DirectX alpha blending (deferred rendering)

I'm having a major issue which has been bugging me for a while now.

My problem is my game uses a deferred rendering engine which makes it very difficult to do alpha blending.

The only way I can think of solving this issue is to render the scene (including depth map, normal map and diffuse map) without any objects which have alphas.

Then for each polygon which has a texture with an alpha component, disable the z buffer and render it out including normals, depth and colour, and wherever alpha is '0' don't output anything to the depth, normal and colour buffer. Perform lighting calculations/other deferred effects on these two separate textures then combine the colour buffers using the depth map to check for which pixel is visible.

This idea would be extremely costly (not to mention has some severe short comings) to do so obviously should only be reserved for as few cases as possible, which makes rendering forest areas out of the question. However if there is no better solution I have one question.

When doing alpha blending with directx is there a shader/device state I can set which makes it so that I can avoid writing to the depth/normal/colour buffer when I want to? The issue is the pixel shader h开发者_如何学Pythonas to output to all its render targets specified, so if its set to output to the 3 render targets it must do it, which will override the previous colour value for that texel in the texture.

If there is no blend state which allows me to do this it would mean I would have to copy the normal, texture and depth map to keep the scene and then render to a new texture, depth and normal map then combine the two textures based on the alpha and depth values.

I guess really all I want toknow is if there is a simple sure-fire and possibly cheap way to render alphas in a deferred renderer?

A usual approach to draw transparent geometry in deferred renderer is just draw them in a separate pass, but using the usual forward rendering, not deferred rendering.