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Packed Raw WebGL Model (PRWM) Specifications (version 1 / final 2017-06-09)

Conventions

  • All signed integer values are stored in two's complement format.
  • All float values are stored following the IEEE 754 spec.
  • The endianness of the file is defined in its header.
  • The format supports indexed and non-indexed geometries.
  • The format is designed to allow any number of custom attributes and doesn't force the use of any pre-defined attributes.
  • The format doesn't support any type of bone-based animations, morphing is achievable with custom attributes.
  • The format prioritizes fast parsing over file weight.
  • The format is specifically designed for WebGL and will probably prove useless for any other platforms.

The general structure of the file is the following : One header (8 bytes) followed by one or more attribute blocks (varying byte count) and, in the case of an indexed geometry, up to one indices block (varying byte count).

Header

  • Version : 1 byte (0 to 255)
  • Indexed geometry : 1 bit
  • Indices type : 1 bit
  • Endianness : 1 bit
  • Number of attributes per vertex : 5 bits (0 to 31)
  • Number of values per attribute : 3 bytes (0 to 16777215)
  • Number of indices : 3 bytes (0 to 16777215)

Version

Indicates the version of the specification to apply while decoding this model.

A value of 0 should be treated as an error by the decoder.

Indexed Geometry

  • 0 : Non-indexed geometry
  • 1 : Indexed geometry

Indices type

  • 0 : Unsigned 16bit integer (2 bytes)
  • 1 : Unsigned 32bit integer (4 bytes)

This should be set to 0 for non-indexed geometries, a value of 1 should be treated as an error by the decoder.

Endianness

  • 0 : Little Endian
  • 1 : Big Endian

Number of attributes per vertex

Indicates the number of attributes per vertex.

A value of 0 should be treated as an error by the decoder.

It should be noted that most WebGL implementations are currently limited to 16 attributes per vertex.

Number of values per attribute

Indicates the number of values per attribute.

Number of indices

The number of indices stored in the index block.

This should be set to 0 for non-indexed geometries, any other value should be treated as an error by the decoder.

Attribute block

Attribute header

  • Name : An ASCII encoded C-string (one byte per character, terminated with a NUL character)
  • Type : 1 bit
  • Normalized : 1 bit
  • Cardinality : 2 bits (0 to 3)
  • Encoding type : 4 bits (0 to 15)
  • Padding : 0 to 3 bytes

Type

  • 0 : Float (should declare the attribute with vertexAttribPointer)
  • 1 : Integer (should declare the attribute with vertexAttribIPointer, if supported)

Normalized

  • 0 : The attribute is not to be normalized.
  • 1 : The attribute should be normalized (see vertexAttribPointer).

This only affects attributes encoded as integers (see below) and typed as floats.

Cardinality

Indicates the number of components per vertex attribute. 0 is scalar, 1 is a 2d vector, 2 is a 3d vector and 3 a 4d vector.

Encoding type

This indicates how each elements are stored in the file and doesn't necessarily match the aforementioned type.

  • 0 : Reserved
  • 1 : Signed 32bit float (4 bytes)
  • 2 : Reserved
  • 3 : Signed 8bit integer (1 byte)
  • 4 : Signed 16bit integer (2 bytes)
  • 5 : Reserved
  • 6 : Signed 32bit integer (4 bytes)
  • 7 : Unsigned 8bit integer (1 bytes)
  • 8 : Unsigned 16bit integer (2 bytes)
  • 9 : Reserved
  • 10 : Unsigned 32bit integer (4 bytes)
  • 11 to 15 : Reserved

Padding

The attribute header block is to be padded with 0 values in such a way as to make the attribute values block starts at a position which is a multiple of 4. For example, if the attribute values block would start at the position 0xF1 without any padding, three padding bytes should be added to the header block as to make it start at the position 0xF4.

Attribute values

The values of the attributes are encoded sequentially with the specified encoding type, making up the rest of the attribute block.

While not directly encoded in the attribute header, the total length of the attribute values can be obtained as such :

Attribute values length = Cardinality * Encoding type byte count * Number of values per attribute

Indices block

This block should only be present for indexed geometries.

It should start with some padding to align the values on position which is a multiple of 4, if needed.

While not directly encoded in the block, the total length of the indices values can be obtained as such :

Indices values length = Number of indices * Indices type byte count