# Bifröst

Ignoring the music that accompanies the hour long video, it is actually a brilliant source of reference on how the water should react to similar sized birds in the water, on a relatively calm surface. It also serves as reference for the animation of the crane and how the water will react when the crane pokes its beak into the water.

Here is the are some progress videos of the water simulation I have been running. Bifröst is very particular about scale. All of its calculations are computer in a real world scale where 1 unit is equal to 1 metre. Now, Maya’s default is 1 unit is equal 1 cm and even if you change this Bifrost works independently of Maya’s units. Because of this, interesting physics and interactions can occur which can be seen in image 1 where there isn’t enough interaction with the water and in image 2 where there is too much interaction. The final image is where I have the scale set properly. Because it’s built at cm scale we have to adjust the particle density and gravity so that the physics are relative to its scale.

• 1 Voxel = 1 Maya Unit
• Voxel = m
• Gravity = 9.8m/s²
• Liquid Density = 1000kg/m³

Convert to cm:

• Gravity at a real world scale is 9.8m/s² and as I am working in a cm scale in maya need to I need to adjust me gravity and density accordingly. So there are 100cm in 1m so: 9.8*100=980m/s²
• The liquid density is set at default to 1000kg/m³ which is approximately the density of water at room temperature. Because I am scaling down and it is being cubed, I need cube root or  1000  1000/1000/1000=0.001

With these adjustments the water now reacts accurately to any motion or force.

The vorticity attribute calculates the accumulation of rotation magnitude (curl of velocity field) within voxels. The vorticity channel is not physically accurate, but can be used to simulate churning when viewing and rendering. I have disabled this as it was giving me some annoying effects of the water churning for too long, and it is not really noticeable when the water is quite still, it would just cause a longer sim time.

Bifrost Liquid Material

Here are my liquid material tests, I’ve been adjusting the Droplet reveal factor, surface radius and smoothing, within the bifrost meshing tab of the liquidShape node. By reducing the droplet radius and increasing the smoothing we get a much smoother surface, see the last image. with higer values the water is rendered in more detail and picks up more ripples. This isn’t exactly desirable for our water, as we want it to have strong reflection to match the still reflective water in the park.

By enabling the Bifrost Meshing attribute, Bifrost converts the voxels into a dynamic mesh, this greatly improves the render quality and also the render time.

• Droplet reveal: Creates and preserves the detailing of the mesh
• Droplet radius: Increases the size of the droplets when the separate from the mesh
• Kernel Factor: Adjusts the width of the surface smoothing kernel
• Smoothing smooths the mesh
• Resolution factor: increases the resolution but can cause long delays if set about 2-3

One problem I did encounter was that we wanted our water to look quite still and reflective to imitate the water of the pond. To achieve this effect I needed to have  relatively low settings, in the ‘surface radius’ and ‘droplet radius’; and the high settings on ‘kernal radius’, this really smoothed out the water surface, but at the cost of causing the water to become quite globular looking. This had a knock on effect on the ripples on the water not being as prominent as I had hoped. However they are still noticeable and having the surface match the live plate was much more important. You can see in the images above, where on the left image it has high droplet and surface radius’ and the right has low settings.

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Here is a list of content I have found to help me troubleshoot any  of the problems i encountered and also what has helped me learn and understand Bifrost’s interface.

The first link is from the autodesk website; and although it does contain information on the whole interface, it does lack any description on how to use attribute.

http://help.autodesk.com/view/MAYAUL/2015/ENU/?guid=GUID-E4D7C5E6-62C4-4DF2-AA3F-5EE0CE11BB90

This video helped to understand the basic concepts of how to set up the scene correctly. It also shows how to set up a guided simulation, whereby the water is driven by some geometry, I, however decided not to use a guided sim because it gave the surface too much motion and wasn’t reflective enough.