Session 3 left the harness on the IPL's 768x512 text timing because no CRTC
values had been derived and guessing them was the failure mode to avoid. This
derives them from MAME 0.277's divisor ladder instead, and the derivation is
self-checking: the 256-wide mode runs at div 6 against the 768 mode's div 2, so
htotal is exactly 1104/3 = 368 dots and every horizontal register divides by
three with no remainder. Only the blanking split rounds. Verified by snapshot:
native 256x512, active area pixel-exact, x=512 wrap gone.
Two things fell out that change numbers elsewhere:
- The palette's shared LSB I must be chosen per entry, not hardcoded to 1.
Doing so lifts the display ceiling from 38.85 to 40.81 dB and is the only way
to reach true black at all, since pal6bit(1) = 4. 102 of 256 entries want
I = 0, so this is not a corner case. Supersedes FINDINGS 22.4; scsi has ~2 dB
more headroom than that section claimed. The encoder does not do this yet.
- Letterboxing costs a palette entry: GVRAM cleared to zero shows entry 0, and
a free mediancut palette puts a real image colour there. 255 colours plus a
reserved black, via prep_frame.py --reserve-black.
MAME's graphics double-scan is phase-shifted one raster line (it halves the
absolute scanline and vbegin is odd), which produced a false failure before it
was understood; the regression test now asserts the shifted pairing explicitly.
Still Lua-side. No 68000 instruction has drawn a pixel; the 38% blit estimate
remains unvalidated. What this buys is a defined geometry for the decoder to
write into: 256 words per row, 1024-byte stride, rows 32..223.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
First pixels on an actual X68000 screen. Everything up to now was Python-side
or a headless -video none run, which cannot snapshot at all.
The blocker was not the video controller. The IPL leaves CRTC R20 = 0x0B16,
and bit 11 is "G-VRAM set to buffer", which makes MAME's draw_gfx() return
early. GVRAM writes still land and read back correctly while the layer is
invisible, so six attempts at $E82400/$E82500/$E82600 all rendered black with
every register holding the value I intended.
Two more facts, both confirmed against MAME 0.277 source rather than assumed:
- $E8E001 monitor contrast is left at 14 by the IPL, scaling all output to
93.3%. The player must set it to 15. Contrast 0 blanks the screen, which is
a free fade-to-black for scene transitions.
- The palette word is GGGGGRRRRRBBBBBI with a shared LSB, expanded as
pal6bit((field<<1)|I). With contrast at 15 the render is pixel-exact, not
merely close, which also confirms the 1024-byte GVRAM line stride.
That exactness gives a new quality ceiling: the 15-bit+I palette alone costs
38.88 dB against the 24-bit palettised source, the same order as the scsi
profile's own codec error. scsi is close to display-transparent on hardware,
which bounds how much further it is worth raising.
Unblocks next step 2, the 68000 decoder skeleton, which now has a known-good
reference image to diff against.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
Answers session 1's critical-path question. Flat 4x4 VQ at k=256 was prototyped
and REJECTED by eye: Dirk's face disintegrates and ink outlines break into
4-pixel stair-steps. The 256-colour palettised frame is excellent, so the
palette was never the problem -- block VQ was.
Replaced it with a Cinepak-style hybrid: each 4x4 block is SKIP, one 4x4
codeword, four 2x2 codewords, or RAW literal pixels, chosen per block by
rate-distortion. The RAW escape makes lam=0 pixel-exact (measured 0.00 dB loss),
so the quality knob spans lossless to heavily-compressed in one bitstream.
Per the user's decision, ships TWO quality profiles from that one codec, one
decoder and one bitstream -- only the rate knob differs:
sasi 45 KB/s lam=300 34.8 dB stock 10MHz ACE/EXPERT
scsi 75 KB/s lam=100 35.9 dB Super/XVI or CZ-6BS1
Three corrections to earlier numbers:
1. Session 1's "183 KB/s at 12fps" was a bad extrapolation. Halving the
framerate does not halve the bitrate -- decimation roughly doubles the
per-frame delta. Re-measured directly: 340 KB/s for session 1's own RLE,
247 KB/s for changed-spans+deflate. The lossless floor is 319 MB.
2. A FOURTH false-good result, same family as the three in FINDINGS 4:
k=1024 codebooks appeared to buy +2.4 dB free, because the rate model
charged 1 byte for a 10-bit index. Charging the true cost reverses the
verdict -- k=256 wins at every matched bitrate, and by 5 dB at the low end
where the SASI profile lives. k=256 ships.
3. Stream inventory: the ~3-5MB clips are 1.2-1.7s, not ~60s, and some 60s
streams are menus, not content. Any survey must classify before averaging.
Also cleared both candidate sources for the game-logic layer: the SNES project
is MIT and DirkSimple is zlib, so the arcade scene graph can be imported and
the two transcriptions diffed against each other.
Encoder is working end-to-end: extract.py -> vq/vq_hybrid/ratectl -> encode.py,
emitting a big-endian DLX1 container the 68000 can parse with plain moves.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
prosolis