOR 1, DCI#
 R3% = DR%
 DO WHILE R3% > 1
@@ -881,13 +998,11 @@ REM  The origin point: erase it where it was, redraw it pulsing on
 REM  SIN(BF#) and strobing through the bar ramp table -- the colours
 REM  of the next scene, because this is where its bars come from.
 LABEL BALLFX
-IF PB% <= 0 THEN GOTO BALLFX2
 DCI# = 1
 DR% = PB%
 DCX% = C1X%
 DCY% = CY%
 GOSUB DISC
-LABEL BALLFX2
 DR% = BBR% + ((0.45 * BBR%) * SIN(1.1 * BF#))
 DCI# = BARC#(MOD(BF#, 20))
 DCX% = C1X%
@@ -920,6 +1035,72 @@ IF SND# = 0 THEN RETURN
 SOUND 3, 4000, 40, 1, 52000, 800
 RETURN
 
+REM  Decode picture strings IM$(SS#) through IM$(SE#) and paint them
+REM  as 5x5 blocks. A run is colour-char then length-char. 'Q' misses
+REM  the colour table, so it advances without drawing -- the skip. 'A'
+REM  is black and DOES draw, which is how a video delta erases. 'R'
+REM  opens a row record: two base-36 digits of row, cursor to column
+REM  zero -- how a delta jumps between the rows that changed.
+LABEL DRAWSTREAM
+X# = 0
+Y# = 0
+S# = SS#
+DO WHILE S# <= SE#
+  T2$ = IM$(S#)
+  TL2# = LEN(T2$)
+  P# = 0
+  DO WHILE P# < TL2#
+    C2$ = MID(T2$, P#, 1)
+    IF C2$ = "R" THEN GOSUB PICROW ELSE GOSUB PICRUN2
+  LOOP
+  GOSUB POLLKEY
+  S# = S# + 1
+LOOP
+RETURN
+
+LABEL PICROW
+Y# = INSTR(LTAB$, MID(T2$, P# + 1, 1)) * 36
+Y# = Y# + INSTR(LTAB$, MID(T2$, P# + 2, 1))
+X# = 0
+P# = P# + 3
+RETURN
+
+LABEL PICRUN2
+CI# = INSTR(CPAL$, C2$) + 1
+LN# = INSTR(LTAB$, MID(T2$, P# + 1, 1)) + 1
+IF CI# > 0 THEN GOSUB PICRUN
+X# = X# + LN#
+IF X# >= 160 THEN X# = 0
+IF X# = 0 THEN Y# = Y# + 1
+P# = P# + 2
+RETURN
+
+REM  One run: LN# cells of colour CI# at cell (X#, Y#), scaled by BS%.
+LABEL PICRUN
+X3% = X# * BS%
+X4% = ((X# + LN#) * BS%) - 1
+Y3% = Y# * BS%
+COLOR 1, CI#
+DRAW 1, X3%, Y3% TO X4%, Y3%
+Y3% = Y3% + 2
+DRAW 1, X3%, Y3% TO X4%, Y3%
+Y3% = Y3% + 2
+DRAW 1, X3%, Y3% TO X4%, Y3%
+RETURN
+
+REM  The rays live outside the encoding, redrawn over every video
+REM  frame: nineteen lines instead of thirty runs a grid row, and they
+REM  are what makes the encoded floor nearly free.
+LABEL RAYS
+COLOR 1, 4
+RI# = 0 - 9
+DO WHILE RI# < 10
+  X3% = (0.5 * W#) + (RI# * 26 * BS%)
+  DRAW 1, 0.5 * W#, (74 * BS%) + 2 TO X3%, H# + 99
+  RI# = RI# + 1
+LOOP
+RETURN
+
 REM  Park the eight letter sprites on a sine.
 LABEL WAVE
 L# = 1
@@ -954,11 +1135,9 @@ RETURN
 REM  One frame of fireball: erase the previous radius, draw the new
 REM  one. PR% carries the radius to erase between calls.
 LABEL BOOMFX
-IF PR% <= 0 THEN GOTO BOOMFX2
 BCF# = 1
 BR2% = PR%
 GOSUB BOOMPASS
-LABEL BOOMFX2
 BCF# = 0
 BR2% = ER%
 GOSUB BOOMPASS
@@ -969,6 +1148,7 @@ REM  The fireball at radius BR2%: three rings, white heart to orange
 REM  rim, and twelve red spokes thrown past the outer ring. BCF# = 1
 REM  draws the whole thing in black, which is the eraser.
 LABEL BOOMPASS
+IF BR2% < 1 THEN RETURN
 R3% = BR2%
 IF BCF# = 1 THEN COLOR 1, 1 ELSE COLOR 1, 2
 CIRCLE 1, CX0%, CY0%, R3%, R3%
@@ -1220,6 +1400,15 @@ FOR K# = 1 TO N# * 2
 NEXT K#
 RETURN
 
+REM  The bird: SPRSAV's type-in form, sixty-three bytes, three per row,
+REM  most significant bit leftmost.
+LABEL LOADBIRD
+FOR I# = 0 TO 62
+  READ D#
+  PBRD#(I#) = D#
+NEXT I#
+RETURN
+
 REM =====================================================================
 REM  Trap handlers for the hardware probes. Each marks its device gone
 REM  and lets the probe fall through to the next line.
@@ -1367,3 +1556,18 @@ REM full stop
 DATA 1, 15,16
 REM exclamation mark
 DATA 2, 10,14, 15,16
+
+REM  The bird, 24 by 21, one bit a pixel:
+REM    ..XXX...........XXX.....
+REM    .X...XX.......XX...X....
+REM    X.....XXX...XXX.....X...
+REM    .......XXXXXX...........
+REM    .........XX.............
+REM  and sixteen empty rows of sky.
+DATA 0, 0, 0, 56, 0, 224, 70, 3, 16
+DATA 131, 142, 8, 1, 248, 0, 0, 96, 0
+DATA 0, 0, 0, 0, 0, 0, 0, 0, 0
+DATA 0, 0, 0, 0, 0, 0, 0, 0, 0
+DATA 0, 0, 0, 0, 0, 0, 0, 0, 0
+DATA 0, 0, 0, 0, 0, 0, 0, 0, 0
+DATA 0, 0, 0, 0, 0, 0, 0, 0, 0