Direct3D: Output 4 color channels and a separate alpha?

Question

Something I'm wondering about, that's probably not possible unfortunately, but thought I'd ask - I'm setting up a deferred lighting shader that takes in 2 textures and outputs to 2 render targets. Their channels are:

Input Tex 1: Color Texture
  Channel 1: R
  Channel 2: G
  Channel 3: B
  Channel 4: A

Input Tex 2: Normal + Specular Texture
  Channel 1: X
  Channel 2: Y
  Channel 3: Z
  Channel 4: Specular Amount


Output Target 1: Color Target
  Channel 1: R
  Channel 2: G
  Channel 3: B
  Channel 4: A

Output Target 2: Normal+Specular Target
  Channel 1: X
  Channel 2: Y
  Channel 3: Z
  Channel 4: Specular Amount

Pretty simple. Except - when outputting to the Normal+Specular target, I want to mask the pixels that are written using the Color texture's alpha channel to control the amount of blending. But - I'm outputting a vector4 as my second color from the shader, whose channels are all used up.

So is there any way of having Direct3D use an alpha value temporarily as a blend parameter, use it to "mask" my 4 norm+spec channels, then discard it after use? Or to have render target 2 use render target 1's alpha as its blend parameter?

I know I can represent a normal vector using only 2 components - e.g. store X & Y then recalculate Z on the fly - but the process would be simpler & probably perform better if I could get away with leaving them unchanged - so hoping to find a solution that way.

Thanks!

Answer 1

So, to follow up on this one - I ended up just encoding the XYZ normals into two components using a Spheremap transform technique (#3 mentioned on the page http://aras-p.info/texts/CompactNormalStorage.html#method03spherical ).

So I'm writing the same alpha value to both targets to control blending - couldn't find any other way.

Answer 2

Interesting problem! Just thinking aloud, I wonder if you could write the mask value to the stencil channel of the depth buffer and make use of that when masking?