converting normals to tangent space

[susiezgreen]

Hi,

I am using Houdini to create a flow map, which is basically a normal map. I am using the normal direction to colour the vertices (there is more information on this here: http://www.valvesoftware.com/publications/2010/siggraph2010_vlachos_waterflow.pdf )

Because I am using an irregular object I need the map to be in tangent space rather than worldspace- again like a normal map.

This is where I am having problems. There doesn't seem to be an easy way to make the conversion in houdini. I have achieved the best results by using polyframe to get tangent, normal and binormal for my original object, then a vopsop with an orient node, using the x,y and z values of the tangent, binormal and normal to orient the normals. This works if I type in the values by hand (for a flat plane) but if I try to use an expression get them on a per point basis:

along the lines of point("../../point2", $PT, "tangentv", 0)

I get zero values for everything

Can anyone come up with a solution for this- or perhaps a better way of doing it?

thanks

[Macha]

Where are you using the expression? It may not work in every node.

[susiezgreen]

Where are you using the expression? It may not work in every node.

I wanted to use it in the transform matrix for an orient node within the vopsop, but this evidently does not work. If I use a fixed value for the point number

ie: point("../../point2", 1, "tangentu", 0)

then I (sometimes) get an error saying "cannot have channels which depend on time" (which they dont)

if I "delete channels" from the orient node then it replaces them with the correct values, and everything works- but only for a flat plane where the tangents, normals and binormals are the same across all points.

If I use the $PT variable it just doesn't work (no error)

I have attached an image showing the network.

I think the best solution would be to write a script to do this for me, but I am not sure where to start- there seem to be a lot of different options for doing this.

Edited January 5, 2012 by susiezgreen

[3dbeing]

if you use import attribute, you can grab all of your attributes there, vops works per point natively. you can then convert your vectors to floats and your floats to a matrix.

[susiezgreen]

if you use import attribute, you can grab all of your attributes there, vops works per point natively. you can then convert your vectors to floats and your floats to a matrix.

That works! Great, thank you very much. I had tried something similar but I was using 'get attribute', not 'import attribute' so it didn't work

[dynamoanders]

Hi Susie.

I am just curious on how you exported the Flow Maps?

I am doing something similar.

Thanks,

Anders