Volumetric hair simulation

• US 7,468,730 B2
• Filed: 03/03/2005
• Issued: 12/23/2008
• Est. Priority Date: 03/25/2004
• Status: Active Grant

• Current Assignee: Pixar (The Walt Disney Company)
• Original Assignee: Pixar (The Walt Disney Company)
• Inventors: Anderson, John R., Petrovic, Lena
• Primary Examiner(s): Chauhan; Ulka
• Assistant Examiner(s): PRENDERGAST, ROBERTA D
• Application Number:
  US11/072,049
• Publication Number:
  US 20050212800A1
• Time in Patent Office: 1,391 Days
• Field of Search: 345/426, 345/424, 345473-475
• US Class Current: 345/473
• CPC Class Codes: G06T 13/40 of characters, e.g. humans, animals or virtual beings
• CPC Class Codes: G06T 15/08 Volume rendering
• CPC Class Codes: G06T 2210/21 Collision detection, intersection
• CPC Class Codes: G06T 2210/56 Particle system, point based geometry or rendering

Abstract

A volumetric representation of a hair simulation model determines collective hair attributes. To determine inter-hair collisions, vertices include average velocities of the adjacent portions of the model. The average velocities determine target velocities. Forces for the model are determined from the target velocity values. To direct hair to a desired pose, vertices include target and current density values representing the density of adjacent portions of the model in the desired pose and current position, respectively. The differences in density values determine pressure forces applied to the model. To determine the illumination of the hair, vertices include density values representing the density of adjacent portions of the model. The density values define a hair surface, and signed distance values relative to the surface are determined for the vertices. Normal vectors are determined from the gradients of the signed distance values at locations corresponding the positions of the hairs.

Claims

1. A method for determining the collective attributes of a set of hairs, the method comprising:

determining a volumetric representation of a hair simulation model, wherein the volumetric representation comprises a plurality of volume elements representing the space occupied by the hair simulation model, wherein at least a first portion of the volume elements each encompasses space including at least two non-colliding hairs of the hair simulation model;

determining a set of target collective attribute values for at least the first portion of the volume elements of the volumetric representation, wherein each target collective attribute value of the first portion of the volume elements is based on at least non-zero numerical contributions from their respective encompassed two non-colliding hairs of the hair simulation model; and

determining a set of forces for the hair simulation model from the set of target collective attribute values.

2. The method of claim 1, further comprising:

simulating the behavior of the hair simulation model under the influence of at least the set of forces.

3. The method of claim 1, wherein the volumetric representation of the hair simulation model is a voxel grid.

4. The method of claim 1, wherein the volumetric representation includes a first set of sample points each including an attribute value representing attributes of an adjacent portion of the hair simulation model.

5. The method of claim 4, wherein the hair simulation model includes a set of hair particles.

6. The method of claim 4, wherein each attribute value represents the density of the adjacent portion of the hair simulation model.

7. The method of claim 6, wherein determining a set of target collective attribute values for the volumetric representation includes:

receiving a target hair pose specifying a desired position for the set of hairs; and

determining a target volumetric representation, wherein the target volumetric representation includes a second set of sample points, each of the second set of sample points including an attribute value representing the density of an adjacent portion of the target hair pose.

8. The method of claim 7, wherein determining the set of forces for the hair simulation model from the set of target collective attribute values includes:

determining, for each portion of the hair simulation model, a pressure force from the difference between at least one adjacent sample point of each of the first and second sets of sample points, wherein the set of pressure forces associated with the hair simulation model are adapted to move the hair simulation model towards a pose approximating the target hair pose.

9. The method of claim 8, wherein the pose approximating the target hair pose corresponds to a volumetric representation of the hair simulation model having a set of density values similar to the corresponding set of attribute values of the target volumetric representation.

10. The method of claim 4, wherein the attribute value represents the velocity of the adjacent portion of the hair simulation model.

11. The method of claim 6, wherein determining the set of target collective attribute values for the volumetric representation includes:

determining a target velocity value for each of the first set of sample points from its attribute value.

12. The method of claim 11, wherein determining a target velocity value for each of the first set of sample points includes:

filtering the attribute values of the first set of sample points.

13. The method of claim 12, wherein determining the set of forces for the hair simulation model from the set of target collective attribute values includes:

determining, for each portion of the hair simulation model, a drag force from the target velocity value of at least one adjacent sample point of the first set of sample points, wherein the drag force is adapted to change the velocity of the portion of the hair simulation model towards a target model velocity value, wherein the target model velocity value is based on the target velocity value of at least one adjacent sample point of the first set of sample points.

14. The method of claim 10, wherein the attribute value of each of at least a portion of the sample points includes an average velocity of at least two hairs of the hair simulation model.

15. The method of claim 6, wherein the density of the adjacent portion of the hair simulation model is based on a quantity of hair particles in the adjacent portion of the hair simulation model.

16. A computer-readable medium including a set of instructions adapted to direct an information processing device to perform an operation comprising:

determining a volumetric representation of a hair simulation model, wherein the volumetric representation comprises a plurality of volume elements representing the space occupied by the hair simulation model, wherein at least a first portion of the volume elements each encompasses space including at least two non-colliding hairs of the hair simulation model;

determining a set of target collective attribute values for at least the first portion of the volume elements of the volumetric representation, wherein each target collective attribute value of the first portion of the volume elements is based on at least non-zero numerical contributions from their respective encompassed two non-colliding hairs of the hair simulation model; and

determining a set of forces for the hair simulation model from the set of target collective attribute values.

17. The computer-readable medium of claim 16, further comprising:

simulating the behavior of the hair simulation model under the influence of at least the set of forces.

18. The computer-readable medium of claim 16, wherein the volumetric representation of the hair simulation model is a voxel grid.

19. The computer-readable medium of claim 16, wherein the volumetric representation includes a first set of sample points each including an attribute value representing attributes of an adjacent portion of the hair simulation model.

20. The computer-readable medium of claim 19, wherein the hair simulation model includes a set of hair particles.

21. The computer-readable medium of claim 19, wherein each attribute value represents the density of the adjacent portion of the hair simulation model.

22. The computer-readable medium of claim 21, wherein determining a set of target collective attribute values for the volumetric representation includes:

receiving a target hair pose specifying a desired position for the set of hairs; and

determining a target volumetric representation, wherein the target volumetric representation includes a second set of sample points, each of the second set of sample points including an attribute value representing the density of an adjacent portion of the target hair pose.

23. The computer-readable medium of claim 22, wherein determining the set of forces for the hair simulation model from the set of target collective attribute values includes:

determining, for each portion of the hair simulation model, a pressure force from the difference between at least one adjacent sample point of each of the first and second sets of sample points, wherein the set of pressure forces associated with the hair simulation model are adapted to move the hair simulation model towards a pose approximating the target hair pose.

24. The computer-readable medium of claim 23, wherein the pose approximating the target hair pose corresponds to a volumetric representation of the hair simulation model having a set of density values similar to the corresponding set of attribute values of the target volumetric representation.

25. The computer-readable medium of claim 19, wherein the attribute value represents the velocity of the adjacent portion of the hair simulation model.

26. The computer-readable medium of claim 21, wherein determining the set of target collective attribute values for the volumetric representation includes:

determining a target velocity value for each of the first set of sample points from its attribute value.

27. The computer-readable medium of claim 26, wherein determining a target velocity value for each of the first set of sample points includes:

filtering the attribute values of the first set of sample points.

28. The computer-readable medium of claim 27, wherein determining the set of forces for the hair simulation model from the set of target collective attribute values includes:

determining, for each portion of the hair simulation model, a drag force from the target velocity value of at least one adjacent sample point of the first set of sample points, wherein the drag force is adapted to change the velocity of the portion of the hair simulation model towards a target model velocity value, wherein the target model velocity value is based on the target velocity value of at least one adjacent sample point of the first set of sample points.

29. The computer-readable medium of claim 25, wherein the attribute value of each of at least a portion of the sample points includes an average velocity of at least two hairs of the hair simulation model.

30. The computer-readable medium of claim 21, wherein the density of the adjacent portion of the hair simulation model is based on a quantity of hair particles in the adjacent portion of the hair simulation model.