OneCAD/docs/design-container.md

Design containers, version 1

New designs use .solidcraft; .solvecraft remains a supported legacy extension. Opening detects the ZIP local-header signature (50 4b 03 04) independently of the name; otherwise it uses the existing JSON design reader and migrations. Plain JSON remains a fully supported, self-contained input/output format. io::write_design keeps its JSON contract; io::write_design_container is the fallible container writer.

Packages have container.json with {"format":"solidcraft-container/1"} and document.json with the versioned parametric document (currently solidcraft/14). The independent versions let document migrations evolve without changing blob encodings. Existing serialized document fields are retained automatically, without a second copy of the document schema or a deep clone of embedded payloads.

Within document.json:

• Import features have empty step and a step_blob reference.
• Mesh imports retain each mesh's name and optional colour, with empty positions and triangles plus a mesh_blob reference.
• Canvases retain placement, format and pixel metadata, with empty data plus an image_blob reference. Raw image bytes replace base64 in the package.

References are blobs/<lowercase SHA-256>.<step|mesh|png|jpeg>, hashed over the exact uncompressed payload. Identical content with the same encoding is written once. Required parts, malformed references, ambiguous embedded payloads, incorrect hashes, invalid mesh data and unsupported versions produce errors before session adoption.

Mesh encoding

Mesh blobs begin with the eight ASCII bytes SCMESH01, followed by little-endian u64 coordinate count and u64 index count. These count scalar values, not vertices or triangles. Next come that many IEEE-754 f64 coordinates, then u32 indices. Both counts must be divisible by three; the byte length must match exactly. Coordinates must be finite and at most 1e9 mm in magnitude; indices must address an existing vertex. Names and colours remain in JSON. Signed zero and full coordinate precision survive.

The default fast policy stores STEP and binary meshes without Deflate for faster saving and reading. Small document metadata uses Deflate level one; PNG/JPEG are stored directly. An explicit compact policy compresses STEP and mesh payloads at level one. Readers support both policies. This deliberately favors interactive file operations over maximum compression; compressed STEP was slower in the storage benchmark. Use "compression":"compact" with file.save or file.save_as to favor file size, or io::write_design_container_with_compression from Rust.

Bounds and recovery

Input and individual expanded parts are capped at 256 MiB. Total declared expanded parts and resolved payloads are each capped at 512 MiB, including repeated references in the latter budget. There are at most 10,000 entries. Duplicate/ambiguous entry names, path traversal, ZIP64 and multi-disk packages are refused. Entries are read in memory; no archive paths are extracted to the filesystem. Exact part lengths and ZIP CRCs are checked, along with blob hashes. Hashes detect correspondence and corruption, not trust.

Saving retains the existing temporary-file/flush/rename protocol and .bak backup. Recovery reads both older JSON and new containers. Browser OPFS, downloads and recovery use the same container reader/writer; JSON export and Save As remain available.

Provisional saved display

Experimental engine saves with preview: true can include cache/preview.json and cache/preview.bin when the evaluated world display meshes are already warm. Ordinary saves omit this redundant display data to avoid extra save work; it is not the proposed evaluated B-rep cache. Saving never triangulates a cold body for this cache. Pending occurrence moves, active sketch edits, feature errors and oversized previews omit it. The cache is bounded to 8 MiB, 1,024 bodies and 100,000 triangles; its manifest is at most 64 KiB. Body names/colours and contiguous binary mesh slices use the same lossless encoding as imported meshes. Extra cache size is optional; it is omitted if it would put the container near the file-size cap.

The manifest includes SHA-256 of the exact document metadata (whose blob references contain content hashes), the preview payload, solidcraft-preview/1 and the application package version. A version mismatch invalidates the display cache. Development builds with the same package version may briefly show an older picture before the authoritative rebuild replaces it; previews never supply model state. Earlier draft containers using solvecraft-container/1 remain readable, with their obsolete previews treated as misses. Missing, obsolete, oversized or malformed preview data is a cache miss, not a failed design open. All archive-wide safety budgets still apply.

Desktop Open can render these meshes on its existing file worker and show a clearly labelled saved picture in the opening dialog before timeline reconstruction finishes. It uses only the normal command's file snapshot, without a second file read. The picture is approximate shading without face paint or picking. A conservative 16-million-pixel raster budget rejects excessive overdraw before rendering, including repeated large triangles in a small hostile cache. The picture is never adopted as editable geometry, used for measurements, or used to seed evaluated model state. The original design remains intact until the complete rebuilt session is ready. Cancellation or any change of active document/revision discards both preview and completion.

Normal design reading, CLI and browser opening still rebuild authoritatively; no evaluated B-rep is deserialized or adopted. Portable evaluated B-rep/state restoration and interactive cached viewport navigation remain further work in #303.