Fat & Skin Workflow

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The latest Ziva VFX release, version 1.4, include 2 new tissue parameters – Surface Tension and Pressure for Tissues – that will eliminate the need for a time-consuming fascia pass. These new “for tissue” parameters are essentially enabling the fat and skin passes, which are tissues, to achieve the dynamic effects associated with fascia, which is a cloth.

  • Pressure is going to function in a similar way as it did with cloth – it’ll apply even force in the normal direction across the tissue.

  • Surface Tension is going to shrink the tetrahedra that are in direct contact with the surface mesh in the direction of the surface tangent. Unlike Pressure, it is non-uniform and isotropic.

‘Fat fitting’ with Ziva VFX 1.4:

Just like before, you’ll want your skin model to be a closed volume with an inner layer, outer layer, and boarder. The Inner Layer is modelled using the cloth solver (this would be the fascia layer), the the Outer Layer is projected back to the skin layer using Wrap3 or similar tools, and lastly, the Border is the bridge between the two layers. Keep in mind, we aren’t removing the need for a fascia-like layer in your build, we’re simply removing the need to simulate it as a cloth and, instead, making it part of the skin/fat layer – so it is all in one and is much faster to simulate.

Once your character’s skin model is setup, you’ll want to convert the entire object (all layers) into tissue and add the necessary attachments. You can learn more about attachments here.

1. Create the inner layer material

  1. Select the Skin
  2. Go to Ziva Menu –> Add Tissue Properly —> Material Layer
  3. Rename the new material property something clear and easy like "skin_innerMaterial”
  4. We suggest reducing the material’s density to prevent excessive sliding and dynamic effects. In our Mr. Ink tutorial example, we changed it to 1/10th the original density. This is not a particularly realistic value, but it will help nonetheless.

2. Apply new material to the inner skin layer

  1. First, we want to make sure the new material is only applied to the inner layer. So, right click on your skin mesh again → Paint → zMaterials → “Skin_innerMaterial Weight”. This will paint your entire inner material layer white (not what you want).
  2. Since we don’t want external layer to be affected, click ‘Flood to Black’ to turn off the material in that area.
  3. Right click on the skin mesh and choose: Paint → zMaterials → “Skin_innerMaterial Weights”
  4. Paint only the inner surface to a value of white.

Now, the interior of the skin layer should be painted white while the exterior is black. This means that any changes you make to the material will only affect the white-painted areas (the inner layer!)

3. Set Effective Fat/Skin Parameters:
  1. Go to the your “Skin_innerMaterial” and change surface tension to, say, 45. This will shrink the corresponding tets along the tangent of the embedded surface.
  2. Now, go to the corresponding “envelope” attribute in the tissue node and set that to 0 for your start frame/frame 1.
  3. Then, introduce the effect over time, right click on “envelope” → “Key selected”
  4. Jump to a different frame – Frame 5 is suggested.
  5. On that frame, change the ‘envelope’ from 0 to 1.
  6. If you forget to ramp on the effects over time, some collisions may get missed as the solver attempts to abruptly reach the 45 value within the first frame of the simulation.

Now, if you play that back, you’ll see the collision proxy pair points that appear as green highlights, and you’ll see the skin mesh follow along nicely as the underlying anatomy moves. If there are any areas that are missing collisions – you may want to consider increasing the contact stiffness or making sure your ‘envelope’ is set up correctly.

Lastly, to improve the muscle definition on your character, use the new Pressure for Tissues function. Much like the Pressure for Cloth, this tool will help guide the tissue into the muscle grooves to decrease the amount of webbing between muscle groups.

  1. We suggest setting your tissue Pressure to -2500. This amount may seem oddly low in comparison to the inputs for Pressure for Cloth - but that is just the nature of tissues.

  2. Then, you’ll want to key it over time as well. A value of 0.0 at frame 1 and a value of 1.0 at frame 5 should suffice.
  1. Play back your simulation and see the results!
Tip: If your character’s tissue seems to be inflating rather than wrapping tighter to the underlying anatomy, double check that you’ve entered a negative value.

One of the great things about Ziva Materials is that you can make as many of them as you want. So, if you wanted to increase the definition just under the arm or at a single joint area, where skin is often softer or tighter to the bone, you can simply make another material layer and apply greater pressure and less surface tension to form a deeper armpit.

The simulation speed improvements you’ll gain from removing the fascia layer and enhancing the fat/skin layer is highly dependant on your unique build. Internally, we've seen upto a ⅓ time reduction in complex simulations. If you have any further questions of the fat/skin workflow or wish to share your results with us, please contact us on the Ziva Community Form.

August 9, 2018

For myself I still solve the fat and the fascia separatly. but before using tissue as fascia layer you still need to model your fascia with cloth system like in te webinar. then you can extrude your fascia and convert it to tissue and simulate the fascia with a tissu or create directy your fat and keep the collision on with no fascia between your muscle and your fat. (hope I'm clear xD)

August 17, 2018

Was working on this creature Simulation for the past 3 - 4 months. Here you can see Fat and Fascia solved with coupled simulation method. I find the couple method more appropriate and convenient than the new ziva 1.4 (omitting the fascia and diminishing the pressure). If Fascia is omitted I am not getting the desired output. Maybe need to do some more tests. I will soon update the details of this breakdown process in my website with all tissue and material properties.