I have trawled forums for ages looking for a possible answer to this.
I am trying to render shadows to a separate layer then recombining them in the compositor with the rest of the image. This is because I would like to have complete control over the shadow layer and ultimately apply a mask to it (to avoid any shadows extending past the render border creating a hard edge).
I have created a simplified setup to demonstrate the issue.
In this simplified setup I have 2 view layers, layer 1 containing a cube and layer two containing a shadow catching plane. You can see that the lines appear at the border between the shadows and the object. I have looked into issues of premultiplied alpha but none of this seem to sort it. They appear despite the condition of either Convert Premul checkboxes on the Alpha Over nodes.
I suspect that it may be to to with the Transparency filter in the Film section of the properties, however altering this leads to poor results.
Does anyone have a solution or some tips on what to investigate. This will ultimately be for an animation and so I'm keen to achieve everything in Blender rather than saving 2 sets of frames for the shadows and for the objects
For reference here is an image of just the first view layer with both the cube and shadow cater being rendered showing no lines
Any help or advise would be massively appreciated