Intrepid/prometeu-runtime

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implements PLN-0073 render contract specs

2026-05-25 11:35:49 +01:00

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GFX Peripheral (Graphics System)

Domain: virtual hardware: graphics Function: normative

Didactic companion: ../learn/mental-model-gfx.md

1. Overview

The GFX peripheral is responsible for generating images in PROMETEU.

DEC-0030 defines the current logical render boundary. PROMETEU render production is split from render implementation:

domain buffers during logical frame
-> RenderManager closes buffers
-> RenderSubmission snapshot
-> render surface / implementation consumes submission
-> RGBA8888 surface publication

The GFX peripheral remains the classic local raster implementation for PROMETEU's 2D output. Logical render APIs do not target Gfx or the framebuffer directly.

RenderSubmission is the closed snapshot passed across the render boundary. It MUST contain a frame id, the active app mode, and one typed packet: Game2DFramePacket for AppMode::Game or ShellUiFramePacket for AppMode::Shell. Once closed, producers MUST treat the submission as immutable. The runtime backpressure policy is latest-complete-submission-wins; it MUST NOT accumulate an unbounded frame queue.

The current 2D graphics model is based on:

framebuffer
tilemaps
tile banks
priority-based sprites
composition by drawing order

2. Resolution and Framebuffer

Base resolution

480 × 270 pixels
aspect ratio close to 16:9
scalable by the host (nearest-neighbor)

Pixel format

RGBA8888
8 bits Red
8 bits Green
8 bits Blue
8 bits Alpha
canonical raw channel order: RGBA

The framebuffer alpha channel may carry meaningful runtime output. The runtime MUST NOT force the published front buffer alpha channel to 255 as a blanket contract.

Color values in the runtime, HAL, host-facing framebuffer, and GFX ABI are logical RGBA8888 values. RGB565 is not a supported runtime framebuffer, palette, host presentation, or compatibility contract.

3. Double Buffering

The render surface implementation maintains the concrete buffers needed to publish a frame. The classic software implementation uses:

Back Buffer — where the frame is built
Front Buffer — where the frame is displayed

Per-frame flow:

Game or Shell code mutates mode-specific domain buffers during the logical frame.
RenderManager closes the active buffers into an immutable RenderSubmission.
The render surface implementation consumes the submission and rasterizes to its back buffer.
The render surface publishes the completed RGBA8888 surface.
The host displays the published surface.

This guarantees:

no tearing
clear per-frame synchronization
deterministic behavior

4. PROMETEU Graphical Structure

The graphical world is composed of:

Up to 16 Tile Banks
4 Tile Layers (scrollable)
1 HUD Layer (fixed, always on top)
Sprites with priority between layers

4.1 Tile Banks

There are up to 16 banks
Each bank has a fixed tile size:
- 8×8, 16×16, or 32×32
A bank is a graphics library:
- environment
- characters
- UI
- effects
assets.pa tile-bank payloads use a serialized representation distinct from runtime memory:
- serialized pixels are 4bpp packed in payload order
- runtime memory may expand pixels to one u8 palette index per pixel after decode

4.2 Layers

There are:
- 4 Tile Layers
- 1 HUD Layer
Each layer points to a single bank
Sprites can use any bank
HUD:
- does not scroll
- maximum priority
- generally uses 8×8 tiles

5. Internal Model of a Tile Layer

A Tile Layer is not a bitmap of pixels.
It is composed of:

A logical Tilemap (tile indices)
A Border Cache (window of visible tiles)
A Scroll Offset

Structure:

bank_id
tile_size
tilemap (large matrix)
scroll_x, scroll_y
cache_origin_x, cache_origin_y
cache_tiles[w][h]

6. Logical Tilemap

The tilemap represents the world:

Each cell contains:

tile_id
flip_x
flip_y
priority (optional)
palette_id (optional)

The tilemap can be much larger than the screen.

7. Border Cache (Tile Cache)

The cache is a window of tiles around the camera.

Example:

Screen: 480×270
16×16 tiles → approximately 30×17 visible
Current runtime cache sizing with 16×16 layers: 35×21 tiles

It stores tiles already resolved from the tilemap.

8. Cache Update

Every frame:

Calculate:

tile_x = scroll_x / tile_size
tile_y = scroll_y / tile_size
offset_x = scroll_x % tile_size
offset_y = scroll_y % tile_size

If tile_x changed:

Advance cache_origin_x
Reload only the new column

If tile_y changed:

Advance cache_origin_y
Reload only the new line

Only one row and/or column is updated per frame.

9. Cache as Ring Buffer

The cache is circular:

Does not physically move data
Only moves logical indices

Access:

real_x = (cache_origin_x + logical_x) % cache_width
real_y = (cache_origin_y + logical_y) % cache_height

10. Canonical Game Projection

Game mode uses a typed Game 2D submission. composer.* owns high-level Game 2D frame composition: scene binding, camera, sprites, HUD, and frame orchestration. gfx2d.* owns Game 2D primitives only. Both domains are mutable while the logical frame is being produced and are closed by RenderManager into a Game2DFramePacket.

For each Game 2D packet:

For each Tile Layer, in order:

Rasterize visible tiles from the cache
Apply scroll, flip, and transparency
Write to the render surface's working buffer

Draw sprites:

With priority between layers
Drawing order defines depth

Draw HUD layer last
Draw buffered gfx2d.* primitives according to the Game 2D packet contract.

This section describes only the Game 2D packet rendering path. Shell/system UI uses gfxui.* and ShellUiFramePacket; it is never part of Game HUD or composer.*.

11. Drawing Order and Priority

There is no Z-buffer
There is no automatic sorting
Whoever draws later is in front

Base order:

Tile Layer 0
Tile Layer 1
Tile Layer 2
Tile Layer 3
Sprites (by priority between layers)
HUD Layer
Buffered gfx2d.* Game primitives

Normative boundary:

Items 1 through 6 belong to composer.* Game-frame composition.
Item 7 belongs to gfx2d.*.
gfx2d.* primitives MUST NOT be interpreted as scene, sprite, camera, HUD, or frame orchestration.
Shell/system UI belongs to gfxui.* and ShellUiFramePacket, not Game HUD.

12. Transparency

Transparency is represented by the alpha channel of the resolved RGBA8888 color.

Palette indices are ordinary indices. The runtime MUST NOT reserve palette index 0 as a special transparent index. A palette entry may still be authored as transparent by setting its alpha channel to 0, but that is ordinary palette data rather than a special index rule.

if resolved_color.alpha == 0:
    skip
else:
    draw_or_blend(resolved_color)

13. Color Math (Discrete Blending)

Inspired by the SNES.

Official modes:

BLEND_NONE
BLEND_HALF
BLEND_HALF_PLUS
BLEND_HALF_MINUS
BLEND_FULL

Alpha is available through RGBA8888 colors. Discrete blend modes remain part of the classic GFX contract, and later optimization work may specialize opaque, masked, and alpha paths.

Everything is:

integer
cheap
deterministic

14. Where Blend is Applied

Blending occurs during drawing
For canonical game composition, the result goes to the back buffer during composition
For gfx2d.* and gfxui.* primitives, the result is applied when the render surface consumes the closed packet for the active app mode
There is no automatic GPU-style post-processing pipeline

15. What the GFX DOES NOT support

By design:

Shaders
Modern GPU pipeline
HDR
Gamma correction
RGB565 compatibility framebuffers
fade fields, fade syscalls, or fade packet members
multi-format backend selection
render-thread ownership as part of this contract

16. Performance Rule

Layers:
- only update the border when crossing a tile
- never redraw the entire world
Rasterization:
- always per frame, only the visible area
Sprites:
- always redrawn per frame

17. Transitions and Removed Fade Contract

The previous special fade model is not part of the canonical render contract. Current render packets, syscalls, and ABI domains MUST NOT expose scene fade, HUD fade, fade levels, fade colors, or equivalent inherited fade state.

Future visual transitions between Shell and Game belong to RenderManager. They are not owned by composer.*, gfx2d.*, gfxui.*, Game2DFramePacket, or ShellUiFramePacket.

Rules:

render domain packets MUST NOT contain fade fields;
public render syscalls MUST NOT expose fade controls;
render implementations MUST NOT treat fade as an inherited post-processing habit;
transition work requires an explicit RenderManager contract or a later decision.

18. Palette System

18.1. Overview

PROMETEU uses exclusively palette-indexed graphics.

There is no direct RGB-per-pixel mode.
Every graphical pixel is an index pointing to a real color in a palette.

18.2. Pixel Format

Each pixel of a tile or sprite is:

4 bits per pixel (4bpp)
values: 0..15

Fixed rule:

Indices 0..15 are ordinary valid palette indices
Transparency comes from the alpha channel of the resolved palette entry

18.3. Palette Structure

Each Tile Bank contains:

64 palettes in runtime-facing v1
Each palette has:
- 16 colors
- each color in RGBA8888 with canonical RGBA channel order

Size:

1 palette = 16 × 4 bytes = 64 bytes
64 palettes = 4 KB per bank
16 banks = 64 KB maximum palettes

18.4. Palette Association

Fundamental Rule

Each tile uses a single palette
Each sprite uses a single palette
The palette must be provided explicitly in every draw

There is no palette swap within the same tile or sprite.

18.5. Where the Palette is Defined

Tilemap

Each tilemap cell contains:

tile_id
palette_id (u8)
flip_x
flip_y

Runtime-facing validity rule for v1:

palette_id values are valid only in the range 0..63

Sprite

Each sprite draw contains:

bank_id
tile_id
palette_id (u8)
x, y
flip_x, flip_y
priority

Runtime-facing validity rule for v1:

palette_id values are valid only in the range 0..63

18.6. Color Resolution

The pipeline works like this:

Read indexed pixel from tile (value 0..15)
Resolve:

real_color = palette[palette_id][index]

Apply:

flip
discrete blend
alpha/skip behavior from the resolved RGBA8888 color
writing to back buffer

In other words:

pixel_index = tile_pixel(x,y)
color = bank.palettes[palette_id][pixel_index]
if color.alpha == 0:
    skip
else:
    draw_or_blend(color)

18.7. Organization of Tile Banks

Tile Banks are "strong assets":

Tiles and palettes live together
Export/import always carries:
- tiles + palettes
In assets.pa v1, the serialized payload is:
- packed indexed pixels for the whole sheet
- followed by the palette table for the same bank
The hardware does not impose semantic organization:
- grouping is the creator's decision
Tooling and scripts can create conventions:
- e.g.: palettes 0..15 = enemies
- 16..31 = scenery
- etc.

Runtime-facing v1 baseline:

sheet pixels are authored and resolved as indexed values 0..15
serialized tile-bank payload uses packed u4 pixel indices
runtime may materialize the decoded bank as expanded u8 pixel indices plus palette table

18.8. Metrics for Certification (CAP)

The system can measure:

palettes_loaded_total
palettes_referenced_this_frame
tiles_drawn_by_palette_id
sprites_drawn_by_palette_id

19. Syscall Return and Fault Policy

Graphics-related public ABI in v1 is split between:

composer.* for Game 2D high-level frame composition;
gfx2d.* for Game 2D primitives only;
gfxui.* for Shell UI primitives only.

composer.* and gfx2d.* are available to AppMode::Game. gfxui.* is available to AppMode::Shell. Renderers and capabilities may differ by app mode.

Only operations with real operational rejection paths return explicit status values.

Fault boundary:

Trap: structural ABI misuse (type/arity/capability/shape mismatch);
status: operational failure;
Panic: internal runtime invariant break only.

19.1 Return-shape matrix in v1

Syscall	Return	Policy basis
`gfx2d.clear`	`void`	no real operational failure path in v1
`gfx2d.fill_rect`	`void`	no real operational failure path in v1
`gfx2d.draw_line`	`void`	no real operational failure path in v1
`gfx2d.draw_circle`	`void`	no real operational failure path in v1
`gfx2d.draw_disc`	`void`	no real operational failure path in v1
`gfx2d.draw_square`	`void`	no real operational failure path in v1
`gfx2d.draw_text`	`void`	no real operational failure path in v1
`gfxui.*` primitives	`void`	no real operational failure path in v1
`composer.bind_scene`	`status:int`	status-returning API, but missing scene glyph dependencies are fatal runtime errors
`composer.unbind_scene`	`status:int`	explicit orchestration-domain operational result
`composer.set_camera`	`void`	no real operational failure path in v1
`composer.emit_sprite`	`status:int`	explicit orchestration-domain operational rejection

19.1.a Primitive domain semantics

The primitive domains have stable operational meaning:

gfx2d.* is Game 2D primitive command buffering;
gfx2d.* is screen-space and primitive-only relative to composer.*;
gfx2d.* is outside scene, camera, sprites, HUD, and Game 2D frame orchestration;
gfxui.* is Shell UI primitive command buffering;
gfxui.* does not contain widget or layout policy.

Callers MUST NOT rely on stable immediate writes to the working back buffer as the public contract for primitive drawing. Primitive calls mutate domain command buffers that close into the active typed submission.

19.1.b Scene dependency fatal boundary

composer.bind_scene remains a status-returning syscall in the public ABI, but scene glyph dependency absence is outside the accepted passive operational-error model.