VIOLIN RBD SHATTER

This project centers on the timescale and control of an RBD simulation of destroying a violin via a golf ball. This tutorial will guide you through preparing and setting up smaller, unique objects for an RBD simulation, while controlling their destruction in slow motion. Follow along with the HIP File for better understanding.

Step 1 - Asset Preparation

To start off any RBD simulation we need to make sure that the model is prepared for destruction. This means closing up any holes as well as fixing any geometry that could cause issues for fracturing. In our case this means ensuring that as much of the model is 3D pieces, ensuring no “disconnected” parts, and recreating missing geometry behind the neck of the violin.

The model has a few issues that have to be fixed. Start off with cleaning up a gap under the neck where it connects to the case which has to be filled so that there are no holes. The side panel is only 2D and needs to be thickened, as well as some smaller fixes such as filling in cylinder-like objects and blasting off some interior floating pieces.

We then just need to group off some parts for easier selection/naming for when we fracture and render. We can then create an extra placeholder name attribute called pname which will hold the original names of the pieces as it will help when naming things for Solaris. There are some additional changes made after due to certain edits for grouping and geometry that need to be made later down the road, but they are minor. For the additional assets create a box with some bevels as a stand, a simple curved backdrop, and a goofball although a plain sphere is also just fine, and we are good to go. Once it is all modelled and placed upon the stand we can move on to fracturing the geometry.

Step 2 - Fracture the Violin

Now that the model is cleaned up its time to begin fracturing the violin to prepare it for the RBD simulation. It is good practice to do the fractures and constraint set up on objects at rest and then simply copy the transformations or animations back onto the object later. Even though our scene is relatively simple, set the violin back to rest and we can start fracturing. 

Fracturing an object like a violin is a bit more complicated than say a rock as with wood you have to take into account grain structures and in our particular case referring back to the reference to see the general shape of the pieces we are looking to create. Using the groups and names from before we can fracture the main violin case needed for the RBD simulation as well as ensure that the parts which are not breaking; in this case the neck, tail and small parts at the bottom are properly prepared for the RBD sim.

RBD Material Fracture on bad geo vs On fixed geo

Starting off, the wood case of the violin will be fractured using a RBD Material Fracture however even if we cleaned up the model sometimes some issues may arise. In this case the RBD Material Fracture node may struggle to properly create fractured geometry, usually due to non-airtight geometry. In order to fix that we will convert the pieces to VDB and back but ensure that we transfer back attributes and especially the UVs as we still want to use the original textures for the violin. This will ensure that the piece has 0 holes and is airtight so the RBD Material Fracture can easily give us good looking fractures. Aim to keep the fractures larger as it makes controlling the simulation easier and the splinters/shards can be done as secondaries. The bridge and soundpost being so small can also cause issues so we will simply enlarge them, fracture them and then scale them back down.

Broken Uvs vs Fixed at top

Once all of the wood pieces have been fractured, we can work on the rest of the pieces which will not be fracturing but still part of the simulation. We simply need to have them named correctly for the RBD Bullet Solver and that means giving each piece a name attribute unique to itself which can be done a variety of ways, in this case we can just turn off the cutters in the RBDMaterialFracture and turn on the Fracture per Piece option. Reassemble the violin and we are ready to set up constraints for the RBD simulation. 

Step 3 - Constraint it all together

Next step is to set up constraints between these pieces. We will be using SOP level tools to set up the connections and then tweak the constraints in order to control where and what pieces are breaking.

Begin by constraining the violins main wooden case, this will be the most complex constraint system but by using the RBD Constraints from Rules and RBD Constraint Properties nodes it will do a lot of the work for us allowing us to focus on figuring out the values for the parameters. Create the constraints for each of the 3 pieces, the front, the sides and the back and then combine them together with a different set of constraints for the pieces making up the case vs the 3 main sections of the violin. Make sure to keep the constraint tags separate so that we can have more control over the pieces themselves versus the connections between them. 

The constraint type should be Glue as it will hold pieces together without letting them move until broken however for anything breaking ensure that Constraint Switching is on and set to Soft as this will allow us to switch pieces that have to be broken to Soft constraints right before impact. I found trying to allow the solver to calculate breaks based on Glue strengths resulted in strange results such as the ball shifting suddenly or coming to a complete stop against the strength of the wood. Soft constraints allow us to skip that and focus on the final feel of the wood shattering. 

Constraints set up exploded view

Ensure that the Override Angular Stiffness and Damping Ratio is on as this will give us better control over the strength and oscillations of the wood that is being bent away due to the impact. It should be weaker than the general Stiffness and Damping Ratio as wood is bendable, and we hope to imitate those aspects.

Step 4 - Shatter

The last step of the simulation is to just set up the RBD Configure and RBD Bullet Solver node as we need to make some tweaks to get the look we are aiming for.

Starting off with the RBD Configure we will set the density of the wood to a large number to add some weight to them as well as making the button at the bottom inactive until right after impact to prevent the violin from flying off the stand. Then just move the violin into position by copying the transform from the asset to a rbdxform node. Finally, take the golf ball and give it a good amount of density and velocity but remember the shot is in slow motion and golf balls are extremely fast off the tee so it will have to be quite fast. We are keeping the density of the violin and ball unrealistically high because of the nature of the shot and details we want to capture. 

Then just move the violin into position by copying the transform from the asset to a rbdxform node, adjust the golf ball and give it a good amount of density and velocity towards the violin. Remember the shot is in slow motion and golf balls are extremely fast off the tee so it will have to be quite fast. We are keeping the density of the violin and ball unrealistically high because of the nature of the shot and details we want to capture.

Finally, onto the RBD Bullet Solver to really set up the controls and look we are going for. Start with the Time Scale, since this is a slow-motion shot set the time scale to 0.1 as that is around where the reference seems to be at, it's hard to tell without exact numbers but it seems about right. The rest of the numbers can take some trial and error to get right however, I found setting the Bullet World Scale to 1000 gave stable results as well as 400 Bullet Substeps and 100 Constraint Iterations. As for drag, set it 0 initially until impact since the golf ball is an active object it'll make it easier to judge how fast it is going at impact and not require us to spend too much time adjusting its values

Onto the Constraints tab where a lot of the setup from earlier comes into use. For the Soft constraints we can set the Break Thresholds to 0.45 for the Angle Threshold and 0.01 for the Distance Threshold as we want to allow pieces to hinge at the point of connections but once separated break. Finally, for the Impact group set a tab to Switch to Next Constraint and set the frame to switch to 24. This ensures that a frame before impact the constraints at the impact location are no longer Glue but now operating as Soft constraints. 

Break Thresholds for the violin

Step 5 - Adding Details and Solaris Prep

Now that we have the RBD simulation completed, we will add some smaller details that are not simulated to further add some weight and feel to the impact before moving on to secondaries.

Start off by unpacking the simulation and setting the names for Solaris as we will be using Karma XPU for the render. Reset the name attribute using the pname attribute that we set at the start will make it very easy as the RBD sim has changed the naming of the pieces. Then, like many other RBD simulations we will switch between the unfractured original asset a frame before impact using a switch to hide some of the imperfections.

We also need to set up proper UVs for the inside of the breaks as this is new geometry we created. Split off the inside group then give it a name so that we can assign the material easily in Solaris, and finally set up UVTextures on a time shifted and frozen version of them before copying the UV attribute back to the simulated geo. We now have rock steady UVs for the inside pieces. Also split off the interior face of the violin so that we can name it correctly for textures later and we are good to go to add some vibrations and a peg twisting animation. 

Bend node set up

For the reverberations we can just select all the pieces that aren't being sent flying from the impact and add some twist and bend using the Bend node. We just need to add some wave noise from CHOPs to the Bend Angle and Twist that is animated towards 0 after a few frames as we want the vibrations to end fairly quickly. Remember to enable Deform in Both Directions and Continuous Twist in Both Directions so that the vibrations are correct. Finally, add a peg twisting animation by extracting the centroid of whichever peg to add the animation to, time shift and then animate the peg twisting, then copy it back to the centroid of the original peg. 

Step 7 - Dust

Now that we have the main portion of the Violin shatter done it's time to add in some secondaries. In this case we will add some particulate debris, some dust and some smaller pieces to help fill out the general debris cloud.

VDB For Pyro Collisions

Start off by taking the simulation of the violin and combining it with the stage and converting both of those to a VDB cache that we can then use to provide collisions for the pyro simulation. Using a VDBFromPolygons node, drop the Voxel Size low enough to keep a good amount of the detail of the violin, and then change the Distance VDB name to collision. Change the Surface Attribute to point.v, the VDB name to collisionvel, and finally the Vector Type to Displacement/Velocity/Acceleration. The collisions are now ready to be cached out to be fed into the pyro simulation.

Step 8 - Debris & Splinters

Points to copy to and further fractured pieces

In the copytopoints turn on the Piece Attribute parameter and set it to piece and check on the Pack and Instance parameter. Finally, add a randomized name to the pieces using an attribrandomize as this will be used to assign materials to the splinters.

Step 9 - Render

Once the materials have been assigned, drop down some lights as well as a karmaFogBox for the hazy feel. Split the render into its separate passes using karmaRenderSettings so that the violin, the dust, the particulate and the splinters are all separate for both of the cameras. For the karmaRenderSettings, we are rendering using the Karma XPU engine and make sure you have Motion Blur enabled as well as the Depth of Field set to off. Turn on Velocity Blur and that is everything needed to recreate this shot! Hope you found this tutorial helpful and thank you for reading!