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Material Viewport
Here the material will be displayed applied on a sphere or a cube (the default).
On the top-left corner of the viewport the file path, relative to the project, of the material is showed in green.
Above the viewport there is the following toolbar:
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Cube
The cube model for representing the material
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Sphere
The sphere model for representing the material
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Thumbnail
Click here to generate a new thumbnail for the material into the
Project Browser
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Background
Click here to show a background behind the material. It is useful in the case of transparent materials.