This article is dedicated to explain how make clothes for the Mixamo Fuse , with one example.
It serves as much for clothes that you do or that you have and want to import to the program.

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In this case I made a simple T-shirt with a serigraphy behind, as shown in the image.
When modeling the shirt with a 3D program, (Maya, blender, 3DMax … )you must use a base body of the program mixamo. In this case I have used a woman-based model.
First I have modeled in low poly, and to give a little more smoothness in the contours I have given a level of subdivision to the polygons. (Beware of subdivisions because Mixamo has a limit).
Also tell you that the open parts like necks, sleeves, etc. close them to integrate them better on the body that will support them.

For more detail I have exported the model to Zbrush where I added wrinkles and seams with the high quality that Zbrush allows from here I get the Normal Map and the Ambient Occlusion that later I will add to the model low poly when importing it to Mixamo.
The textures should be in dimensions of 2048 x 2048 and 8bits RGB.
Mixamo Fuse Clothing Download Photos
I leave you the link of the platform mixamo where to consult this data.
In addition to the mapping of textures for clothing, it is convenient to do in the body on which a mask is to be placed that hides the polygons that will be below, so that there are no conflicts of body that protrudes. In such a way as in the image below what is hidden will remain in black and what is seen in white.
Another thing when import to mixamo is to know that the program offers different materials, textures … with which you can customize your model and for this gives some colors of masks with Photoshop o similar program so that you can divide your model to taste. In my case I have only used two masks for the color, one for the base and another for the serigrafy.
Just import the clothes, the following image shows the steps that you will ask the program mixamo, choose your mesh, what type of category (if it is top, bottom, shoes …) The base of the body, textures and finally on the chosen mask colors you can give the name you like.
Thank you very much and I leave you the l link of the platform mixamo where to consult these and other data.