I believe I have a specific enough problem to ask this question.
Basically, I want to render the crossroad object in front of the black border of the image. but I also want it to be where it is because it's important that the people and bikes go through it. Is there a way to achieve that in compositing (or some other way)?
I'll admit I'm quite new to compositing and I already have some compositing setup there.
Yes, you can do this with compositing. The general setup is:
Create separate collections for the "Border", "Crosswalk", rest of the "Scene", and the "Studio" for camera and lighting.
Create a new "Crosswalk" view layer. In this layer, uncheck the "Border" collection to disable it for the layer, and toggle on "Holdout" for the "Scene" collection. (The "Holdout" icon can be enabled in the filter pull-down above the icons.) When this view layer is rendered, the crosswalk will be drawn as if the border wasn't there, but any other parts of the scene that overlap the crosswalk will be held out as alpha=0 areas of the image.
Composite the two layers with an "Alpha Over" node. The crosswalk "foreground" will be placed on top of everything else, but the parts of the scene that should appear in front of the crosswalk will be visible through the alpha=0 holdouts.
Here are the original and composited scenes side by side:
Note that anything that appears on top of the crosswalk under the border area needs to be part of the "Crosswalk" collection. (If not, the border will show through those held out areas, instead of the intended object.) In your scene, you may need to separate the rain into the parts that appear over the (ends of the) crosswalk, and make them part of the "Crosswalk" collection, versus the rest of the rain which should be part of the "Scene" collection so it will be covered by the border wherever there is no crosswalk.
My example blend file:
