Basic vops question

Dean_19 · Posted: Mon Oct 03, 2011 4:21 am GMT Post subject: Basic vops question

Hello,

Can someone point me the right way to normalizing a custom attribute in vops? Usually I do this is sops, by getting the oldmin, oldmax values using attrib promotes and then fitting the original attribute between these and then specifying 0 and 1 for the newmin, newmax values.

I want to do this in vops as its quicker (I've been told). I tried to do it by looping through each point in my geometry, pulling in the attribute value for each point and the ptnum + 1 and doing min and max on that and so forth, but I can't work out how to use the for loop vop properly! It seems sort of non-nonsensical to me.

A simple example would be great. Thanks!

arctor · Posted: Mon Oct 03, 2011 5:21 am GMT Post subject:

have you tried the normalize VOP?

Dean_19 · Posted: Mon Oct 03, 2011 5:37 am GMT Post subject:

Yes, this was the first thing I tried. But it didn't seem to output what I expected. Say for example I create a random $CR value on a sphere using a point sop with rand($PT) in the first component of 'add colour', then I multiply it by something random, say 56. This gives me $CR values ranging between around 55.9 and 0.1.

Taking that into vops and plugging it into the normalize vop then outputting it back to Cd simply gives me 1 for every $CR value.

I messed about with it a little by converting Cd into a 'proper' vector (as its a 3 component float) but still the same output.

eetu · Houdini Expert Joined: 15 May 2007 Posts: 316

I would think that getting the min and max with attrib promotes is the way to go.
You could then read that value in as a parameter in VOPs and do your per-point math there.

Doing a loop for finding min and max in VOPs is a bad idea. the VOP code is run separately for each point (in a simd/vector -friendly way), so you would end up running the loop for each and every point.

Dean_19 · Posted: Mon Oct 03, 2011 5:54 am GMT Post subject:

But surely attrib promote has to do some per-point attribute comparing of its own to determine the min and max values too? Although attrib promote does seem to be very quick whenever I use it, so it must do something pretty clever...

Regardless of the 'best' way though, how would this be done in vops using the for loop vop? Does a vops savvy person out there have any ideas?

I'm looking to kill two birds with one stone, getting an example of using the for-loop vop for my own knowledge and a vops method of normalizing attributes.

Thanks!

axebeak · Talkative Joined: 03 Oct 2006 Posts: 45

This thread has some examples of both approaches, attrPromote->fit to range and loop over points in VOP:
http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=23294

axebeak · Talkative Joined: 03 Oct 2006 Posts: 45

Dean_19 · Posted: Mon Oct 03, 2011 6:15 am GMT Post subject:

Ah I see. Thanks for the explanation.

Dean_19 · Posted: Mon Oct 03, 2011 6:16 am GMT Post subject:

...but this thread raises another question for me...

Why doesn't the normalize vop work for this?!

Erik_JE · Posted: Mon Oct 03, 2011 6:38 am GMT Post subject:

Dean_19 · Posted: Mon Oct 03, 2011 6:42 am GMT Post subject:

Aha, of course. Thanks.

jkim7979 · Lurker Joined: 27 Mar 2009 Posts: 10

Hello!
I have a simple vop for loop question. I am trying to use vop for loop to smooth a mesh multiple times by embedding for loop inside of another for loop, but it seems like final global point position value doesn't update. Can someone take a look at my file and see what I am missing? Thanks so much!