BURNING CG GRASS
One of the shots in Litter Kills involves a cigarette, which is thrown out of a moving car and into a patch of dead  grass, which ignites and engulfs the screen in fames. This part of the website is dedicated to breaking down how this shot was made into its most important elements. As you read, you will start with observation and analysis of reference footage of flames and begin to think about the possibilities that Houdini's 3d space holds. It is possible to simulate almost any physical system in Houdini because of the open ended nature of the program. For example, Maya's metaballs (known as blobby surfaces) are not editable by the user; they exist only while Maya is rendering out your particles. In Houdini, you can instance these metaballs to a particle system and then deform them based on their age or any other attribute. This is how the flames were made for Litter Kills. Their motion and growth is the other part of the story and is certainly more important. The particles actually turn geometry, that they come in contact with, into particle emitters, which in turn makes particles that do the same thing. In addition, the flames appear to deform the grass blades, like those in the reference footage. This interaction should be the goal.
Houdini Particle system with instanced metaballs and a custom fire shader.  The flame gathers points along the grass blades and turns them into new flame emitters. The flame also deforms the grass geometry and the shader where it makes contact.
first: concept development / research   second: creating the effect
reference footage

notes / analysis

 
motion            [screenshot] meta instancing    [screenshot] the shader
A sine wave in the X and Z components of the initial velocity of the particle system: sin($F)  usually does the trick. Make sure you have a value like 4 in the Y component so that the flame will rise. You can get more added realism by adding an interact POP  to the network and setting the strength in X and Z to -.1 and 0 in Y. This will pull the particles toward each other and generate more fluid-like motion. Bring the POP network into the SOP level by making a POP merge node and pointing it at the particle system. Create a Metaball node and then pipe the Metaball into the left input of a copy SOP and pipe the POP merge node into the right input of the same copy SOP node. Next, pipe the copy into a convert meta POP and pipe the convert meta POP into a peak POP and adjust the peak value until the deformation of the Metaballs looks right.

I used a modified X-ray shader for the flames. There is an excellent tutorial online at www.techimage.co.uk

spreading effect geometry deformation