OneCAD/docs/render-backends.md

Render backend capabilities (R1)

render.backends discovers render metadata without opening a window, probing a GPU, evaluating the model or changing the document, camera, dirty state or undo/redo history. It also leaves a model made stale by a live sketch gesture stale.

solidcraft-cli exec render.backends '{}'

The control channel and MCP execute use the same engine command. Parameters must be an empty object; a null parameter value is normalized to {} by the engine. Other parameters fail with a normal command error.

Discovery schema

The command returns this JSON object (schema version 1):

{
  "schema_version": 1,
  "default_backend": "builtin",
  "backends": [{
    "id": "builtin",
    "label": "Integrated",
    "availability": {"status": "available"},
    "executors": [{
      "id": "cpu-raster",
      "availability": {"status": "available"},
      "profiles": [{
        "mode": "preview",
        "operation": "image",
        "formats": ["png"],
        "features": []
      }],
      "limits": {
        "max_dimension": 4096,
        "max_pixels": 4194304,
        "max_scratch_bytes": 268435456
      },
      "scratch": {
        "sample_scale": 2,
        "sample_bytes_per_pixel": 8,
        "output_bytes_per_pixel": 8,
        "encoding_bytes_per_pixel": 8
      }
    }]
  }]
}

An unavailable backend or executor instead has "availability": {"status": "unavailable", "reason": "..."}. These are runtime descriptors, not design-file settings. Executor IDs are scoped to their backend. Backend/executor IDs are 1–64 ASCII bytes: a lowercase letter followed by lowercase letters, digits, ., _ or -. Both construction and deserialization validate IDs.

The integrated backend currently describes only the existing CPU raster snapshot path. Its preview/image profile exports PNG; it does not claim HDRI, PBR, refraction, progressive rendering or a final-quality integrator. The existing GPU viewport remains separate and is not advertised here until a GPU host can register and probe its actual capabilities. CPU and GPU are execution paths, not separate user-selectable engine families.

Pure request resolution

solidcraft_render::resolve_request accepts a descriptor slice and a RenderRequest. It validates and resolves metadata only: it does not allocate an image, start a job, reroute snapshot or call a renderer. A request can be deserialized from:

{
  "backend": "builtin",
  "mode": "preview",
  "operation": "image",
  "width": 640,
  "height": 480,
  "format": "png",
  "features": [],
  "policy": "strict"
}

backend defaults to builtin when absent/null. features defaults to [], and policy defaults to strict. Mode, operation and positive integer dimensions are required. An image requires a supported format; a viewport requires absent/null format. Unknown fields, enum values, invalid IDs and incorrectly typed values fail deserialization. Dimensions are checked by the resolver. No floating-point request fields are introduced.

Modes are quick, preview, final; operations are viewport, image; formats are png, exr. Required features can be hdri, pbr, refraction, progressive. These names define requests, not promises of support. Each executor lists complete mode/operation profiles with their formats and features. Capabilities are never combined across executors to create an unsupported profile.

Resolution rejects duplicate backend IDs, executor IDs within a backend and duplicate mode/operation profiles. Registry sizes are bounded to 32 backends, 16 executors per backend and 16 profiles per executor; a request allows at most 32 feature entries. Available executors are considered in lexicographic ID order, and the first executor that satisfies the entire request is selected. Registry order does not convey priority. Returned profile formats/features are sorted, so their declaration order does not change the resolved result. If every candidate fails, the first matching executor's error is returned; unsupported profile is reported when none has that combination.

The resolved value contains backend, executor, profile, width, height, format, estimated_scratch_bytes and warnings. Under strict policy warnings are empty and no requested setting is downgraded. Errors are typed ResolveError values: invalid IDs/request/registry, duplicate IDs, unknown/unavailable backend, no available executor, unsupported profile/format/feature, invalid dimensions, exceeded limits or size overflow. There is no fallback policy or public job API in R1.

Resource admission policy

The new resolver's default limits are conservative policy values, not measured adapter capacity: 1–4096 pixels per dimension, at most 4,194,304 output pixels and a 256 MiB estimated pixel-buffer budget. Each executor supplies configurable RenderLimits and a ScratchLayout. Checked arithmetic covers dimensions, supersampling, buffer products, sums and the target's addressable allocation size, including wasm32.

For the existing CPU raster path the estimate is 48 bytes per output pixel: four supersamples at 8 bytes each (RGBA8 plus f32 depth), an 8-byte output canvas, and an 8-byte allowance for an RGBA copy and encoded output. At 640 × 480 this is 14,745,600 bytes; at 2048 × 2048 it is 201,326,592 bytes. A 4096 × 4096 request exceeds the pixel limit despite each dimension being permitted.

This is an admission estimate, not a total process-memory bound. It excludes scene, texture, projected-geometry, acceleration-structure and codec-internal allocations. Future execution hosts must enforce their own complete allocation/cancellation policy. The legacy rasterizer, CLI snapshots and GPU viewport keep their existing behaviour and limits; none is routed through this resolver in R1.

The follow-on Render workspace, persistent setups, job hosts and optional engines are tracked in #620. The implemented foundation is scoped by #642.