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I'm trying to assign a second material to top and front faces (using 'assign' button in the material collection, as explained here). But instead the faces become transparent:

enter image description here

They are transparent in the preview and in the rendering as well. If I remove the second material from the collection (slots?), the faces are visible again.

Things I did:

  • Reinstalled Blender to be sure.
  • Swapped materials for faces, faces with material from slot 2 are transparent regardless of material in slot 2.
  • Added a third slot, and linked faces to random slots. Slot 1 always work, other work randomly, not always the same which work.

Using Blender 2.81a, with a default new scene, cube and default material except base color.

File: http://pasteall.org/blend/index.php?id=52755

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  • $\begingroup$ Hello :). You can share your .blend file to see if others can replicate your issue. $\endgroup$ – Jachym Michal Jan 5 at 21:32
  • $\begingroup$ @ᴊᴀᴄʜʏᴍᴍɪᴄʜᴀʟ: Done! $\endgroup$ – mins Jan 6 at 0:34
  • $\begingroup$ Hello again :). I checked your file, everything works as it should on my end (Blender 2.81). You can check if 2.82 works better for you, or revert temporarily back to 2.8. $\endgroup$ – Jachym Michal Jan 6 at 9:01
  • $\begingroup$ @ᴊᴀᴄʜʏᴍᴍɪᴄʜᴀʟ: Thanks for your time. I checked again and confirm the random nature of the problem on my side. By assigning the second material to some faces (or all), the transparent faces change or disappear. Also tried Cycles which works normally. I'll try to revert to 2.8 a bit later. Again thanks for your help! $\endgroup$ – mins Jan 6 at 10:18
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This is a known issue when using multiple materials with a number of older GeForce, Quadro and other end of life graphics cards even though they seem to meet the minimum requirements.

Blender Known Issue: Drawing error with multi-material object. Faces disappear or showing artifacts with 2 or more material slots applied to same object. When trying to use a second material or more on an object, the faces where it is applied become transparent or in some cases they are distorted as black rectangles that stretch to the origin (this second occurs above all with more complex models than a cube). It happens in Blender 2.81 as well as in Blender 2.82, but in Blender 2.80 there is no such error.

While the bug report lists multiple cards, the list is not exhaustive (I have this issue on my card, the GeForce 9800 GTX+, which is not listed). The core issue is the recentness of the video driver. In the case of NVIDIA GPUs, they have to be able to run NVIDIA drivers greater than v400. The highest driver that I can install on the GeForce 9800 GTX+ is GeForce v342.01.

I can confirm that as listed in the bug report, I can use multiple materials in Blender 2.80 without a problem.

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  • $\begingroup$ That seems to be the right explanation. I'd this problem with a Quadro FX 880M. Thanks also for the tip about going back to 2.80. $\endgroup$ – mins Mar 5 at 21:13
  • $\begingroup$ I had the same issue from 2.8 to 2.81, where the assign option was used in 2.8 and it is not available in 2.81 / 2.82, so I was forced out of necessity to make faces a separate object and re-assign the materials as a separate object material. I still have .OBJ issues from assigned materials on assets in 2.82 also, as a weird transparent area in some places at some angles. $\endgroup$ – dave44 Mar 5 at 21:31
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There could be several issues be the case. It is possible that in the blue material, you have enabled alpha, which makes it transparent. Have you tried creating a new material for those faces? Do they also become transparent?

As Jachym suggests, you can upload your blend file so we may take a closer look.

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