Open Brush File Format

The .tilt file format can also be parsed by the Open Brush Toolkit.
A .tilt is a zip-format file with a prepended header:
uint32 sentinel ('tilT')
uint16 header_size (currently 16)
uint16 header_version (currently 1)
uint32 reserved
uint32 reserved
Inside the zip, the strokes are stored in a custom binary format in a file, "data.sketch":
uint32 sentinel
uint32 version
uint32 reserved (must be 0)
[ uint32 size + <size> bytes of additional header data ]
int32 num_strokes
num_strokes * {
    int32 brush_index
    float32x4 brush_color
    float32 brush_size
    uint32 stroke_extension_mask
    uint32 controlpoint_extension_mask
    [ int32/float32 for each set bit in stroke_extension_mask & ffff ]
    [ uint32 size + <size> bytes for each set bit in stroke_extension_mask & ~ffff ]
    int32 num_control_points
    num_control_points * {
        float32x3 position
        float32x4 orientation (quat)
        [ int32/float32 for each set bit in controlpoint_extension_mask ]
    }
}
The orientation is that of the controller. Curve and surface frames must be reconstructed.
Stroke extensions:
0
uint32 bitfield.
Bit 0: IsGroupContinue
Control point extensions:
0
float pressure, in [0,1]
1
uint32 timestamp, in milliseconds
​
Also inside the zip is "metadata.json", the metadata for the sketch in json format. Here are some of the fields that can be found there:
"Authors": an array of author names.
"SceneTransformInRoomSpace": the transform of the scene relative to the room.
"ThumbnailCameraTransformInRoomSpace": the transform used to generate the sketch as an array of:
translation (array of 3 floats)
rotation quaternion (array of 4 floats)
scale (single float)
"ModelIndex": an array of models imported into the sketch. Each model can have the following:
"FilePath": location of the model
"PinStates": an array to indicate whether each instance of the model should initially be pinned.
"RawTransforms": an array of transforms for each instance of the model.

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