I'm currently working on a XNA project where I need to create a Picture in picture Overlay to displaying a 3D scene from multiple angles. currently I'm trying to use 2 viewports to do this. The main one fills whole screen and is working as desired. The second is placed in one of the corners of the first (overlapping that corner) and is less than a 5th of the size as the first. apart from the size and placement of the viewports the only thing really difference between the 2 is the placement of their cameras with-in the scene.
As long as the second viewport is drawn second and there are no objects close to the camera of the first viewport in the overlapping corner this actually works greate. However if there is an object close to the camera and in the corner of the first viewport objects seem to experience occlusion culling as a result of the first viewport's object. The occluding object of the first viewport is not shown in the second viewports space though.
My question is how would one prevent the "cross viewport culling" from happening? I've searched all over and the closes threads I could find suggest drawing the second viewport to a RenderTarget2D and using a SpriteBatch to display the resulting texture. Though doing so does fix the occlusion issue it does mayhem on the z ordering, CCW culling, and my water effects all of which I've never had issue with using the default render target.
This issue, at least in my case, Seems to have been resolved by simply clearing the depth buffer between drawing the 2 Viewports. I did this by adding the following line between the function calls that draws the individual Viewports
GraphicsDevice.Clear(ClearOptions.DepthBuffer, Color.Black, 1, 0);
I ran in to this solution while trying to use the Stencil buffer to prevent the Main viewport from drawing under the second, but everything I did before this discovery did not really have a noticeable effect. After I removed all the Stencil code and it looks like I'm getting the desired effect. Sorry but I can't really explain how or why this works without messing up the first Viewport cause I have no clue myself.