View Full Version : Targeting Alpha/Opacity..

08 August 2007, 01:43 PM
Anyone know if its possible to use the alpha channel or non-transparent area of an objects material to target for `find-target` or `speed by surface` operators?

What i have is a plane with a video mapped onto it and i want particles to `swarm` just the part of the plane that is not transparent..


08 August 2007, 02:35 PM
Duplicate the object, make it non renderable. Then put a volume select on the model and set it to use texture as the selection type - put a copy of your oacity map into this and set the selection level to face / polygon - your texture map will select all of the faces touched by the opaque areas of the map. Then tick on the invert option in the volume select so it selects everything that isn't touched by the opaque bits and pop on a delete mesh modifier at the top - you'll have an object that only has the opaque bits left and you can use this as your pflow target.

To get enough accuracy you might need to put on a tesselate modifier on the copied object so that the volume select has plenty of small faces to be accurate to the map with.

08 August 2007, 08:44 PM
This is genious! Thanks! Only thing is, like i said, it`s a video! Would it work in that case and update every frame change?

I`ll try this tomorrow i guess and find out..

Thanks again,

08 August 2007, 12:33 PM
Yup - volume select will use animated images or texture maps - you can do some cool stuff with it.

08 August 2007, 05:12 PM
Thanks again that works amazingly! Shame and surprising there isnt a much less cpu intensive way within pflow to just speed by material opacity?!

I have another problem now. I`m using a spline with an edit-poly on it as the geometry for a `speed by surface` - the problem is i can`t get it to follow the surface closely enough. It works ok when the particle speed is low, but when i have them moving at a higher speed they don`t follow the surface well - even if i bump up the `Accel limit` on the continuous speed control..

Is there a way or is this just a limitation of pflow?

08 August 2007, 05:22 PM
You may need to extend PFlow capabilites by Box#1 (Lock/Bond for surface control) and Box#3 (fast speed by material opacity via Data Operators)

Oleg B.

08 August 2007, 07:19 PM
That`s a pain. PFlow should be able to do this no problem IMO.. its pretty basic..

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08 August 2007, 07:19 PM
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