Thursday, January 16, 2014

Breaking Ego out of his prison

Hello. So I did this shot last year in June and I wanted to do a little write up about how I went about solving the shot.

In this shot we had Ego, a dragon, breaking out of it's prison. The main problem I faced with the shot was that I got the final animation of the dragon around a week before deadline and I just didn't have the time to refine the dust and debris and render them out. So, the output here has been achieved in a little over 3 days. In the end, we were not even able to render out the dust passes. Anyway, this is how I went about solving it.

The models were done by Ross Hildick. You can go and check out his blog here:

This is the final result I was able to produce:

The final result

When I was handed the shot, the first thing I analysed was the structure of the building. After having a discussion with the director, we came to a conclusion that this building was an old castle sort of structure and didn't have any iron reinforcements inside. So the only material used was stone. This meant that I didn't need to worry about the bending and deformation of metal rods.

I looked at the animation of the dragon and I judged the force with which he was coming out. I then divided the upper part of the tower into different parts according to the amount of impact they would be receiving from the dragon's body. This can be seen in the image below:

Geometry broken down into collision groups

Here we can clearly see the parts I broke the geometry into. The red one is the main impact group which takes the direct hit from the dragon. These rocks would be completely broken. Next would be the secondary main impact group. These bricks received impact as well, but not direct impact. So, the fractured, but into bigger chunks. Moving further out is the secondary impact group. These rocks didn't receive direct impact but were unsettled by the impact to the neighboring rocks. So, they just fall off. Beyond this I made several other groups which would stay glued together but will form tumbling geometry.

I used the RBD solver for this because I wasn't really happy with the result the Bullet engine was giving me. After breaking these areas down, I imported the geometry into Houdini as separate groups. I then started working from inwards to outside. I fractured the main impact group first. After I was happy with the result. I wrote it out through a ROP output driver. In the next file when doing the next group I imported this first geometry as collision geometry and went ahead with solving the shot, working outwards constantly.

In the picture below you can see all the major groups:

Various collision groups

After this, ideally, I would have done a dust and debris solution, but due to the lack of time, I just added a simple smoke solution on top of this. After 3 passes of smoke was added and all the layers were brought together, this is how the final output looked like.

Smoke passes and final output

Below are some screenshots of the scene graphs:

The overall scene graph

The DOP network

SOP-network for one of the groups. This shows how I handled fracturing

So that was about it. It was a really simple fracture. I wouldn't say I am proud of it, but it is something. Feel free to message me if you want any clarification on this.

Happy fracturing!