Bug or missunderstanding (Viewport render to texture)?

comboy

I'm trying out custom post processing and rendering to textures. Not sure if I stumbled over a bug in 3.1 Beta or just don't grasp the concept yet.

I have the following scene:

Inside the first viewport is just a camera and 2 identical meshes with an alpha faded texture.

Mesh1 is on culling layer 1 Mesh2 on culling layer 2

The main Camera sees only culling layer 1

then I have another Viewports without shader and a transparent background It has another camera that only sees culling layer 2

My goal was to only process one of the 2 meshes with a post processing shader

Since the ViewportContainer for camera2 has no shader yet, I would expect to see 2 identical meshes. But it seems the viewport messes up the alpha blending somehow as the mesh on culling layer 2 darkens the background.

Do I just missunderstand the concept or is this a bug?

TwistedTwigleg

On your Viewport, do you have transparent_bg enabled?

Looking at the sphere on the left, it looks like it is blending with a darker color, which is especially noticeable at the bottom. It could be that it is not using a transparent background, and in Godot when a Viewport does not have a transparent background it renders what would be transparent black.

Another potential issue I could see is that the Viewport is in its own world, and so it is not blending the background/sky simply because it does not exist in the Viewport. If you disable own_world then it should make it where it uses the same World resource as the main scene, which should make it use the same background/sky.

If transparent_bg is enabled and own_world is disabled on the Viewport, then I have no idea as it should be rendering both spheres exactly the same. If neither fix works I would suggest opening a issue on the GitHub repository.

comboy

It seems to be a known issue https://github.com/godotengine/godot/issues/17574

Due to non pre multiplied alpha. I managed to nearly fix it by manually doing the pre multiplication in the shader code of the object and the ViewportContainer. That only leaves a small artifact.

A premultiplied alpha option is on the roadmap for 3.2 as I understand. So until then one has to be carefull with transparent objects in viewports.