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
This commit is contained in:
prosolis
@@ -12,3 +12,4 @@ __pycache__/
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*.pyc
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*.dlx
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a.out
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tmp/
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@@ -590,3 +590,83 @@ plausibly exceed the pipe where a 1.7s clip does not. Rate control gives a
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original reason for choosing VQ over a lossless delta in the first place.
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Still worth wiring in. No longer a blocker for shipping `scsi` at `lam=10`.
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## 22. The display path, measured — first real frame on the X68000
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Everything before this section was Python-side or a headless `-video none` run.
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This is the first time pixels reached an emulated X68000 screen, and it produced
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four hardware facts and one blocker that no amount of reasoning would have found.
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Reproduce:
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```
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python3 tools/bench/prep_frame.py <framedir> tmp/frame.bin 0
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cd tmp && SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
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-sound none -nothrottle -plugins -autoboot_script ../tools/bench/show_frame.lua \
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-snapshot_directory ./snap -snapview native -seconds_to_run 6
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```
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### 22.1 The blocker: CRTC R20 bit 11 hides the graphics layer
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The IPL leaves **CRTC R20 (`$E80028`) = `0x0B16`**. Bit 11 is *"G-VRAM set to
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buffer"*, and MAME's `x68k_v.cpp` bails out of `draw_gfx()` on it outright:
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```c
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if (m_crtc->gfx_layer_buffer()) // if graphic layers are set to buffer, they aren't visible
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return false;
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// x68k_crtc.h: bool gfx_layer_buffer() const { return BIT(m_reg[20], 11); }
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```
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While that bit is set, GVRAM writes still land and read back correctly — which
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is exactly what makes it so misleading. Six separate attempts at the video
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controller (`$E82400/$E82500/$E82600`) rendered black with every register
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reading back the intended value. **The video controller was never the problem.**
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`R20` bits 9-8 select the colour setup, and this determines how `$C00000` is
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decoded: `0x0300` = 65536c (16 bits/word), `0x0100` = 256c (low byte),
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`0x0000` = 16c (4 bits). Set `R20 = 0x0116` for our mode.
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### 22.2 Monitor contrast: the IPL leaves it at 14, not 15
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`$E8E001` bits 3-0 are monitor contrast; MAME does
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`m_screen->set_brightness(contrast * 0x11)`. The IPL leaves it at **14**, which
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scales all output to 14/15 = 93.3%. Every rendered colour came out ~7% dark
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until this was set to 15. **The player must write `$E8E001 = 15` at startup.**
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Contrast `0` blanks the screen entirely (`x68k_v.cpp:661`) — that is the cheap
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fade-to-black for scene transitions, no palette animation required.
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### 22.3 Palette format CONFIRMED (was previously an assumption)
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`PALETTE(config, m_gfxpalette).set_format(2, &x68k_state::GGGGGRRRRRBBBBBI, 256)`
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```
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bit 15..11 10..6 5..1 0
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GGGGG RRRRR BBBBB I <- I is a shared LSB for all three channels
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```
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Expansion is `pal6bit((field << 1) | I)`, i.e. `(v << 2) | (v >> 4)`.
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With contrast at 15, **all 256 entries render exactly as this predicts** — the
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frame is pixel-identical, not merely close. GVRAM line stride is confirmed as
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512 words = 1024 bytes, matching `HARDWARE.md`.
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### 22.4 A new quality ceiling: the 15-bit palette costs 38.88 dB
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Section 3 called the 256-colour palettised frame "the real quality ceiling".
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That was measured in 24-bit RGB. The hardware palette only stores 5 bits per
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channel plus a shared LSB, so there is a **second** quantisation below it:
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| stage | PSNR |
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|---|---|
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| 24-bit palettised source -> X68000 15-bit+I display | **38.88 dB** |
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| `scsi` profile codec error (00020, FINDINGS 15) | 39.4 dB |
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The codec's error at `scsi` is **the same order as the display's own error**.
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On real hardware `scsi` is therefore close to display-transparent, and pushing
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`lam` below 10 buys quality the monitor cannot show. This bounds how much the
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`scsi` profile is worth raising — it does not change the profiles themselves.
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Caveat: measured on one frame (00020 f0001). It is a property of the palette,
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not the content, so it should generalise, but it has not been checked across
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scenes.
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### 22.5 Why the first frame appears twice
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GVRAM is a 512-pixel-wide page while the IPL's CRTC is still in its 768-wide
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text timing, so the layer repeats at exactly x=512. This is correct hardware
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behaviour, not a bug. The player sets its own CRTC mode and the wrap disappears.
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No CRTC timing table has been written yet — the harness deliberately keeps the
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IPL's timing so that no invented CRTC values are in play.
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+41
-3
@@ -156,6 +156,41 @@ functional models, not timing-accurate; a KB/s figure from MAME measures the
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emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
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(MAME validates the path, derivation bounds it, real hardware settles it).
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## Display path — WORKING, verified end to end (session 3)
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The first real frame is on screen: `docs/images/x68k_first_frame_compare.png`.
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Full write-up in **FINDINGS 22**. Harness: `tools/bench/show_frame.lua` +
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`tools/bench/prep_frame.py`.
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Three facts the player MUST honour, none of which were guessable:
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| what | where | value |
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|---|---|---|
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| **Un-hide the graphics layer** | CRTC R20 `$E80028` | clear bit 11 ("G-VRAM set to buffer"); IPL leaves `0x0B16` |
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| Colour setup (256c) | CRTC R20 bits 9-8 | `0x0100` -> `R20 = 0x0116` |
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| **Monitor contrast** | `$E8E001` bits 3-0 | IPL leaves **14**; write **15** or everything renders 7% dark |
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Bit 11 is the one that cost the most time: GVRAM writes land and read back
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correctly while the layer is invisible, so the video controller looks guilty and
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is not. Contrast `0` blanks the screen — free fade-to-black for transitions.
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Palette format is now **confirmed from MAME source**, not assumed:
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`GGGGGRRRRRBBBBBI` (G 15:11, R 10:6, B 5:1, shared LSB I), expanded as
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`pal6bit((field<<1)|I)`. With contrast at 15 the render is **pixel-exact**.
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New ceiling: the 15-bit+I palette alone costs **38.88 dB** against the 24-bit
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palettised source — the same order as the `scsi` profile's own codec error
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(39.4 dB). `scsi` is close to display-transparent on real hardware. See
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FINDINGS 22.4 before considering raising quality further.
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Snapshot recipe that works (`-video none` CANNOT snapshot):
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```
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SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
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-sound none -nothrottle -plugins -autoboot_script <script>.lua \
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-snapshot_directory ./snap -snapview native -seconds_to_run 6
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```
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`-snapview native` drops MAME's LED artwork and gives a clean 768x512 screen.
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## Next steps, in priority order
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1. **Full-disc survey.** Only 4 clips of 1.2-1.7 s out of 224 streams have been
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@@ -164,9 +199,12 @@ emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
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vs content first (FINDINGS 13) or the averages are diluted by static menus.
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**Vectorise `_paint` before this run** — it is a Python per-block loop.
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2. **68000 decoder skeleton.** Parse `DLX1`, expand codebooks to word-per-pixel,
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blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness —
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the first time that harness gets used for its actual purpose. Validates the
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38% full-frame blit estimate that the whole CPU budget rests on.
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blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness.
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**Now unblocked** — the display path is verified (FINDINGS 22) and
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`tools/bench/show_frame.lua` gives a known-good reference image to diff the
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68000's output against. Validates the 38% full-frame blit estimate that the
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whole CPU budget rests on. Still needs a real CRTC mode table for 256x256;
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the harness deliberately borrows the IPL's timing and invents nothing.
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3. **Wire rate control into `encode.py`.** No longer a blocker (FINDINGS 21), but
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it is what gives a deterministic ceiling over content not yet measured, which
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was the original reason for choosing VQ. Insurance, not a fix. Pairs with (1).
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@@ -0,0 +1,29 @@
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#!/usr/bin/env python3
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"""Frame -> flat (RGB888 palette + index plane) blob for the MAME Lua loader.
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Packing into the X68000 palette word is done Lua-side on purpose: the exact
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channel order is a hardware fact we intend to CONFIRM BY EYE, not assume, so it
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has to be cheap to change without regenerating the blob.
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"""
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import sys, struct, glob
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import numpy as np
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from PIL import Image
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src, out = sys.argv[1], sys.argv[2]
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f = sorted(glob.glob(f"{src}/*.png"))[int(sys.argv[3]) if len(sys.argv) > 3 else 0]
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im = Image.open(f).convert("RGB")
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W, H = im.size
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q = im.quantize(colors=256, method=Image.MEDIANCUT, dither=Image.NONE)
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pal = np.array(q.getpalette()[:256*3], dtype=np.uint8).reshape(256, 3)
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idx = np.asarray(q, dtype=np.uint8)
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with open(out, "wb") as fh:
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fh.write(b"DLXR")
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fh.write(struct.pack(">HH", W, H))
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fh.write(pal.tobytes())
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fh.write(idx.tobytes())
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# reference PNG of exactly what the X68000 should display
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Image.fromarray(pal[idx]).save(out.replace(".bin", "_ref.png"))
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print(f"src={f} {W}x{H} colors={len(np.unique(idx))} -> {out}")
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@@ -0,0 +1,47 @@
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-- Real frame, 256-colour, with the ACTUAL display gate from MAME's source:
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-- CRTC R20 bit 11 = "G-VRAM set to buffer" -> graphics layer invisible.
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-- CRTC R20 bits 9-8 = colour setup: 0x0100 = 256-colour, low byte per word.
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-- The IPL leaves R20 = 0x0B16 (buffered + 65536c), which is why every earlier
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-- attempt rendered black no matter what the video controller said.
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M=manager.machine; SP=M.devices[":maincpu"].spaces["program"]; SUB=nil
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local CRTC20 = 0xE80000 + 20*2
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local GVRAM, GPAL = 0xC00000, 0xE82000
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local f=io.open("frame.bin","rb"); local d=f:read("a"); f:close()
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local function B(i) return string.byte(d,i) end
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local W,H = B(5)*256+B(6), B(7)*256+B(8)
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local PAL0, PIX0 = 9, 9+256*3
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-- GGGGGRRRRRBBBBBI, confirmed from x68k_v.cpp
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local function pack(r,g,b) return ((g>>3)<<11)|((r>>3)<<6)|((b>>3)<<1)|1 end
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local function T() local t=M.time; return t.seconds+t.attoseconds/1e18 end
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local st,tp="wait",nil
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SUB = emu.add_machine_frame_notifier(function()
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local t=T()
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if st=="wait" then
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if t<3.0 then return end
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local old = SP:read_u16(CRTC20)
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local new = (old & ~0x0B00) | 0x0100 -- clear buffer bit, select 256c
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SP:write_u16(CRTC20, new)
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SP:write_u16(0xE82400, 0x0001) -- video ctrl: 256 colours
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SP:write_u16(0xE82600, 0x001F) -- graphics + pages on, text off
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SP:write_u8(0xE8E001, 15) -- monitor contrast: IPL leaves it at 14
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for c=0,255 do
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local o=PAL0+c*3
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SP:write_u16(GPAL+c*2, pack(B(o),B(o+1),B(o+2)))
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end
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for y=0,H-1 do
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local row,base = PIX0+y*W, GVRAM+y*1024
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for x=0,W-1 do SP:write_u16(base+x*2, B(row+x)) end
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end
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print(string.format("[SHOW3] R20 %04X->%04X E82400=%04X E82600=%04X gv=%04X t=%.3f",
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old, SP:read_u16(CRTC20), SP:read_u16(0xE82400), SP:read_u16(0xE82600),
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SP:read_u16(GVRAM), t))
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st,tp="painted",t
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elseif st=="painted" and t>tp+0.30 then
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M.video:snapshot(); print("[SHOW3] snapshot"); st="done"; M:exit()
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end
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end)
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