Page 48 - 3D World - October 2017

TuTorials
Grow procedural plants in Houdini

use wrangle
nodes
Wrangle nodes are
the Swiss Army Knife
of Houdini. Adding
attributes, sampling
values, setting colours,
removing points, can
all be done in a single
line of VEX.

finAl foRce soften the chAnge
11Add the last force to the VEX loop by sampling 12 The vines still don’t look natural. This is because
the value and gradient VDB of the ruin. This gives you they can change direction rapidly on each step. Soft
the distance between point and wall as well as the wall’s blend between the directions of the previous and the
normal direction. Calculate a force that points towards current step to smooth that out. You also want to stop
the wall and add it to the grow direction. Sample the VDB growing if the position moves too far off the walls. See if
a second time in the new position to check if the point is the wall distance you get from the VDB sample exceeds a
inside the wall. If so, just push it out along the normal. certain threshold. Just break out of the for loop if it does.

bRAnching
13 behAviouR
Another aspect of climbing
plants we still have to add
– branching. But this is very
straightforward to achieve. Tag
each growth step with a certain
probability to be a branch
point. Then run the growth
algorithm again on each of
those marked points, using the
point itself as the root.
It’s a good idea to tweak
the growth parameters on
the second pass in order to
be more wandering and less
upwards, to fill up the areas
along the walls.

finish the bRAnches instAnce the leAves
14 We now have created sequences of points resembling branch growth. 15 Instancing leaves works a bit like the grass strands
Make use of the id attribute on the point and create an Add node to turn them earlier. We define a distribution attribute along the curves
into individual curves. coming out of the growth algorithm. This is zero at the
A UV Texture set to Rows&Columns will provide uvs along the curve length, start of each branch and then 1 at the tip. Use a scatter
which you then ramp along their individual length to an attribute called pscale. to place points along the curve based on that attribute.
A Polywire node turns curves into tubes where you use $PSCALE for thickness. It looks better to vary the pscale. Use a copy stamp to
Add a Mountain and a Facet after, to finalise the branches. instance the prepared leaves onto those points.

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