Unexpected transparency in alpha map from RenderTarget

[stephent]

This may be a three.js issue rather than rtf specific, but thought I'd put it out there:

I’m using a RenderTarget to generate a simple alpha map using a Cylinder. The Cylinder uses a basic material with color white, which should be fully opaque in the alpha map, per the docs. However, when applied to a geometry, the area corresponding to white in the mask is not rendered fully opaque, as shown in this example:

https://codesandbox.io/s/r3f-programmatic-alpha-map-f8lhi?file=%2Fsrc%2Findex.js

The scene setup is:

• Red plane masked to a cylinder via the alpha mask - expected to be fully opaque
• Behind that, a white box - it should not be visible at all from default camera position, but it is.

Does anyone have any insight as to why the white alpha mask leaves the Red plane with non-zero transparency (i.e. NOT fully opaque)?

Thanks!

Stephen

[stephent]

Possibly a three.js issue? mrdoob/three.js#21987