   Render to texture, then put onto backbuffer with alpha
  I am currently rendering to a texture by grabbing its surface and rendering to that.  What I would like to do is have any of the untouched pixels become transparent in the texture, so that when I use a sprite to render it to the backbuffer, it isn't the model I rendered with a big black rectangle around it.  I have tried clearing the surface with an alpha value specified, but it doesn't seem to work.The only thing I could think of was to render the entire backbuffer first, then copy a rect from the backbuffer to the texture, then render to that texture and copy it back to the backbuffer.  The only function I could find to do this was to use D3DLoadSurfaceFromSurface.  (I couldn't satisfy the memory location requirements of UpdateSurface for going in both directions).  This is incredibly slow, though, and thus dropped my frame rate from 90+ to 2.  However, if there is a way to do this without a noticable slowdown, I could go this route as well.Thanks for any help.   
  I cant think of a good way to do it... should work->Render the scene to a texture->Render a screen aligned quad (position+texture), throwing the previous rendertarget as the texture, to a second render target.  ->in the pixel shader check to see if its the 'clear' color, if so set its alpha to 0;  Thanks for the reply.  I'm trying to avoid using a pixel shader because nothing else in my game uses one, and I'm not sure I want to add the extra complexity to the code just for this one viewport.Is there any way to do it without using a pixel shader?   