|Name:||glTF 2.0 format|
|Authors:||Julien Duroure, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Khronos Group, Mozilla|
glTF™ (GL Transmission Format) is used for transmission and loading of 3D models in web and native applications. glTF reduces the size of 3D models and the runtime processing needed to unpack and render those models. This format is commonly used on the web, and has upcoming support in native 3D engines such as Unity3D and Unreal Engine 4.
This importer/exporter supports the following glTF 2.0 features:
- Materials (Principled BSDF) and Shadeless (Unlit)
- Punctual lights (point, spot, and directional)
- Animation (keyframe, shape key, and skinning)
glTF’s internal structure mimics the memory buffers commonly used by graphics chips when rendering in real-time, such that assets can be delivered to desktop, web, or mobile clients and be promptly displayed with minimal processing. As a result, quads and n-gons are automatically converted to triangles when exporting to glTF. Likewise, curves and other non-mesh data are not preserved, and must be converted to meshes prior to export.
The core material system in glTF supports a metal/rough PBR workflow with the following channels of information:
- Base Color
- Baked Ambient Occlusion
- Normal Map
The glTF material system is different from Blender’s own materials. When a glTF file is imported, the add-on will construct a set of Blender nodes to replicate each glTF material as closely as possible.
The importer supports Metal/Rough PBR (core glTF), Spec/Gloss PBR (
and Shadeless (
Examining the result of the material import process is a good way to see examples of the types of material nodes and settings that can be exported to glTF.
The exporter supports Metal/Rough PBR (core glTF) and Shadeless (
It will construct a glTF material based on the nodes it recognizes in the Blender material.
The material export process handles the settings described below.
When image textures are used by materials, glTF requires that images be in PNG or JPEG format. The add-on will automatically convert images from other formats, increasing export time.
To create Shadeless (Unlit) materials, use the Background material type.
The glTF base color is determined by looking for a Base Color input on a Principled BSDF node. If the input is unconnected, the input’s default color (the color button next to the unconnected socket) is used as the Base Color for the glTF material.
If an Image Texture node is found to be connected to the Base Color input, that image will be used as the glTF base color.
Metallic and Roughness¶
These values are read from the Principled BSDF node. If both of these inputs are unconnected, the node will display sliders to control their respective values between 0.0 and 1.0, and these values will be copied into the glTF.
When using an image, glTF expects the metallic values to be encoded in the blue (
and roughness to be encoded in the green (
G) channel of the same image.
If images are connected to the Blender node in a manner that does not follow this convention,
the add-on may attempt to adapt the image to the correct form during exporting (with an increased export time).
In the Blender node tree, it is recommended to use a Separate RGB node
to separate the channels from an Image Texture node, and
connect the green (
G) channel to Roughness, and blue (
B) to Metallic.
The glTF exporter will recognize this arrangement as matching the glTF standard, and
that will allow it to simply copy the image texture into the glTF file during export.
The Image Texture node for this should have its Color Space set to Non-Color Data.
Baked Ambient Occlusion¶
glTF is capable of storing a baked ambient occlusion map.
Currently there is no arrangement of nodes that causes Blender
to use such a map in exactly the same way as intended in glTF.
However, if the exporter finds a custom node group by the name of
glTF Metallic Roughness, and
finds an input named
Occlusion on that node group,
it will look for an Image Texture attached there to use as the occlusion map in glTF.
The effect need not be shown in Blender, as Blender has other ways of showing ambient occlusion,
but this method will allow the exporter to write an occlusion image to the glTF.
glTF stores occlusion in the red (
R) channel, allowing it to optionally share
the same image with the roughness and metallic channels.
To use a normal map in glTF, connect an Image Texture node’s color output to a Normal Map node’s color input, and then connect the Normal Map normal output to the Principled BSDF node’s normal input. The Image Texture node for this should have its Color Space property set to Non-Color Data.
The Normal Map node must remain on its default property of Tangent Space as this is the only type of normal map currently supported by glTF. The strength of the normal map can be adjusted on this node. The exporter is not exporting these nodes directly, but will use them to locate the correct image and will copy the strength setting into the glTF.
Blender’s Cycles rendering engine has a Bake panel that can be used to bake tangent-space normal maps from almost any other arrangement of normal vector nodes. Switch the Bake type to Normal. Keep the default space settings (space: Tangent, R: +X, G: +Y, B: +Z) when using this bake panel for glTF. The resulting baked image can be saved and plugged into to a new material using the Normal Map node as described above, allowing it to export correctly.
See: Cycles Render Baking
An Image Texture node can be connected to an Emission shader node, and optionally combined with properties from a Principled BSDF node by way of an Add shader node.
If the glTF exporter finds an image connected to the Emission shader node, it will export that image as the glTF material’s emissive texture.
The Double Sided setting is controlled from the Normals panel of the Mesh settings tab. In Blender, this is a per-mesh setting, but in glTF, it is a per-material setting.
Blender also has a setting called Backface Culling in the Shading panel of the 3D viewport. This setting has no effect on the glTF export, however standard glTF viewers will automatically turn on backface culling per-material for any glTF materials that are not double-sided.
The Base Color input can optionally supply alpha values. How these values are treated by glTF depends on the selected blend mode.
With the Eevee rendering engine selected, each material has a Blend Mode on the material settings panel. Use this setting to define how alpha values from the Base Color channel are treated in glTF. Three settings are supported by glTF:
- Alpha values are ignored (the default).
- Alpha Blend
- Lower alpha values cause blending with background objects.
- Alpha Clip
- Alpha values below the Clip Threshold setting will cause portions of the material to not be rendered at all. Everything else is rendered as opaque.
Be aware that transparency (or Alpha Blend mode) is complex for real-time engines to render, and may behave in unexpected ways after export. Where possible, use Alpha Clip mode instead, or place Opaque polygons behind only a single layer of Alpha Blend polygons.
Control over UV map selection and transformations is available by connecting a UV Map node and a Mapping node to any Image Texture node.
Settings from the Mapping node are exported using a glTF extension named
There is a mapping type selector across the top. Point is the recommended type for export.
Texture and Vector are also supported. The supported offsets are:
- Location - X and Y
- Rotation - Z only
- Scale - X and Y
For the Texture type, Scale X and Y must be equal (uniform scaling).
These nodes are optional. Not all glTF readers support multiple UV maps or texture transforms.
Any Image Texture nodes may optionally be multiplied with a constant color or scalar. These will be written as factors in the glTF file, which are numbers that multiply with specified image textures. These are not common.
A single material may use all of the above at the same time, if desired. This figure shows a typical node structure when several of the above options are applied at once:
Certain features require extensions to the core format specification. The following glTF 2.0 extensions are supported:
glTF allows multiple animations per file, with animations targeted to particular objects at time of export. To ensure that an animation is included, either (a) make it the active Action on the object, (b) create a single-strip NLA track, or (c) stash the action.
Only certain types of animation are supported:
- Keyframe (translation, rotation, scale)
- Shape keys
- Armatures / skinning
Animation of other properties, like lights or materials, will be ignored.
Custom properties on most objects are preserved in glTF export/import, and may be used for user-specific purposes.
File Format Variations¶
The glTF specification identifies different ways the data can be stored. The importer handles all of these ways. The exporter will ask the user to select one of the following forms:
glTF Binary (
This produces a single
.glb file with all mesh data, image textures, and
related information packed into a single binary file.
Using a single file makes it easy to share or copy the model to other systems and services.
glTF Separate (
.bin + textures)¶
This produces a JSON text-based
.gltf file describing the overall structure,
along with a
.bin file containing mesh and vector data, and
optionally a number of
.jpg files containing image textures
referenced by the
Having an assortment of separate files makes it much easier for a user to go back and edit any JSON or images after the export has completed.
Be aware that sharing this format requires sharing all of these separate files together as a group.
glTF Embedded (
This produces a JSON text-based
.gltf file, with all mesh data and
image data encoded (using Base64) within the file. This form is useful if
the asset must be shared over a plain-text-only connection.
This is the least efficient of the available forms, and should only be used when required.
- Pack Images
- Pack all images into the blend-file.
- How normals are computed during import.
- See: File Format Variations
- Selected Objects
- Export selected objects only.
- Apply Modifiers
- Apply modifiers (excluding armatures) to mesh objects.
- Y Up
- Export using glTF convention, +Y up.
- Custom Properties
- Export custom properties as glTF extras.
- Remember Export Settings
- Store export settings in the Blender file, so they will be recalled next time the file is opened.
- Legal rights and conditions for the model.
- Export UVs (texture coordinates) with meshes.
- Export vertex normals with meshes.
- Export vertex tangents with meshes.
- Vertex Colors
- Export vertex colors with meshes.
- Export materials.
- Export cameras.
- Punctual Lights
- Export directional, point, and spot lights. Uses the
- Use Current Frame
- Export the scene in the current animation frame.
- Exports active actions and NLA tracks as glTF animations.
- Limit to Playback Range
- Clips animations to selected playback range.
- Sampling Rate
- How often to evaluate animated values (in frames).
- Always Sample Animations
- Apply sampling to all animations.
- Export skinning (armature) data.
- Bake Skinning Constraints
- Apply skinning constraints to armatures.
- Include All Bone Influences
- Allow >4 joint vertex influences. Models may appear incorrectly in many viewers.
- Shape Keys
- Export shape keys (morph targets).
- Shape Key Normals
- Export vertex normals with shape keys (morph targets).
- Shape Key Tangents
- Export vertex tangents with shape keys (morph targets).
This importer/exporter is developed through the glTF-Blender-IO repository, where you can file bug reports, submit feature requests, or contribute code.
Discussion and development of the glTF 2.0 format itself takes place on the Khronos Group glTF GitHub repository, and feedback there is welcome.