lukas-shawford/gltflib lukas-shawford/gltflib

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Library for parsing, creating, and converting glTF 2.0 files in Python 3.6+.

gltflib

Library for parsing, creating, and converting glTF 2.0 files in Python 3.6+.

Overview

This library is intended for working with glTF 2.0 at a fairly low level, meaning you are responsible for managing the actual geometry data yourself. This library facilitates saving this data into a properly formatted glTF/GLB file. It also helps with converting resources inside a glTF/GLB file between external files or web URLs, data URLs, and embedded GLB resources.

Installation

This library can be installed using pip:

pip install gltflib

Usage

The examples below illustrate how to use this library for a couple sample scenarios. The example models come from the Khronos glTF-Sample-Models repository available here:

https://github.com/KhronosGroup/glTF-Sample-Models

Parsing a glTF 2.0 Model

To load a glTF 2.0 model:

from gltflib import GLTF

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')

The GLTF.load static method supports loading both the JSON-based .gltf format, as well as the binary .glb format. The type of the file will be determined based on the filename extension. Alternatively, you can use GLTF.load_gltf(filename) or GLTF.load_glb(filename).

After loading, you can inspect the model structure by accessing the model property:

print(gltf.model)
# GLTFModel(extensions=None, extras=None, accessors=[Accessor(extensions=None, extras=None, name=None, bufferView=0, byteOffset=0, componentType=5123, ...

You can also inspect the various model properties:

print(gltf.model.buffers[0].uri)
# BoxTextured0.bin

A glTF 2.0 model may contain resources, such as vertex geometry or image textures. These resources can be embedded as part of the model file, or (as with the above example) be referenced as external file resources.

In either case, the resources are parsed alongside the model structure into the resources property after loading a model:

print(gltf.resources)
# [FileResource(CesiumLogoFlat.png), FileResource(BoxTextured0.bin)]

Note that the actual content of these external file resources is not loaded by default when loading a model. You can load the resource into memory in one of two ways. One way is to call the load() method on the resource:

resource = gltf.resources[0]
resource.load()     # Assumes resource is a FileResource

Another way is to pass the load_file_resources flag when calling GLTF.load():

gltf = GLTF.load(filename, load_file_resources=True)

In either case, now the file resource data can be accessed via the data property:

print(resource.data)
# b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80?\x00\x00\x00\x00\x00\x00\...

Embedded resources in binary GLB files are also parsed into the resources list, but they will be of type GLBResource instead of FileResource:

glb = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF-Binary/BoxTextured.glb')
print(glb.resources)
# [<gltflib.gltf_resource.GLBResource object at 0x7f03db7c1400>]

For embedded resources, the content is parsed into memory automatically. The binary data can be accessed using the data property:

resource = glb.resources[0]
print(resource.data)
# b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80?\x00\x00\x00\x00\x00\x00\...'

Exporting a glTF 2.0 Model

To export a model, call the GLTF.export() instance method in the GLTF class.

The example below creates a simple glTF 2.0 mode in memory (consisting of a single triangle), then exports it as a glTF file named triangle.gltf (alongside with an external file resource named vertices.bin):

import struct
import operator
from gltflib import (
    GLTF, GLTFModel, Asset, Scene, Node, Mesh, Primitive, Attributes, Buffer, BufferView, Accessor, AccessorType,
    BufferTarget, ComponentType, GLBResource, FileResource)

vertices = [
    (-4774424.719997984, 4163079.2597148907, 671001.6353722484),
    (-4748098.650098154, 4163079.259714891, 837217.8990777463),
    (-4689289.5292739635, 4246272.966707474, 742710.4976137652)
]

vertex_bytearray = bytearray()
for vertex in vertices:
    for value in vertex:
        vertex_bytearray.extend(struct.pack('f', value))
bytelen = len(vertex_bytearray)
mins = [min([operator.itemgetter(i)(vertex) for vertex in vertices]) for i in range(3)]
maxs = [max([operator.itemgetter(i)(vertex) for vertex in vertices]) for i in range(3)]
model = GLTFModel(
    asset=Asset(version='2.0'),
    scenes=[Scene(nodes=[0])],
    nodes=[Node(mesh=0)],
    meshes=[Mesh(primitives=[Primitive(attributes=Attributes(POSITION=0))])],
    buffers=[Buffer(byteLength=bytelen, uri='vertices.bin')],
    bufferViews=[BufferView(buffer=0, byteOffset=0, byteLength=bytelen, target=BufferTarget.ARRAY_BUFFER.value)],
    accessors=[Accessor(bufferView=0, byteOffset=0, componentType=ComponentType.FLOAT.value, count=len(vertices),
                        type=AccessorType.VEC3.value, min=mins, max=maxs)]
)

resource = FileResource('vertices.bin', data=vertex_bytearray)
gltf = GLTF(model=model, resources=[resource])
gltf.export('triangle.gltf')

As with load, the export method infers the format based on the filename extension (.gltf vs .glb). However, you can also call export_gltf or export_glb to manually force the format.

In the above example, the export will produce two files: triangle.gltf and vertices.bin. However, it is possible to bypass saving external file resources by setting the save_file_resources flag to False when calling export:

gltf.export('triangle.gltf', save_file_resources=False)

To export the model as a binary GLB instead, simply change the extension when calling export, or use export_glb:

gltf.export('triangle.glb')

Note that when exporting as a GLB, all resources will be embedded by default (even if they were instantiated as a FileResource). This is generally the desired behavior when saving as a GLB.

However, it is possible to force some or all resources to remain external when exporting a GLB. To do so, you must call export_glb (instead of export), and setting either embed_buffer_resources or embed_image_resources (or both) to False:

resource = FileResource('vertices.bin', data=vertex_bytearray)
gltf = GLTF(model=model, resources=[resource])
gltf.export_glb('triangle.glb', embed_buffer_resources=False, embed_image_resources=False)

In this case, you will also need to ensure that the associated buffers still have the appropriate uri set in the model:

model = GLTFModel(
    ...,
    buffers=[Buffer(byteLength=bytelen, uri='vertices.bin')],

The model will be exported as a binary GLB, but with external file resources. These file resources will be saved by default when exporting the model. However, it is also possible to bypass saving external file resources by setting the save_file_resources to False when calling export_glb:

gltf.export_glb('triangle.glb', embed_buffer_resources=False, embed_image_resources=False,
                save_file_resources=False)

Converting Between glTF and GLB

To convert a glTF model to GLB, simply load it and export it using the glb extension:

from gltflib import GLTF

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')
gltf.export('BoxTextured.glb')

This will automatically convert all external file resources to become embedded GLB resources.

The reverse conversion is also possible, though with some caveats. Since a non-binary glTF model may not have embedded binary data, the GLBResource must first be converted to a different resource type. The section on Resources below goes into more details, but here is a quick example where the GLBResource is first converted to a FileResource with the filename BoxTextured.bin prior to exporting to glTF:

from gltflib import GLTF

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF-Binary/BoxTextured.glb')
glb_resource = gltf.get_glb_resource()
gltf.convert_to_file_resource(glb_resource, 'BoxTextured.bin')
gltf.export('BoxTextured.gltf')

Note that a GLB file typically contains a single binary GLB chunk that combines data from multiple buffers (which are then consumed by multiple buffer views, images, and accessors). Currently, when converting a GLB to glTF, the entire GLB chunk can be converted to a resource of a different type, but the resource cannot be split out into multiple resources (e.g., separate resource per buffer).

Resources

glTF and GLB models can refer to embedded or external resources (via the buffer or image URIs, or in the case of GLB, by leaving the first buffer's URI undefined). These resources are represented in this library using subclasses of the GLTFResource base class. These resources will be parsed when loading a model, and must be properly instantiated and added to the model prior to exporting.

There are 4 resource types that are supported by this library:

  • FileResource: File resources are resources that refer to a file path.
  • Base64Resource: Resources that are embedded directly in the glTF (or GLB) file using a Base64-encoded data URI.
  • GLBResource: Used only by GLB files, this resource type represents the binary GLB chunk that is embedded directly in the GLB file.
  • ExternalResource: External resources refer to external web URLs.

A reference to a particular resource can be obtained if its URI is known by calling get_resource:

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')
logo = gltf.get_resource('CesiumLogoFlat.png')
print(logo)
# FileResource(CesiumLogoFlat.png)

Alternatively, a list of all resources in a model can be obtained using the resources list on the loaded model:

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')
print(gltf.resources)
# [FileResource(CesiumLogoFlat.png), FileResource(BoxTextured0.bin)]

The GLTF class provides helper methods that can be used to convert a resource from one type to another. Some of these methods require some additional information to do the conversion; for instance, the filename when converting to a FileResource, or the MIME type when converting to a Base64Resource.

The sections below go into more detail about each resource type, including their caveats and limitations, as well as how to convert a given resource to that type.

File Resources

File resources are denoted using the FileResource class, and represent resources that refer to a file path (generally a relative path, though absolute file paths are also supported).

When loading a model, these resources are parsed by looking at the uri property on buffers and images; however, their content is not automatically loaded unless the load_file_resources flag is set to True when calling GLTF.load():

gltf = GLTF.load(filename, load_file_resources=True)

Alternatively, the load() method can be called on a FileResource instance to load the data into memory:

resource = FileResource('triangleWithoutIndices.bin')
resource.load()

Once the file resource is loaded into memory, its content is accessible via the data property:

print(resource.data)
# b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80?\x00\x00\x00\x00\x00\x00\...

When exporting a model, file resources will be written to disk by default. However, this can be bypassed by setting the save_file_resources flag False when calling export:

gltf.export(filename, save_file_resources=False)

When creating a model instance manually, if the intention is to also save file resources, then there must be a corresponding FileResource in the resources list for every buffer or image that references a file path (otherwise, an error will be raised when attempting to export):

resource = FileResource('buffer.bin')
model = GLTFModel(asset=Asset(version='2.0'), buffers=[Buffer(uri='buffer.bin', byteLength=18)])
gltf = GLTF(model=model, resources=[resource])
gltf.export('model.gltf')

When instantiating a FileResource, if the content of the file is known, it can be provided via the data constructor parameter:

resource = FileResource('buffer.bin', data=b'binary content here')

A resource of another type can be converted to a FileResource using the convert_to_file_resource helper method on the GLTF class. This method requires a filename as a parameter, and returns the converted FileResource instance:

resource = gltf.resources[0]
file_resource = gltf.convert_to_file_resource(resource, 'BoxTextured.bin')

Note the file will not be created until the model is saved (with save_file_resources flag set to True). Also, note that the resource to be converted must be part of the resources list in the model (otherwise an error will be raised).

If the resource is already a FileResource and the filename matches, no action is performed. If the filename is different, then the filename will be updated on any buffers and images that reference it.

If the resource to be converted is a GLBResource or Base64Resource, it will be un-embedded and converted to an external file resource, and any buffers that reference the resource will be updated appropriately. Any embedded images that reference the resource will be updated. If the image previously referenced a buffer view, it will now reference a URI instead; the corresponding buffer view will be removed if no other parts of the model refer to it. Further, after removing the buffer view, if no other buffer views refer to the same buffer, then the buffer will be removed as well.

If the resource to be converted is an ExternalResource, this method will raise an error (accessing external resource data is not supported).

Base-64 (Data URI) Resources

glTF supports embedding a resource directly into a JSON-based glTF file (or a GLB file, though it's not as common) using a data URI. In this scenario, the resource is defined as part of the URI itself, allowing the model to be self-contained without necessarily using the GLB format:

{
  ...
  "images": [
    {
      "uri": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAQAAAAEACAYAAABccqhmAAAEDW..."
    }
  ],
  ...
}

When loading such a model, a resource of type Base64Resource will be instantiated and added to the model's resources list. The uri property of the resource will contain the original data URI, while the data property can be used to access the decoded binary data:

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF-Embedded/BoxTextured.gltf')
logo = gltf.resources[1]
print(logo.data)
# b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80?\x00\x00\x00\x00\x00\x00\...

To instantiate a Base64Resource, there are two options. One is to use the constructor to pass in the binary data and MIME type (which defaults to application/octet-stream if not provided):

resource = Base64Resource(b'sample binary data', mime_type='application/octet-stream')

The other way is to use the Base64Resource.from_uri factory method and pass in the data URI:

resource = Base64Resource.from_uri('data:application/octet-stream;base64,c2FtcGxlIGJpbmFyeSBkYXRh')

To convert a resource of another type to a Base64Resource, use the GLTF.convert_to_base64_resource helper method. This method accepts an optional mime_type parameter if the MIME type of the original resource is known (defaults to application/octet-stream if not provided):

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')
logo = gltf.get_resource('CesiumLogoFlat.png')
gltf.convert_to_base64_resource(logo, 'image/png')
gltf.export('BoxTexturedBase64.gltf')

If the resource to be converted is already a Base64Resource, no action is performed.

If the resource is a FileResource, then it will be converted to a Base64Resource. The data for the FileResource will be loaded from disk if not already loaded (which may raise an IOError if the file does not exist).

If the resource is a GLBResource, it will be converted to a Base64Resource. The GLB buffer will be replaced with a buffer with a data URI (or removed entirely if it is only used by images). Any images that refer to the resource via a buffer view will instead refer to the image directly via a data URI, and the corresponding buffer view will be removed (if it is not also referenced elsewhere). Further, if no other buffer views refer to the same buffer as the removed buffer view, then the buffer will be removed entirely as well.

If the resource is an ExternalResource, this method will raise an error (accessing external resource data is not supported).

GLB Resources

GLB Resources are resources that are embedded directly in a GLB file as binary chunks. These resources can only be used with a GLB file (if saving to glTF, these resources must first be converted to a different type).

There is generally one GLB chunk in a file (with the chunk type BIN), though it is valid to have multiple GLB chunks if they have a different chunk type. This library supports loading and saving these additional GLB chunks, though no assumptions are made about their content.

A reference to the GLBResource corresponding to the primary GLB chunk (with the chunk type BIN) can be obtained by calling get_glb_resource on a model instance, and its data can be accessed via the data property:

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF-Binary/BoxTextured.glb')
glb_resource = gltf.get_glb_resource()
print(glb_resource.data)
# b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80?\x00\x00\x00\x00\x00\x00\...

Additional GLB chunks can be referenced by calling get_glb_resource with a resource_type parameter set to the chunk type:

my_custom_glb_resource = gltf.get_glb_resource(resource_type=123)

An individual resource of another type can be converted to a GLBResource using the embed_resource helper method. This allows embedding a particular resource while leaving others external when exporting to GLB (in this scenario, ensure to use export_glb instead of export, and set both embed_buffer_resources and embed_image_resources to False to prevent the other resources from also being automatically embedded):

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF-Binary/BoxTextured.glb')
logo = gltf.get_resource('CesiumLogoFlat.png')
gltf.embed_resource(logo)
gltf.export_glb('BoxTexturedPartial.glb', embed_buffer_resources=False, embed_image_resources=False)

However, the most common scenario is to embed all resources regardless of their type, which happens automatically when calling export with a .glb extension (or when calling export_glb with the default set of parameters).

Note that embedding an ExternalResource is not supported because its data is not accessible (this library does not support loading resources from an external web URL).

As explained in the other sections, converting a GLBResource to a resource of another type (i.e., "un-embedding" a resource) will typically not only replace the URIs on the corresponding buffers and images, but may also result in removing the GLB buffer and buffer views entirely if they are not also referenced elsewhere.

External Resources

External resources (represented by the ExternalResource class) are resources that have an external web URL. While this library is able to load models with external web URLs, the resource itself will not be fetched. A resource of type ExternalResource will be instantiated with the corresponding URI, but the library will not perform any web requests to load the resource data. Likewise, the library supports saving a model containing ExternalResource instances, but again, no web requests will be performed.

A resource of another type can be converted to an ExternalResource using the GLTF.convert_to_external_resource helper method, which accepts a URL:

gltf = GLTF.load('glTF-Sample-Models/2.0/BoxTextured/glTF/BoxTextured.gltf')
logo = gltf.get_resource('CesiumLogoFlat.png')
gltf.convert_to_external_resource(logo, 'http://www.example.com/image.png')
gltf.export('BoxTexturedExternal.gltf')

Again, since this library does not handle calling out to external resources, this is strictly a bookkeeping operation. It is the responsibility of the caller to ensure that the resource exists externally. Note when converting a resource to an ExternalResource, the resource data becomes inaccessible.

If the resource is already an ExternalResource and the URI matches, no action is performed. If the URI is different, then the URI will be updated on the resource instance as well as on any corresponding buffers or images in the model.

If the resource is a FileResource or Base64Resource, then it will be converted to an ExternalResource, and all buffers and images will be updated appropriately.

If the resource is a GLBResource, it will be converted to an ExternalResource. The GLB buffer will be replaced with a buffer with a data URI (or removed entirely if it is only used by images). Any images that refer to the resource via a buffer view will instead refer to the image directly via a data URI, and the corresponding buffer view will be removed (if it is not also referenced elsewhere). Further, if no other buffer views refer to the same buffer as the removed buffer view, then the buffer will be removed entirely as well.

Credits

This project is based on the pygltflib library by dodgyville available here:

https://gitlab.com/dodgyville/pygltflib

Specifically, this project is based on a much earlier version of pygltflib at a time when it didn't seem to be actively maintained. I used that library as a starting point and added some features I needed for my own work. Since then, the original pygltflib project has been revived, but our implementations have diverged significantly. So now there are two :-)

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