shading: vector pointing in increasing u/v direction

[rendermaniac]

If you want to find out the curvature, you can get the second derivative of u or v (or more likely s or t). There is information on how to do this here:

http://studentpages.scad.edu/~mparro21/curvature/curvature.htm
http://www.affine.com/curvature.html

This works well on NURBs surfaces, but gets a little more tricky on subdivs and polygons.

Simon

[Gravedigger]

hei rendermaniac

thanks a lot for your answer. i'm not looking for the curvature

i'd like to have a vector pointing in the flow direction. imagine a guy sitting on his boat on a river. the vector i'm looking for is the velocityvector of him

as i said i have setup the uv-layout like descriped above and i hope that it's possible to get this vector defined at every position of the river

grs
Gravedigger

[rendermaniac]

Yeah - just noticed that.

You may be able to use dPdv*dv - dPdv gives you the direction and dv gives you the scale.

Again NURBs will work best.

Simon

[Gravedigger]

hei simon

thanks a lot for your anwer. i'll try that this afternoon and report my results

grs
Gravedigger

[noizFACTORY]

So if the river is flowing down the v direction, you would want to stretch your textures in the same direction?

-Sachin

[Gravedigger]

hei Sachin

thanks for your anwer. this is what sounds pretty plausible but u and v are not vectors but scalars. this is why simon suggests to take the derivative which points to the greatest change which in this case is exactly the direction i was looking for

[noizFACTORY]

hei Sachin

thanks for your anwer. this is what sounds pretty plausible but u and v are not vectors but scalars. this is why simon suggests to take the derivative which points to the greatest change which in this case is exactly the direction i was looking for

Yes, I was merely confirming if thats what you wanted to do. dPdu or dPdv is what you'll probably need to use to do this. Interesting shader though. I'd like to have a go at it too.

-Sachin

[Gravedigger]

yeah i was playing around because i need a simple river in a project. stretching the fractal really adds a lot of detail

what i did is that the sides of the river are shifted slowly while the middle part of the river is shifted fast. this results in a nice dynamic V-shape. after a few secs the river starts to be stretched too much. this is where i use modulus to blend in a less stretched version. like this it looks pretty nice

if you guys have some other ideas please share :D

grs
Gravedigger

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