Subject: Additive on Macs 2004-02-27 - By NoiseCrime
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Hi Mal,
First to those who are interested, the artcile and source to the demo Mal
posted can be found here.
http://www.noisecrime.com/develop/tutorials/additiveblend/
To recap
To date no PC running in DX5 has failed to produce additive blending using
maxTextureUnits+1 to place the additive texture into.
On a PC, any card that supports 8 texture units will not be able to produce
additive blending in DX7
On a Mac additive blending is supported in openGL as long as the card has
less than 8 texture units. However the gethardware() command is unreliable
and does not return the correct maxTextureUnits.
When getting the image of the sw3d member on a mac, you always get back a
software openGL version (not software), which invalidates automatic
checking.
This means on a Mac, there is no automatic way of determing if it supports
additive blending, you have to ask the user, and basically cycle through
which texture unit to place the additive texture into.
Most Mac cards, if not all will support aditive belnding, its just the
developer has no gauranteed method of knowing which layer to place the
additive texture. One idea was to use the cards name as a look up into a
database where you would store the known maxTextureUnits. Sadly though
naming appears in consistant and so is not gauranteed to work.
Basically then Macs are a bit of a bugger for additive blending. It will
work, but you need to invent some method of getting the user to identify
which layer provides the belnding affect. My outlook on this was that
additive benldning would be off by default on macs, and the user had to turn
it on through a preference screen. This screen would display a pre-generated
additive blend of some scene or object, and then the w3d trying to implement
additive blending. They have a simple 'yes' 'no' decision to make if the
images match or not. If they don't match move onto the next texturelayer, if
they do, then you can stop because you know which texture layer to use.
Two tips
Firstly don't forget the user may have overriden the renderer API in the SW
preferences
Secondly that the num of texture units is API dependant. So if running in
DX5 you will always get 1 for the maxtextureunits.
I've got another 50-75 results that I'll try and add to the tutorial results
over the weekend.
Hope this helps
Noisecrime 2004
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