Cauldron Ambiance and Explosion

List of Effects

Part of my visual effects list was cauldron smoke. We have a large and a small cauldron in the scene. Initially I wanted the smoke to interact with the witch; using her hand as an active collider against the smoke. I did not have enough time to explore this possibility. Adding her and masking in the render was not the issue. It was the behaviour of the smoke that changed with it being interacted with, which needed constant tweaking. I had to prioritise the best rendering settings along with optimal light settings. I had already completed the fire underneath the large cauldron (smoke coming from ended up not being used, as it appeared too busy). The list of all effects are:

• Small cauldron smoke (until Bunsen turns off)
• Small cauldron explosion
• Large cauldron constant smoke
• Fire underneath the large cauldron
• Bunsen burner 
• Transformation smoke

These were the most important effects I needed to complete. Anything extra could be done in After Effects.

Large Cauldron

Since the fire is constant, the large cauldron must emit steam constantly through the film. I took a list of all the shots that have the cauldron on screen, which ended up being 11. The first mistake I made was that I did not make the smoke to scale. Maya’s fluids do not like to be resized. I learned it the hard way, where I spent a long time coming up with beautiful effects for them to be unusable in the end. Even if you start all over again and copy the same settings with correct sizing, the result is still different due to density and heat attributes. 

This was a playblast of the first cauldron smoke I attempted. Alec informed me that fluids must be cached out as their true result will not be visible in a simple playblast. It took me a total of seven attempts at cauldron effects to get the desired effect. To get the exact sizing and location of the cauldrons, I exported both the small and large one directly from the scene. 

I followed  a tutorial by Osmel Carrizo on YouTube to kickstart my smoke effects. The tutorial is in Spanish, but the steps were clear enough to follow. I did not find any tutorials online to create smoke results I was going for so I simply used this tutorial as a secure base and then played with values to get a decent effect.

Above is an example of the work-flow of my work. It is a tedious process, especially when you cache out once and then you’re unable to see changes in the smoke when you change values, unless you re-cache and blend the results together. I also found that rendering the smoke out also gave a different result, but you cannot render out an example unless you cache your fluids. This is the biggest reason for me restarting seven times. I would often forget which value I changed before caching and then it would often completely throw off my progress and I would have to start again. It was frustrating to work on something for an hour, just to start over again due to one value being off and me not being able to test before caching. I did end up working without caching as long as possible, before the computer could no longer handle it. 

I cached about 700 frames for both cauldrons. This gave it enough time to “bubble” and I could manually change the frames it began and ended. This gave me more control.

I imported the cameras into the cauldron file and rendered the exact frames our main film had. If the scene involved a character going over the smoke, I had to import that file too and add a “matte” overrride in Arnold settings. This acted as an alpha map.

Above is what the cat render looks like if it did not have a matte override on. Also an example of the cauldron with/without the matte override. It was important to select every single piece of geometry and add tick matte. If I even missed one tooth or a whisker it would get picked up in the render. This also took me a long time to set up due to our characters having so many separate pieces of geometry. I also had to select everything visible in the scene (walls, bookcases, books, floors etc.) to hide. Especially if there was a camera change and the objects were directly in front of the smoke. 

My render settings were all set to 1 in Arnold apart from camera which was at 4. There was one Ai light in the scene so one frame took around a minute or less to render. The biggest issue I ran into was camera’s being tweaked in the original render but not saved. Two scenes do not match perfectly which is a shame. I cannot redo them as the fixed camera is not saved sadly.

This will be a learning experience to save every little thing you change, even the most minor like camera focal length.

Despite the two scenes not matching, everything else turned out lovely! The large cauldron was the most time consuming to prepare (masking every piece of geometry in scene). Small cauldron was quicker to complete as it was only active in 4 scenes.

Following the same tutorial, I set up the small cauldron. I made it act as a collider to ensure the smoke does not fall out of the cauldron. I was never able to fix the clipping issue but it ended up not being obvious when comped in anyway. Above is me playing with the settings to achieve an “explosive” effect when the witch’s experiment goes wrong in the first scene. 

Here you can see my constant smoke render and the explosive separate one in the same scene. The explosion is yellowy-green to symbolise that something went wrong. I timed it so it explodes exactly at the time the witch retracts back. I think it turned out nicely! Michelle colour-corrected all of my smoke/explosion effects during the compositing stage. The biggest lesson I have learned doing these effects is that you must work to scale, save the files correctly in-case you need to get back to them and do not cache anything out until you’re satisfied with the result.