I guess this is a question of understanding, but perhaps I'm overlooking something which might solve my hassle with rendering of objects with transparency in cycles.
I'm working on a project with large scale landscapes and found out that the original asset, if close to the world origin renders just fine, but when used in the actual scene transparent parts of the object (e.g. a decal) render black. This happens already at a distance of about 400 m from the world origin. However, this happens to cycles only, but renders correctly in eevee. Below is an example of a decal rendered in cycles and eevee:
The same decal in cycles:
I figured out that when I move the decal further away from the background object it renders correct again in cycles.
I've also observed problems with glass shaders that are too far from the world origin, where the refraction renders incorrectly and looks faceted or blocky.
Besides moving everything which is in focus of the camera closer to the world origin, is there any other trick I don't know of to make this work? Imagine my hassle of creating an asset with roughly 30 decals, already appended to four different scenes (because linking and library overrides has tons of other problems) only to find out that for the final render in cycles it does not work out!
And my final question, why is it working just fine with eevee but not in cycles, where is the difference (technically)?
Linked below is a very simple scene with a cube and a decal. With the cube on the world origin it renders fine. Move the cube 100 m in X and transparency renders black. The camera and light is parented to the cube to make this as simple as possible.
Edit #2: Seems to be a long known issue with blender :(: https://developer.blender.org/T43835