I am trying to export a glass material in gltf. Here is the material principled shader, notice I have transmission set to 1.0:

principled shader

this gives me this result in cycles (the glasses have the glass material):

van with glass

However when I export this to gltf, and view it in the gltf viewer or load it in my webgl application, this is the result:

webgl van

How do I export glass?

I already have this in my render options:

render transparency

I am using blender 2.8 beta, all the other questions I could find all refer to an older version of blender.

EDIT: I don't even have the Blend Mode dropdown in my shader settings:

shader settings

  • $\begingroup$ A big comprehensive write up in blender docs would be good for this challenging transition where V8 and Blender are tops $\endgroup$ – Cymatical Sep 3 at 17:04

glTF 2.0 has an alpha blending mode available, but it's intent is just to be "coverage" (like medical gauze tape), not a physically correct optical translucency (as the latter was considered too high a target for the initial release of 2.0). Basically this means that low alpha values will dissolve the entire material, including specular reflections. Even so, the mode is often used to emulate clear materials like glass.

To control the blend mode, look for a settings panel near the bottom of the material properties set of panels. Blender offers many choices here. glTF supports three of them: Opaque, Alpha Blend, and Alpha Clip (along with the "Clip Threshold" value in that case).

The alpha values can come from an RGBA "Base Color" texture assigned to the Principled BSDF node. If no texture is assigned, you can use the color picker on the Base Color disconnected input itself, and note the color picker there includes an alpha slider.

NOTE: At some point in the Blender 2.8x series, the glTF exporter began using the "Alpha" input on Principled BSDF, rather than the alpha channel of the base color, for textures but not yet for default values. In Blender 2.9x, it will exclusively use the alpha input for both textures and scalar values.

The following Blend Mode must be set (in Eevee, and each material remembers this setting even if you switch back to Cycles) for alpha blending to take effect:

Blend Mode

For folks looking for a more advanced, physically plausible optical translucency, take a look at a glTF extension called ADOBE_materials_thin_transparency. Adobe submitted this in support of the translucency available in Adobe Dimension, and Microsoft worked with them to add support to BabylonJS. But, not many other glTF viewers have added support for this yet, outside of Dimension and Babylon.

UPDATE: (July 2020) There's now an effort underway to bring an extension similar to ADOBE_materials_thin_transparency to Khronos as a new extension draft called KHR_materials_transmission. Blender support is under development in PR #1094.

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  • $\begingroup$ Thanks for the insight! However I am not finding the Blend Mode setting, I even downloaded the latest beta right now, do I need to do something specific? i.imgur.com/dzoTVVa.png $\endgroup$ – Pontiacks Jan 25 '19 at 22:32
  • $\begingroup$ Hmm, it's there when Eevee is selected as the rendering engine (in the Scene tab), but when I switch to Cycles, these options disappear. This might need to be filed as an exporter issue, not accounting for blend modes in Cycles. I guess the workaround would be to temporarily switch to Eevee to edit the material settings. $\endgroup$ – emackey Jan 28 '19 at 15:28
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    $\begingroup$ Ok, I've been able to make the material transparent also, in eevee, following this guide, however the gltf export still exports an opaque material. Should I report this bug to the blender issue tracker? blender.stackexchange.com/questions/81851/… $\endgroup$ – Pontiacks Feb 10 '19 at 18:36
  • $\begingroup$ When you switched to Eevee, did you find the "blend mode" -> "alpha blend" setting on the material, similar to the screenshot above? This, plus an alpha channel in the base color texture, should cause the current exporter to ask for blending in glTF. If you still need to file an issue, the tracker is here: github.com/KhronosGroup/glTF-Blender-IO/issues $\endgroup$ – emackey Feb 11 '19 at 13:00

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