Fading a 3D object into the background, using D3D9, SH3 & HLSL

Question

I have a simple program that renders a couple of 3D objects, using DirectX 3D 9 and HLSL. I'm just starting off with HLSL, I have no experience with 3D rendering.

I am able to change the texture & color of the models and fade between two textures without problems, however I was wondering what the best way to simply fade a 3D object (blend it with the background) would be. I would assume that it wouldn't be done as fading between two textures (using lerp), since I want the object faded to the entire background, so there would be many different textures behind it.

I'm using the LPD3DXEFFECT as my effect class, DrawIndexedPrimitive as the drawing function in each pass, and I only have a single pass. I'm also using Shader Model 3, as this is an older project.

The only way that I thought it possible would be to simply get the color of the pixel before you apply any changes, and then do calculations on it with the color of the texture of the model to attain a faded pixel. However, after looking over the internet, it does not appear that it's actually possible to get the color of a pixel before doing anything to it with HLSL.

Is it even possible to do something like this using HLSL? Am I missing something that could assist me here?

Any help is appreciated!

Answer 1

Forgive me if I'm misunderstanding, but it sounds like you're trying to simulate transparency instead of using built-in transparency.

If you're trying to get the color of the pixels behind the object and want to avoid using transparency, I'd start by trying to use the last rendered frame as a texture, then reference that texture in your current shader. There may be some way to do it within the same frame - to force all other rendering to go first, then handle the one object - but I don't know it.