Legacy SKP Format (SketchUp 3–2020)

SketchUp save targets from version 3 through 2020 use a pre-ZIP serialized object graph. SketchUp 2021 and later use the ZIP/VFF container described in Modern SKP format.

This page is the entry point for legacy-format users and contributors. Detailed wire layouts have been moved into focused references:

Reference	Contents
Container and archive	Header, strings, version map, references, primitive values, and root order
Class catalog	Class-to-model mapping and schema tables by save generation
Field layouts	Confirmed entity, material, scene, and rendering fields

Evidence and scope

The implementation is an independent interoperability effort based on files created through documented public APIs, user-owned compatibility samples, and observable file behavior. These notes are not an official specification.

Every statement in the detailed reference is either:

• confirmed by controlled fixture generation or a cross-version sample;

• required to consume a complete file without losing stream alignment; or

• explicitly marked unknown or inferred.

Unobserved class/schema combinations fail with a specific legacy-format error. They are not decoded using a nearby layout.

Identifying the container

Both container families begin with legacy UTF-16 product and version strings. The bytes immediately after those strings distinguish them:

Property	Legacy (3–2020)	Modern (2021+)
Boundary after product/version	16-byte model identifier	VFF marker
Main body	Serialized object graph	ZIP containing model.dat
Per-file schema selection	CVersionMap	TLV tags and namespaces

skppy.load() performs this check automatically. File extension and product text alone are insufficient.

Parser model

The archive is a graph, not a list of independent records. Class definitions, class references, object references, and null values share one ordered archive session. A later entity may point to a vertex, edge, material, definition, or layer decoded earlier.

The parser therefore has three stages:

1. Read the header and the file’s CVersionMap.

2. Decode the graph while preserving archive identity and unresolved links.

3. Resolve links into the same public Model classes used by the modern parser.

Technical archive indexes never become public entity IDs. Public IDs are allocated during model assembly and references are translated afterward.

Public model coverage

Legacy data	Public skppy result
Vertices, edges, loops, faces, curves, arcs	Entities geometry
Definitions, instances, groups, placed images	Component/entity classes
Materials, textures, embedded image data	Material and Texture
Layers and SU2020 layer groups	Layer and LayerFolder
Guides and section planes	Construction entity classes
Cameras and saved pages	Camera and Scene
Rendering, shadows, axes, options	Shared model metadata
Text, dimensions, fonts, styles	Annotation and style classes
Attribute and relationship maps	Public dictionaries and entity relationships

Parser-only state is retained only when needed for graph identity, unresolved references, exact payload boundaries, or data without a lossless public owner.

Version-aware behavior

The file’s CVersionMap is the primary selector for body layouts. The target generation is also required for a small number of confirmed down-save differences, including early construction lines and section-plane names.

Important public capability boundaries include:

Capability	First confirmed save target
Entity-owned attribute dictionaries	SketchUp 4
Component-instance names	SketchUp 5
Model-level dictionaries	SketchUp 7
Named section planes	SketchUp 2018
Custom line styles	SketchUp 2019
Layer folders	SketchUp 2020
Generated folder membership retained by current public fixture path	SketchUp 2021 (modern container)

Older files keep older semantics; the parser does not synthesize a modern value merely to make versions look alike.

Validation

The companion skppy-tests repository generates controlled save targets with the documented SketchUp C API and runs version-aware semantic checks. Focused raw-byte tests cover wire branches that the public API cannot generate directly.

When adding legacy support:

1. add an independent raw fixture or controlled generated sample;

2. record the schema boundary in the class reference;

3. assert complete payload consumption;

4. verify the resulting public model rather than archive-only state; and

5. reject layouts for which evidence is incomplete.

Known boundaries

• The legacy writer is intentionally out of scope; skppy writes the confirmed modern container.

• Some historical runtime-only values have no public model counterpart and are consumed without assigning speculative names.

• Interactive-only texture control points are covered by raw fixtures because the documented geometry API emits affine projections.

• Embedded thumbnails are metadata and are not treated as model textures.

See the class catalog for the supported schema matrix and field layouts for confirmed details.