How to do a custom shadow mesh an actual shadow?

Hi argos93, welcome to Studio forums. Sims 4 has several different shadows and I cannot tell from your explanation which shadow you mean. There is one that shows up outdoors that is cast when sun hits the object, there is one that appears on the floor under floor items or on the wall behind wall items, and there is one that appears on the wall cast by large floor items when light hits them. The answer to your question depends on which of these shadows you're trying to alter.

You need to clone something that has a shadow mesh, whatever you used does not have one. If you clone something like this shelf you can replace the shadow plane with your mesh



If you upload the package Ill show you what I mean.

hi! If you say that the package doesn't recognize the new shadow then we're going going to able to see "your" shadow on the blend since the package isn't importing it correctly. That's why should upload the blend file to.

Having said that, you need to change the cut numbers of the new shadow to match the original shadow of the package.

It's on the number 6.B from this beginners tutorial.
In this case, the cut number is 0. but it's not always 0 since it depends on the item.

Hi argos93, Sims 4 has a shadow system that handles the wall shadows that lie behind items hung on a wall like your rifle is. This system involves assigning the shadow group to a specific shadow shader that uses what has come to be known as the "blob" sheet because it looks like a large white image with blobs of grey all over it. To see this image and work with your mesh using it select the shadow group behind your rifle, go to edit mode, select all the vertices of your shadow plane so they light up in orange, click the "Browse image to be linked" menu button and select the image shown:



Once you do that you will see the map for your shadow plane laying on top of EA's shadow shader image.  



Generally, I find it easiest to get the shadow roughly how I want it by editing the mesh and then tweaking the appearance of it with small changes to the map as needed:




There are multiple tutorials in our tutorial section that go over how to work with EA's shadow planes: my start to finish object tutorial, the tutorial showing how to change the shape of a painting, the tutorial showing how to convert a Sims 2 item to Sims 4, and the one titled How to fix every kind of Sims 4 object shadow.  The Build/Buy/Tuning tutorial index is HERE so you can easily find these tutorials going over how to work with EA's shadow system.

With an item like this one, that isn't completely square, round, oval, or rectangular the blob sheet isn't going to make a perfectly matching shadow with a plane that has been subdivided only a couple times as this one is (and as most EA shadow plane meshes are).  If you want a more detailed shadow you can add more vertices to the plane which will give you greater control over the shadow.  One thing to keep in mind is that, in the game, the object's shadow will be getting help from the game engine so it isn't always necessary to make a highly detailed shadow plane shadow to get a nice appearance for the item in the game.