Texture Blending

The key word here is "PASSES", *multiple* pass rendering means to do something like:

Set base texture
DrawPrimitive() // draw the object
Set light map texture
Set lightmap blending
DrawPrimitive() // draw the object AGAIN

i.e. drawing of the SAME object takes place multiple times. You can do as many passes as you want. The only consequence is the time it takes to re-render each pass.

D3DRS_SRCBLEND, D3DRS_DESTBLEND and D3DRS_BLENDOP renderstates are the main things you use for multiple pass rendering.



What I think you''re thinking of is *single pass multitexture*, where all of the blending is done in one pass using multiple texture stages. This is also possible in D3D, but how many textures can be used simultaneously depends on your hardware.

The texture stage states [SetTextureStageState()] are what you use to control single pass multiple texturing.


You can also combine the two.

--
Simon O''''Connor
Creative Asylum Ltd
www.creative-asylum.com

I'', sorry but I can''t find the the render states you are talking about in my directx sdk documentation (using dx7 sdk). Is this a functionality which was introduced later?

They had different names in DX 7...

D3DRS_ used to be D3DRENDERSTATE_


The SRCBLEND and DESTBLEND render states allow the following type of operation with the frame buffer:

RESULTrgb = (SRCBLEND * SRCrgb) + (DESTBLEND * DESTrgb)

The DESTrgb is the RGB value of the pixel which is already in the frame buffer (i.e. the result of the previous pass).

The SRCrgb is the RGB value of the pixel from the draw call (e.g. DrawPrimitive).

The SRCBLEND and DESTBLEND states allow different operations. Some examples of the kind of flexibility of operations you can do are:

a)
SRCBLEND = D3DBLEND_ONE
DESTBLEND = D3DBLEND_ONE

The operation performed is:
RESULTrgb = (1 * SRCrgb) + (1 * DESTrgb)


b)
SRCBLEND = D3DBLEND_SRCALPHA
DESTBLEND = D3DBLEND_INVSRCALPHA

The operation performed is:
RESULTrgb = (SRCa * SRCrgb) + ((1-SRCa) * DESTrbg)


c)
SRCBLEND = D3DBLEND_DESTCOLOR
DESTBLEND = D3DBLEND_ZERO

The operation performed is:
RESULTrgb = (DESTrgb * SRCrgb) + (0 * DESTrgb)


D3DRS_BLENDOP is a new state which was introduced in DX8 which extends it to:

RESULTrgb = (SRCBLEND * SRCrgb) BLENDOP (DESTBLEND * DESTrgb)

The new OPs let you replace the + with things like -, min/max operations, reverse subtracts etc.


The SetTextureStageState() values are all present in DX7. (Although a few new ones have been added in DX8).




--
Simon O''''Connor
Creative Asylum Ltd
www.creative-asylum.com

About that new state, D3DRS_BLENDOP. Is there a way with Direct3D 7 to do the same as D3DBLENDOP_REVSUBTRACT? I am trying to do subtractive blending of a textured poly over the frame buffer.