Distant coloured billboards fade out on Android due to incorrect mipmap generation or rendering

[wilbefast]

Godot version

v3.3.3.stable.official [b973f99]

System information

Android 8.1.0 (Nexus 5X) - GLES2

Issue description

Billboards that use coloured textures fade out as they move away from the camera on Android 8.1.0. Bizarrely billboards with black and white textures do not exhibit this behaviour. The following videos were captured using the minimal sample project attached below.

Windows (expected behaviour):

minimal-windows.mp4

Android (unexpected behaviour):

minimal-android.mp4

Steps to reproduce

1. Add a Sprite3D, set to billboard mode, to a scene.
2. Use the default Godot icon.png as a texture for the billboard.
3. Add a Camera to the scene, facing the billboard.
4. Have a script move the two apart.
5. Notice that, only on Android, the billboard fades out as it moves away from the camera.

Minimal reproduction project

GodotAndroidBugMinimal.zip

[Calinou]

This is likely due to erroneous mipmap generation or rendering. Does it occur if you disable Mipmaps in the texture's import options, then export the project to Android again?

[wilbefast]

@Calinou I can confirm that disabling mipmaps in the import settings solves the issue 👍

edit: even so I would assert that this is still a bug, as the behavior, for a given set of import settings, should be the same on all platforms.

[Calinou]

@wilbefast Does this occur if you keep Mipmaps enabled, but change the texture compression mode to Lossless? Perhaps ETC1-compressed texture mipmaps are incorrectly generated or stored.

VRAM compression uses S3TC on desktop platforms, ETC2 on mobile/web platforms when using GLES3 and ETC1 on mobile/web platforms when using GLES2. ETC1 compression in particular does not support an alpha channel, so you want to use lossless compression for transparent textures instead.

[wilbefast]

@Calinou I can confirm that with Mipmaps enabled and Lossless texture compression the issue does not occur. Your theory seems solid, though I'm not a GLES expert by any means so I wouldn't really know. In any case so far it is standing up to scrutiny 👍

[Calinou]

Also confirmed to occur in HTML5 on Android: https://godotengine.org/qa/120596/transparent-objects-fading-out-from-camera-mobile-browser