A texture atlas is an image that can gather several textures or images.
Instead of having one texture per mesh, you can have a larger texture shared by several meshes.
This allow to reduce the quantity of textures to load for the app, and to have an efficiently optimized 3D model.
To create your atlas, you shall proceed to these steps:
Mark seams on your mesh in a way that prevent distortions, and separate the elements on your model.
Unwrap/Unfold the UVs.
Distribute by selecting islands/clusters which have different textures and pack to optimize space on the UV grid.
Depending on your model, it is possible to unfold only the relevant parts of mesh, and skip the rest of the unfold. For instance, on a cooler with buttons and details on the front, the front can be unfolded while the rest of the object can be skipped.
Keep in mind that it is possible to set different UV sizes by scaling some islands, depending on the level of detail you need on each part. For example, a main color can be in the texture on a few pixels while a command panel with small writings can take the main space on the whole image.
Regroup islands/clusters from meshes that will share the same texture by overlapping them in one, use the snap tools from your 3D software to fit them properly. If these islands/cluster shapes share the same texture, you may need to check the mesh for good texture fitting.
If you wish to gather several objects' UVs in one atlas, you can organize the UVs in one corner of the UV square. In the example, the bottles were place in the top left corner, while the rest of the texture map will be used for the cooler. However, check correctly the positions of islands/clusters to prevent from overlapping between différent objects.
As for any UV unfold, but particularly if you will combine several objects' UVs together, be careful about the spacing between the islands/clusters. Always reserve an empty border to allow the option of extra border (padding) around the parts and prevent from overlapping.
Export the result as an image before creating your 2D texture. If several meshes are concerned, the several UVs images can then be combined.
You can then design the textures for your atlas in your preferred software.
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To learn about textures in Augment, check here.
To learn about textures resizing, check here.