.SKP File Format – Object stream

SketchUp 3 to 2016 model

Everything after the version map is the model, written as an MFC CArchive stream. The first parts, up to the options, have the same layout in every file seen and are described field by field below. The objects after them follow the class versions in the map and are not decoded here. All numbers are little-endian, and every string is a CString.

Layout

PartSizeContents
Model fields12Three u32 values of unknown meaning.
ThumbnailvariesA CDib object: a new class record, an image type, a length and a PNG.
Unknown1401 00 00 00 and ten zero bytes.
OptionsvariesNamed groups of typed values: units, pages, slideshow and others.
Objectsto the endMaterials, layers, component definitions, entities and attributes.

Class tags

Wherever the archive stores an object, it first writes a 16-bit tag that says what follows. The tags are MFC’s, described in Microsoft’s technical note TN002:

TagMFC nameMeaning
0000wNullTagA null pointer: no object.
FFFFwNewClassTagA class seen for the first time. The schema (u16) and the class name follow; in these files the name is preceded by its length as a u16: FF FF, schema, length, name in ASCII. The object’s own data comes next.
8000 | indexwOldClassTagA new object of a class already seen. The low 15 bits are the class’s index. The object’s data follows.
7FFFwBigObjectTagFollowed by a 32-bit index, for indexes above 7FFE.
any otherA reference to an object already in the archive, by its index.

The indexes (MFC calls them persistent IDs) are handed out in order, starting at 1, to every new class and every new object. So in a SketchUp 8 file, the CVersionMap class takes 1, the map itself 2 and the CDib class of the thumbnail 3; a JPEG texture later in the same file is introduced by 03 80, a new CDib object, which fits. The first material after the CMaterial class record is tagged 0F 80 in that file: the material class has index 15.

The schema number in a class record is the class’s layout version. In the files seen it is the same number as in the version map: 3 for CDib, 12 for CMaterial and 2 for CLayer.

Model fields

Straight after the map’s end entry come three 32-bit numbers. Their meaning is not known.

OffsetSizeTypeNameSeen
04u32Unknown 11 or 2.
44u32Unknown 21200 (B0 04 00 00) in every file.
84u32Unknown 3A small number: 1, 2 or 6 in the classic files seen.
 
01 00 00 00 B0 04 00 00 01 00 00 00
The model fields of a SketchUp 8 file

Three fields lead straight to the thumbnail’s class record. The explorer’s parser checks this: a file whose map is followed by five fields is a SketchUp 2017 to 2020 model.

Thumbnail

The first object after the map is a CDib, the picture SketchUp shows in its file dialogs. It holds a complete PNG file.

OffsetSizeTypeNameMeaning
010class recordClassFF FF 03 00 04 00 43 44 69 62: a new class, schema 3, the 4-letter name CDib.
104u32Image type4 for the PNG thumbnail. JPEG textures inside materials use the same class with type 1.
144u32LengthThe size of the image in bytes.
18lengthbytesImageA complete PNG file, starting 89 50 4E 47.
 
FF FF 03 00 04 00 43 44 69 62 04 00 00 00 19 11 00 00 89 50 4E 47 …
A thumbnail in one of the classic files seen: the class record, type 4, a 4,377-byte PNG

The thumbnails seen are PNGs of 4 KB to 90 KB. The SketchUp 8 file’s is 128 × 69 pixels (its IHDR chunk holds 00 00 00 80 00 00 00 45), the 2015 and 2016 files’ 256 × 256. A JPEG texture in the SketchUp 8 file starts 03 80 01 00 00 00 E2 32 00 00 FF D8 …: a new CDib object, type 1, 13,026 bytes, then the JPEG.

The explorer shows the thumbnail straight from the file.

After the thumbnail

Fourteen bytes whose meaning is not known: 01 00 00 00 and then ten zero bytes, in every file seen. The explorer skips to the next string marker and reports a run of another length.

01 00 00 00 00 00 00 00 00 00 00 00 00 00
The 14 bytes after the thumbnail

Options

Next come the model’s options, in named groups.

PartTypeMeaning
Unknown stringCStringEmpty in every file seen.
Unknownu322 in every file seen.
Group countu32The number of option groups that follow: 4 or 5 seen.
GroupsEach a CString name, then its options, then an empty CString that ends the group.

Each option is a key, a type byte and a value:

PartTypeMeaning
KeyCStringThe option’s name.
Typeu8How the value is stored (below).
ValuevariesThe value.
TypeValue
4i32, a signed 32-bit integer
6f64, a 64-bit floating-point number
7u8, true or false: 0 false, 1 true
10CString

Two options from the PageOptions group, ShowTransition set to true and TransitionTime set to 2.0:

FF FE FF 0E 53 00 68 00 6F 00 77 00 54 00 72 00 61 00 6E 00 73 00 69 00 74 00 69 00 6F 00 6E 00 07 01
FF FE FF 0E 54 00 72 00 61 00 6E 00 73 00 69 00 74 00 69 00 6F 00 6E 00 54 00 69 00 6D 00 65 00 06 00 00 00 00 00 00 00 40
A type 7 option and a type 6 option

Groups seen:

GroupOptions seen
PrintOptionsnone (an empty group)
PageOptionsShowTransition, TransitionTime
SlideshowOptionsLoopSlideshow, SlideTime
UnitsOptionsLengthPrecision, LengthFormat, LengthUnit, LengthSnapEnabled, LengthSnapLength, AnglePrecision, AngleSnapEnabled, SnapAngle, SuppressUnitsDisplay, ForceInchDisplay, AreaUnit, VolumeUnit, AreaPrecision, VolumePrecision and others
NamedOptions

The same key, type and value form appears elsewhere in the stream, in attribute dictionaries: LastModifiedByKey with type 10 and an empty string (0A FF FE FF 00), VersionKey with type 4 and 1000 (04 E8 03 00 00).

To read the options: read the empty string, the u32 2 and the group count. For each group, read its name; then, until the next string is empty, read a key, a type byte and a value of that type; then read the empty string. A type other than 4, 6, 7 or 10 is not known, and the explorer stops there.

The objects

The rest of the file is the model itself as CArchive objects: materials, layers, component definitions, groups, faces, edges, vertices, text, dimensions, scenes and their attributes, each introduced by a class tag and laid out according to its class version in the map. These layouts are not decoded here, and the explorer shows the rest of the file as one range. Two lists can still be found in it by searching.

Materials

The materials are written as a u32 count, then the first material with the CMaterial class record (FF FF 0C 00 09 00 and the name), then the others, each with that class’s 8000 | index tag. Each material starts with a two-byte preamble, 00 00 when it carries no attributes, and then its name as a CString.

0D 00 00 00 FF FF 0C 00 09 00 43 4D 61 74 65 72 69 61 6C 00 00 FF FE FF 05 62 00 6C 00 61 00 63 00 6B 00 …
0F 80 00 00 FF FE FF 08 3C 00 42 00 65 00 69 00 67 00 65 00 3E 00 31 00 …
In a SketchUp 8 file: 13 materials, the first “black”; a later one, “<Beige>1”, with the tag 0F 80

A material without a texture in the SketchUp 8 file goes on after its name with a u16 0, four colour bytes (such as D3 BD 90 FF), an empty CString, 16 bytes and a u8. OpenSKP reads the 16 bytes as 8 bytes and an f64 opacity; that has not been checked here. The classic files seen hold from 4 to 59 materials.

Layers

The layers (called tags in later SketchUp) follow the same pattern: a u32 count, the first layer with the CLayer class record (FF FF 02 00 06 00 43 4C 61 79 65 72), the others with its tag, and each with the same preamble and its name.

How the explorer finds them

  1. Search the object stream for the class name with its length before it (for materials 09 00 and CMaterial), and check for FF FF and a schema in front of it.
  2. Read the u32 count just before the class record.
  3. After the name, skip up to eight zero bytes to the first CString: the first name.
  4. Look for the next tag with the top bit set that is followed by zero bytes and a string: that is the class’s tag and the second name.
  5. Search for that tag again, read the name after it, and repeat until the count is reached.

So the lists are found by searching, not by reading every object, and the explorer labels them so. In most files seen every name is found. An object that carries attributes has a longer preamble, and its name is missed; the explorer then shows how many of the count it found.