All images from The Magic of Houdini by Will Cunningham. Reprinted with permission.

This is the next in a series of excerpts from the Thomson Course Technology book The Magic of Houdini by Will Cunningham. In the next few months VFXWorld readers will learn the basics of the dominant tool that has been used in the creation of some of the most awe-inspiring animation and cinematic effects ever made.

Brush operations in Houdini are a great way to interactively affect geometry. You can sculpt as a sculptor does. You can paint as a painter does. You can comb as a hairstylist does. Welcome to marvelous country. You’ve come a long way, baby! You’ll be using a mouse in the following exercise. But, if you have Wacom tablet, plug it in and take it for a spin. You can get a very fine degree of control with a tablet rig by modulating the application pressure.

Sculpt It! Comb It! Paint It!
Yes, work with me here darling! You are going to work the magic, as they say. You are going to take a bland and boring flat grid and through the miracle of cosmetic brush applications, turn it into a maaaaarvelous looking bit of scenery. Oh yes, darling. Do come along. You simply must see this! Figure 1 shows the end result of the "from flat to fabulous" miracle makeover.

[Figure 1] I just want to jump around with my shoes off and let it tickle my toes!

1. Fire up a new Houdini session. Drop a Geometry and rename it terrain. Jump into it and delete the file node if it exists. Drop a grid operation. Change the Primitive Type to NURBs and the Rows to 30 and the Columns to 30.

2. Over the viewport, choose a Sculpt operation and choose the entire grid to apply it to. Move your pointer over the grid and you’ll see the dotted outline of a circle. That is the size of your brush. Hold down the LMB and move around the grid. It looks kind of like in the cartoons when a certain rabbit is digging his way to the destination. Release the LMB. Hold it down again and continue sculpting. If you keep clicking in one spot, you can pull the geometry up to towering heights.

3. Now, hold down the MMB and do the same thing. The MMB will push geometry back down. You can change the amount of displacement per click by adjust the FD value for the LMB and the BD value for the MMB. When you to start from a clean slate again, click the Reset All Changes button and you get to start from scratch.

[Figure 2] The result of the sculpt operation.

4. Adjust the size of the Radius up and down to change the size of your brush. Go to the Brush tab and change Shape to Square. Reset the Changes and play around with that. The brush is now more angular. Even cooler, change the Shape to Bitmap and the Radius to 0.5. Reset the changes and just click once in the center of the grid. Your painting with a butterfly brush now! You can choose which channel of the bitmap you want to paint with. By default, it is set to Alpha. Feel free to take a few minutes to play with the various parameters to get a feel for what each does. When you are finished, change back to the Circle brush and change the Radius to 0.1. Reset the changes and sculpt something like what is shown in Figure 2. It by no means needs to be exacting. Basically, just sculpt some mountains and leave a flat valley area up front.

5. Append a Convert SOP change and change the Level of Detail in U and V to 1.4. This converts the NURBs grid to a polygonal grid and increases the smoothness a little.

[Figure 3] The grid with point normals displayed.

6. Append a Point SOP. The Point SOP is one the most powerful SOPs available because it allows you to modify attributes on a per point basis. As you continue through the book, you’ll often come back to this operation because it is so useful for a wide variety of things. For now, you are going to color every point on the grid black and give each point a Point Normal which points up in the Y axis. Change Keep Color to Add Color and delete the channels by RMB on the Color label and choosing Delete Channels. Change the triplet (r, g, b) to 0, 0, 0, which turns the grid to black. Go into wireframe shading to make it easier to see. Change Keep Normal to Add Normal and delete the channels. Set this triplet to 0, 1, 0. This gives every point on the grid a point normal pointing up in the Y direction. To verify this, turn on the display of point normals in the right stowbar of the Viewer pane. They may turn out to not be large enough or entirely too large to be very helpful. Over the viewport, press d to bring up the Display Options and go to the Miscellaneous tab. Adjust the Scale Normals slider to around 0.15. MMB on the node to verify that you have added two point attributes being color (Cd) and normal (N), each of which are comprised of three components. Figure 3 shows the grid with point normals displayed and scaled down a bit.