Wind shadow lets you use collision geometry to block wind from affecting the vellum geometry (External Shadow) and vellum geometry to block wind from affecting other parts of the vellum geometry (Self Shadow). External shadow is useful when simulating something like a character walking somewhere windy, then ducking behind a tree. Everything else in the scene will continue to be affected by wind, but the area behind the tree is blocked from the wind. This is also useful when there is no wind but you have local movement, such as a cape on a moving character. Their body will block the air from affecting the cape, resulting in a much more realistic simulation. Self shadow is useful when simulating something like a skirt, where the front side blocks wind from affecting the back side.

You can apply wind shadow in DOPs using POP Mask from Shadow, POP Attribute Blur, and POP Wind Shadow.

• POP Mask from Shadow sends rays backwards according to targetv & v and updates a windshadow_mask and windshadow_v with what it learns. This creates a mask based on whether particles are occluded by geometry.

• POP Attribute Blur is the POP equivalent of the Attribute Blur SOP, which lets you average out wind shadow effects over connected areas or nearby points.

• POP Wind Shadow includes the mask from wind to do both external and self shadowing, blurs the result, and updates targetv to reflect the shadowed wind.

Alternatively, you can do all of this in SOPs in the Wind Shadowing section of the Forces tab of the Vellum Solver SOP.

How to ¶

To...	Do this
Change the direction and intensity of the wind	Use the Built-in Wind parameter on the Forces tab of the Vellum Solver SOP.
Visualize what is being affected by wind	Put down a Color SOP and connect it to the first output of the Vellum Solver SOP. Type windshadow_mask in the Attribute field.
Fix the problem of an object not blocking the wind	Increase the Max Distance parameter on the Vellum Solver SOP. This will extend the area of obstruction. For example, if Max Distance is set to 10, the object will look back 10 units to see if there is anything that could be blocking the wind. Increasing this to 20 will look back 20 units, so objects that are further away can still block the wind. For a moving object, make sure whatever is blocking the wind has a velocity attribute on it. In order for this to work, the cloth needs to look at the blocking object and know the speed it’s moving at.
Soften hard edges	Increase the Shadow Angle parameter on the Vellum Solver SOP and turn on Blur Iterations to smooth out the noise. This creates a more realistic look by suggesting wind is going around the blocking geometry.
Have nearby objects affected	Set the Blur Influence Type to Proximity. This will cause the blur to look across any gaps in connectivity, so objects that are not directly blocked will still be affected based on proximity.

Note

These examples are based on Vellum cloth, but wind shadow can also be used for grains, particles, and hair.