brickboy-dmg (Analogue Pocket openFPGA core)

A port of the DMG display and audio model from brickboy to an Analogue Pocket openFPGA core. This is not an independent implementation: the maths, the pass order and the constants come from brickboy's src/display/ and src/ui/speaker-model.ts.

DMG only. SGB and GBC were removed from the upstream core, and the freed resources went into the panel model.

What it does

The Pocket renders the game at 4x — one Game Boy dot becomes a 4x4 block — and the whole panel model runs at that resolution in hardware, in the same order brickboy's shader chain runs it.

Stage	Ported from
Colour correction	shaders.ts FRAG_COLOR_CORRECT (dmg-lut path)
Passive-matrix crosstalk	shaders.ts FRAG_COLUMN_REDUCE
LCD persistence	shaders.ts FRAG_GHOST
Dot structure and drop shadow	shaders.ts FRAG_GRID
Reflector grain	reflector.ts
Screen-space finish	shaders.ts FRAG_PASSTHROUGH
Dead electrode lines	shaders.ts FRAG_DEFECTS
Speaker and case	speaker-model.ts plus the APU's output DC blocker

Order is load-bearing. Persistence sits after the grid, the faults after persistence, and crosstalk inside the colour pass — moving any of them changes what the panel looks like.

Details worth knowing:

• Crosstalk is bidirectional. A dark block bleeds down strongly and up weakly (0.4x), which is what makes it read as crosstalk surrounding dark content rather than a second copy of the drop shadow. A top-to-bottom scan cannot see the rows below, so the upward field is computed by a pre-pass that walks the frame bottom-to-top during blanking.
• Persistence is asymmetric. Darkening is driven and fast; lightening is relaxation and slow. Getting that backwards reads as input lag.
• The drop shadow has two layers, a sharp umbra at 1.35 dots and a broad penumbra at 3.24, which is what makes the air gap between the element plane and the reflector readable.
• The reflector grain has three bands — half a dot, one to three dots, and eighteen to thirty-six. Only the middle one reads as grain at the Pocket's pixel pitch; the finest is below what the panel resolves and the coarsest is mottling.

Deviations from brickboy

Every one of these is a hardware constraint or a difference in the display, not a redesign. They are marked in the source at the point they matter.

What	Why
Persistence state is kept at native resolution, before the grid	brickboy keeps it post-grid; at 4x that needs 8.85 Mbit against a 3.15 Mbit device
The grain's coarse band is baked on a 4-dot lattice and the finest band is a runtime hash	The full texture is 3.2 Mbit
Dead lines are substituted into the cell before the grid	The shader is a screen-space pass and re-applies a dot mask; a dead electrode is a fact about the element, so doing it there gets the dot structure and the neighbours' shadows for free
The 4-way D-pad uses last-input priority	brickboy's 4-way also resists turning (R_COMMIT_4WAY), which exists because a thumb drifts on glass. A physical cross-pad has no drift
The speaker runs the biquads directly	brickboy synthesises an impulse response and convolves it, which is how you get a fixed filter into a Web Audio graph. The response is identical
The module margin is not drawn	The Pocket's bezel plays that part, matching brickboy's own Fill mode
Panel trim dials	The Pocket's LTPS LCD and the phone OLED brickboy is authored against have different primaries, and Analogue publishes no colorimetry. Guessing a correction into the pipeline would corrupt the reference, so it rides on top as a dial

The panel trim, ink tone, grain contrast and off-element tint dials are not part of the port. At their Normal positions the pipeline is brickboy's, unchanged.

Not yet implemented: dead-line flicker (deadFlicker). Vinegar syndrome is out of scope.

Verification

The port was checked against brickboy itself, not against a reimplementation.

• tools/bb_render.mjs drives brickboy's own ?renderfarm=1 harness — the same six-pass WebGL chain the app puts on screen — and renders arbitrary frames at the Pocket's 4x. Those pixels are the reference.
• tools/color_model.py, grid_rtl.py and frame_model.py are bit-exact models of the RTL, so a change can be measured before a 20-minute synthesis run.
• tools/bake_grain.py, bake_audio.py and bake_deadlines.py evaluate brickboy's own functions to produce the baked tables, so the constants are derived rather than transcribed.

Measured against brickboy at 4x, on flat fields:

shade   grid modulation        cell luminance
        brickboy  this core    brickboy       this core
  0       3.20%     1.98%      [178 166 115]  [176 164 112]
  1       5.46%     4.46%      [143 151  99]  [142 151  97]
  2      15.85%    15.42%      [112 123  79]  [106 119  74]
  3      31.39%    33.09%      [ 98 110  71]  [ 91 105  67]

The audio chain matches its float model to within 0.01 dB across the audible band.

Building

Quartus Prime Lite (tested on 25.1std) for a Cyclone V 5CEBA4F23C8:

cd src && quartus_sh --flow compile ap_core
python3 build/rbf_reverse.py src/output_files/ap_core.rbf build/brickboy.rbf_r

Regenerate the baked tables after changing a profile constant:

cd tools && python3 bake_grain.py && python3 bake_audio.py && python3 bake_deadlines.py

Installing

Copy Cores, Platforms and Assets to the root of the SD card. Place the Game Boy BIOS in /Assets/gb/common as gb_bios.bin, and ROMs in the same folder.

Licence

GPL-3.0-or-later. The upstream core carries GPL-3.0-or-later files (the OpenGateware audio filters) alongside MIT ones, so the combined work is GPL-3.0-or-later.

Provenance:

• Analogue Pocket core framework: Analogue openFPGA
• Game Boy core: budude2/openfpga-GBC, itself ported from Gameboy_MiSTer
• Panel and speaker model: brickboy (not public)

Neither budude2/openfpga-GBC nor Gameboy_MiSTer declares a repository-level licence; the file headers do, and this repository follows them.

Support

This is free, and it stays free. But it was a lot of measuring against real hardware to get right, and if any of it is useful to you, a little help would genuinely make me happy.

Sponsor @kathoc

No pressure at all — a star or a bug report is welcome too.