Texture stages and backbuffer

Sure you can:

Pass #1
P = T1 * T2
ONE:ZERO blendmode

Pass #2
P = T3
ZERO:SRCCOLOR blendmode (or DESTCOLOR:ZERO)


assuming the whole operation is supported in hardware (isn''t on some really old chips)

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

So there''s nothing that can be done single-pass that can''t be done identically in multiple passes? Apart from DOT3 maybe?

On modern chips which support all the nice frame buffer blending operations, you can do most things multipass.

However things which need a "sign" bias (e.g. DOT3), or have a dependency between multiple textures (e.g. EMBM) etc aren''t so easy to work out with multipass - though if the back buffer format supports destination alpha you can use that to great effect too.

Stuff with the Z buffer and things like stencil buffers can end up a bit of a git though which is why I''d personally never aim to make a system which can emulate any multitexture setup with multipass. I''d prefer to just pre-can a few versions (a la effects & techniques), say crap chip, 2 texture unit chip, 3+ texture unit chip, shader chip.


At the end of the day it''s all a case of reorganising the maths and deciding which bit of the maths happens where. For example your maths is P = T1 * T2 * T3

All I did is reorganised that to fit the blendops available:
P = 0 * T3 + T3 * (T1*T2)

or the alternative:

P = (T1*T2) * T3 + 0 * (T1*T2)


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