Drawing 3D in front/behind sprites in XNA/WP7?

This is kind of frustrating me as I've been grizzling over it for a couple of hours now.

Basically I'm drawing 2D sprites through spritebatch and 3D orthographically projected geometry using the BasicEffect class.

My problem is controlling what gets rendered on top of what. At first I thought it would be simply controlling the render order, i.e. if I do:

Draw3DStuff()

Sp开发者_JAVA技巧riteBatch.Begin(...)
Draw2DStuff();
SpriteBatch.End();

It would mean the 2D stuff would render over the 3D stuff, however since I don't control when the device begins/ends renders this isn't the result. The 3D always renders on top of the 2D elements, no matter the projection settings,world translation, the z components of the 3D geometries vertex definitions and the layer depth of the 2D elements.

Is there something I'm not looking into here? What's the correct way to handle the depth here?

OK I figured it out 2 seconds after posting this question. I don't know if it was coincidence or if StackOverflow has a new feature granting the ability to see future answers.

The Z position of spritebatch elements are between 0 and 1 so they're not directly comparable to the z positions of orthographic geometry being rendered.

When you create an orthographic matrix however you define a near and far clip plane. The Z pos you set should be within this clip plane. I had a hunch that the spritebatch class is effectively drawing quads orthographically so by extension that 0 to 1 would mean 0 was representing a near clip and 1 a far clip, and the depth was probably being rendered into the same place the 3D geometry depth is being rendered to.

Soooo, to make it work I just figured that the near/far clips I was defining for the orthographic render will be measured against the near/far clips of the sprites being rendered, so it was simply a matter of setting the right z value, so for example:

If I have a near clip of 0 and a far clip of 10000 and I wanted to draw it so that it would correspond to 0.5f layer depth and render in front of sprites being drawn at 0.6 and behind sprites being drawn at 0.4 I do:

   float zpos = 0.5f;
   float orthoGraphicZPos = LinearInterpolate(0, 10000, zpos);

Or just zpos * 10000 :D

I guess it would make more sense to have your orthographic renderers near/far clip to be 0 and 1 to directly compare with the sprites layer depths.

Hopefully my reasoning for this solution was correct (more or less).

As an aside, since you mentioned you had a hunch on how the sprite batch was drawing quads. You can see the source code for all the default/included shaders and the spritebatch class if you are curious, or need help solving a problem like this:
http://create.msdn.com/en-US/education/catalog/sample/stock_effects

The problem is that the spritebatch messes with some of the renderstates that are used when you draw your 3d objects. To fix this you just have to reset them before rendering your 3d objects like so.

GraphicsDevice.BlendState = BlendState.Opaque;
GraphicsDevice.DepthStencilState = DepthStencilState.Default;
GraphicsDevice.SamplerStates[0] = SamplerState.LinearWrap; 

Draw3DStuff()

SpriteBatch.Begin(...)
Draw2DStuff();
SpriteBatch.End();

Note that this is for xna 4.0 which I am pretty sure your using anyway. More info can be found on shawn hargreaves blog here. This will draw the reset the render states, draw the 3d objects, then the 2d objects over them. Without resetting the render states you get weird effects like your seeing.