I do a lot of product renders with shaped die cut labels applied to bottles. I handle this with a slightly larger cylindrical mesh around the bottle and create the die cut shape with an alpha stencil that makes the unwanted parts of the label mesh invisible. Worked like a charm…until now.
Problem: Models that worked fine in Blender 3 versions now exhibit a shadow or darkening effect in areas overlapped by the invisible geometry when opened in the new Blender 4 versions. It is as if the invisible areas are either casting a shadow or diminishing the light coming through them.
This is only visible on the nearby surface of the bottle. If I remove the bottle and leave just the label object, no effect is seen agains the more distant background. The alpha stenciled areas are again totally invisible.
Something has changed in the newer Blender. Anyone encountered this yet; or know what is causing it?
I began to wonder whether this darkening was an unwanted shadow, or a filtering of light passing through the parts of the mesh blocked by the alpha. To test if it was a shadow, I selected the label mesh and turned off Ray Visibility>Shadow in the Object Properties panel.
The darkening disappeared completely. While this won’t work if you need the non-alpha’d parts of a mesh to still cast a shadow, in the case of a thin label that needs to look as if adhered to a surface, that’s not a problem.
So, to anyone experiencing a similar problem in a similar use case, this is the workaround.
I suspect some shadow handling feature was altered in the Blender 4 revision that has caused this behavior to change. If anyone can throw further light on it, I love to hear about it.
Source of the issue seems to be that in Blender 4, setting the Alpha value to 0 does not render an object totally transparent. It will still cast a slight shadow.
Simple test: place a plane object a little above a surface and assign a new Principled Shader. You get an opaque object with a shadow.
Now, set the Alpha value to 0. In previous versions of Blender, the object becomes totally invisible. In Blender 4 versions, the object disappears, but it leaves behind a faint shadow of itself, like the Cheshire Cat’s smile.
This same behavior applies to the Transparent shader node as well.
Wouldn’t this qualify as a bug?
I think so, and have tried to report it via the Bug Report link in Blender, but the system doesn’t seem to be sending out the email confirmation messages properly. I’ve checked spam and junk mail folders to no avail. I have also tried a second email address. No dice. Can’t complete my log in to be able to send them the bug report.
