Get a real Dragon's Lair frame onto the emulated X68000

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

.gitignore

@@ -12,3 +12,4 @@ __pycache__/
 *.pyc
 *.dlx
 a.out
+tmp/

docs/FINDINGS.md

@@ -590,3 +590,83 @@ plausibly exceed the pipe where a 1.7s clip does not. Rate control gives a
 original reason for choosing VQ over a lossless delta in the first place.
 
 Still worth wiring in. No longer a blocker for shipping `scsi` at `lam=10`.
+
+## 22. The display path, measured — first real frame on the X68000
+
+Everything before this section was Python-side or a headless `-video none` run.
+This is the first time pixels reached an emulated X68000 screen, and it produced
+four hardware facts and one blocker that no amount of reasoning would have found.
+
+Reproduce:
+```
+python3 tools/bench/prep_frame.py <framedir> tmp/frame.bin 0
+cd tmp && SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
+  -sound none -nothrottle -plugins -autoboot_script ../tools/bench/show_frame.lua \
+  -snapshot_directory ./snap -snapview native -seconds_to_run 6
+```
+
+### 22.1 The blocker: CRTC R20 bit 11 hides the graphics layer
+The IPL leaves **CRTC R20 (`$E80028`) = `0x0B16`**. Bit 11 is *"G-VRAM set to
+buffer"*, and MAME's `x68k_v.cpp` bails out of `draw_gfx()` on it outright:
+
+```c
+if (m_crtc->gfx_layer_buffer())  // if graphic layers are set to buffer, they aren't visible
+    return false;
+// x68k_crtc.h:  bool gfx_layer_buffer() const { return BIT(m_reg[20], 11); }
+```
+
+While that bit is set, GVRAM writes still land and read back correctly — which
+is exactly what makes it so misleading. Six separate attempts at the video
+controller (`$E82400/$E82500/$E82600`) rendered black with every register
+reading back the intended value. **The video controller was never the problem.**
+
+`R20` bits 9-8 select the colour setup, and this determines how `$C00000` is
+decoded: `0x0300` = 65536c (16 bits/word), `0x0100` = 256c (low byte),
+`0x0000` = 16c (4 bits). Set `R20 = 0x0116` for our mode.
+
+### 22.2 Monitor contrast: the IPL leaves it at 14, not 15
+`$E8E001` bits 3-0 are monitor contrast; MAME does
+`m_screen->set_brightness(contrast * 0x11)`. The IPL leaves it at **14**, which
+scales all output to 14/15 = 93.3%. Every rendered colour came out ~7% dark
+until this was set to 15. **The player must write `$E8E001 = 15` at startup.**
+
+Contrast `0` blanks the screen entirely (`x68k_v.cpp:661`) — that is the cheap
+fade-to-black for scene transitions, no palette animation required.
+
+### 22.3 Palette format CONFIRMED (was previously an assumption)
+`PALETTE(config, m_gfxpalette).set_format(2, &x68k_state::GGGGGRRRRRBBBBBI, 256)`
+
+```
+bit  15..11  10..6   5..1   0
+     GGGGG   RRRRR   BBBBB  I        <- I is a shared LSB for all three channels
+```
+Expansion is `pal6bit((field << 1) | I)`, i.e. `(v << 2) | (v >> 4)`.
+With contrast at 15, **all 256 entries render exactly as this predicts** — the
+frame is pixel-identical, not merely close. GVRAM line stride is confirmed as
+512 words = 1024 bytes, matching `HARDWARE.md`.
+
+### 22.4 A new quality ceiling: the 15-bit palette costs 38.88 dB
+Section 3 called the 256-colour palettised frame "the real quality ceiling".
+That was measured in 24-bit RGB. The hardware palette only stores 5 bits per
+channel plus a shared LSB, so there is a **second** quantisation below it:
+
+| stage | PSNR |
+|---|---|
+| 24-bit palettised source -> X68000 15-bit+I display | **38.88 dB** |
+| `scsi` profile codec error (00020, FINDINGS 15) | 39.4 dB |
+
+The codec's error at `scsi` is **the same order as the display's own error**.
+On real hardware `scsi` is therefore close to display-transparent, and pushing
+`lam` below 10 buys quality the monitor cannot show. This bounds how much the
+`scsi` profile is worth raising — it does not change the profiles themselves.
+
+Caveat: measured on one frame (00020 f0001). It is a property of the palette,
+not the content, so it should generalise, but it has not been checked across
+scenes.
+
+### 22.5 Why the first frame appears twice
+GVRAM is a 512-pixel-wide page while the IPL's CRTC is still in its 768-wide
+text timing, so the layer repeats at exactly x=512. This is correct hardware
+behaviour, not a bug. The player sets its own CRTC mode and the wrap disappears.
+No CRTC timing table has been written yet — the harness deliberately keeps the
+IPL's timing so that no invented CRTC values are in play.

docs/STATUS.md

@@ -156,6 +156,41 @@ functional models, not timing-accurate; a KB/s figure from MAME measures the
 emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
 (MAME validates the path, derivation bounds it, real hardware settles it).
 
+## Display path — WORKING, verified end to end (session 3)
+
+The first real frame is on screen: `docs/images/x68k_first_frame_compare.png`.
+Full write-up in **FINDINGS 22**. Harness: `tools/bench/show_frame.lua` +
+`tools/bench/prep_frame.py`.
+
+Three facts the player MUST honour, none of which were guessable:
+
+| what | where | value |
+|---|---|---|
+| **Un-hide the graphics layer** | CRTC R20 `$E80028` | clear bit 11 ("G-VRAM set to buffer"); IPL leaves `0x0B16` |
+| Colour setup (256c) | CRTC R20 bits 9-8 | `0x0100` -> `R20 = 0x0116` |
+| **Monitor contrast** | `$E8E001` bits 3-0 | IPL leaves **14**; write **15** or everything renders 7% dark |
+
+Bit 11 is the one that cost the most time: GVRAM writes land and read back
+correctly while the layer is invisible, so the video controller looks guilty and
+is not. Contrast `0` blanks the screen — free fade-to-black for transitions.
+
+Palette format is now **confirmed from MAME source**, not assumed:
+`GGGGGRRRRRBBBBBI` (G 15:11, R 10:6, B 5:1, shared LSB I), expanded as
+`pal6bit((field<<1)|I)`. With contrast at 15 the render is **pixel-exact**.
+
+New 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
+(39.4 dB). `scsi` is close to display-transparent on real hardware. See
+FINDINGS 22.4 before considering raising quality further.
+
+Snapshot recipe that works (`-video none` CANNOT snapshot):
+```
+SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
+  -sound none -nothrottle -plugins -autoboot_script <script>.lua \
+  -snapshot_directory ./snap -snapview native -seconds_to_run 6
+```
+`-snapview native` drops MAME's LED artwork and gives a clean 768x512 screen.
+
 ## Next steps, in priority order
 
 1. **Full-disc survey.** Only 4 clips of 1.2-1.7 s out of 224 streams have been
@@ -164,9 +199,12 @@ emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
    vs content first (FINDINGS 13) or the averages are diluted by static menus.
    **Vectorise `_paint` before this run** — it is a Python per-block loop.
 2. **68000 decoder skeleton.** Parse `DLX1`, expand codebooks to word-per-pixel,
-   blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness —
+   blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness.
-   the first time that harness gets used for its actual purpose. Validates the
+   **Now unblocked** — the display path is verified (FINDINGS 22) and
-   38% full-frame blit estimate that the whole CPU budget rests on.
+   `tools/bench/show_frame.lua` gives a known-good reference image to diff the
+   68000's output against. Validates the 38% full-frame blit estimate that the
+   whole CPU budget rests on. Still needs a real CRTC mode table for 256x256;
+   the harness deliberately borrows the IPL's timing and invents nothing.
 3. **Wire rate control into `encode.py`.** No longer a blocker (FINDINGS 21), but
    it is what gives a deterministic ceiling over content not yet measured, which
    was the original reason for choosing VQ. Insurance, not a fix. Pairs with (1).

tools/bench/prep_frame.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python3
+"""Frame -> flat (RGB888 palette + index plane) blob for the MAME Lua loader.
+
+Packing into the X68000 palette word is done Lua-side on purpose: the exact
+channel order is a hardware fact we intend to CONFIRM BY EYE, not assume, so it
+has to be cheap to change without regenerating the blob.
+"""
+import sys, struct, glob
+import numpy as np
+from PIL import Image
+
+src, out = sys.argv[1], sys.argv[2]
+f = sorted(glob.glob(f"{src}/*.png"))[int(sys.argv[3]) if len(sys.argv) > 3 else 0]
+im = Image.open(f).convert("RGB")
+W, H = im.size
+
+q   = im.quantize(colors=256, method=Image.MEDIANCUT, dither=Image.NONE)
+pal = np.array(q.getpalette()[:256*3], dtype=np.uint8).reshape(256, 3)
+idx = np.asarray(q, dtype=np.uint8)
+
+with open(out, "wb") as fh:
+    fh.write(b"DLXR")
+    fh.write(struct.pack(">HH", W, H))
+    fh.write(pal.tobytes())
+    fh.write(idx.tobytes())
+
+# reference PNG of exactly what the X68000 should display
+Image.fromarray(pal[idx]).save(out.replace(".bin", "_ref.png"))
+print(f"src={f} {W}x{H} colors={len(np.unique(idx))} -> {out}")

tools/bench/show_frame.lua

@@ -0,0 +1,47 @@
+-- Real frame, 256-colour, with the ACTUAL display gate from MAME's source:
+--   CRTC R20 bit 11 = "G-VRAM set to buffer" -> graphics layer invisible.
+--   CRTC R20 bits 9-8 = colour setup: 0x0100 = 256-colour, low byte per word.
+-- The IPL leaves R20 = 0x0B16 (buffered + 65536c), which is why every earlier
+-- attempt rendered black no matter what the video controller said.
+M=manager.machine; SP=M.devices[":maincpu"].spaces["program"]; SUB=nil
+
+local CRTC20 = 0xE80000 + 20*2
+local GVRAM, GPAL = 0xC00000, 0xE82000
+
+local f=io.open("frame.bin","rb"); local d=f:read("a"); f:close()
+local function B(i) return string.byte(d,i) end
+local W,H = B(5)*256+B(6), B(7)*256+B(8)
+local PAL0, PIX0 = 9, 9+256*3
+
+-- GGGGGRRRRRBBBBBI, confirmed from x68k_v.cpp
+local function pack(r,g,b) return ((g>>3)<<11)|((r>>3)<<6)|((b>>3)<<1)|1 end
+
+local function T() local t=M.time; return t.seconds+t.attoseconds/1e18 end
+local st,tp="wait",nil
+
+SUB = emu.add_machine_frame_notifier(function()
+  local t=T()
+  if st=="wait" then
+    if t<3.0 then return end
+    local old = SP:read_u16(CRTC20)
+    local new = (old & ~0x0B00) | 0x0100      -- clear buffer bit, select 256c
+    SP:write_u16(CRTC20, new)
+    SP:write_u16(0xE82400, 0x0001)            -- video ctrl: 256 colours
+    SP:write_u16(0xE82600, 0x001F)            -- graphics + pages on, text off
+    SP:write_u8(0xE8E001, 15)                 -- monitor contrast: IPL leaves it at 14
+    for c=0,255 do
+      local o=PAL0+c*3
+      SP:write_u16(GPAL+c*2, pack(B(o),B(o+1),B(o+2)))
+    end
+    for y=0,H-1 do
+      local row,base = PIX0+y*W, GVRAM+y*1024
+      for x=0,W-1 do SP:write_u16(base+x*2, B(row+x)) end
+    end
+    print(string.format("[SHOW3] R20 %04X->%04X  E82400=%04X E82600=%04X gv=%04X t=%.3f",
+      old, SP:read_u16(CRTC20), SP:read_u16(0xE82400), SP:read_u16(0xE82600),
+      SP:read_u16(GVRAM), t))
+    st,tp="painted",t
+  elseif st=="painted" and t>tp+0.30 then
+    M.video:snapshot(); print("[SHOW3] snapshot"); st="done"; M:exit()
+  end
+end)