Local, deformable precomputed radiance transfer

• US 20060214931A1
• Filed: 03/22/2005
• Published: 09/28/2006
• Est. Priority Date: 03/22/2005
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation), ROHM Co., Ltd.
• Original Assignee: Microsoft Corporation
• Inventors: Sloan, Peter-Pike J., Snyder, John M., Luna, Ben F.
• Granted Patent: US 7,633,503 B2
• Time in Patent Office: Days
• US Class Current: 345/426
• CPC Class Codes: G06T 15/50 Lighting effects

Abstract

Computer graphics image rendering techniques render images using a precomputed radiance transfer (PRT) to model local effects such as bumps, wrinkles, or other detailed features on an arbitrarily deformable model'"'"'s surface. The techniques apply zonal harmonics (ZH) which approximate spherical functions as sums of circularly symmetric functions around different axes. By spatially varying both the axes and coefficients of these basis functions, approximations can fit to spatially varying transfer signals. Compared to the spherical harmonic (SH) basis, the ZH basis yields a more compact approximation, and can be rotated at a low computational expense suitable for dense per-vertex or per-pixel evaluation. This allows PRT to be mapped onto deforming models which re-orient the local coordinate frame.

Claims

1. A computer graphics image rendering method, comprising:

calculating data of radiance transfer for a plurality of points over a surface of a modeled object for a rest coordinate frame, where the radiance transfer data is represented as a function of at least a sum of one or more products of coefficients and circularly symmetric basis functions;

based on the radiance transfer data, evaluating radiance transfer over at least a portion of the surface of the modeled object from a lighting environment for a view direction; and

producing an image of the modeled object as lit according to the radiance transfer evaluation.

2. The computer graphics image rendering method of claim 1 wherein the circularly symmetric basis functions are zonal harmonics.

3. The computer graphics image rendering method of claim 1 wherein the calculating radiance transfer data is performed as a precomputation for the points sampled over a surface of the modeled object.

4. The computer graphics image rendering method of claim 1 wherein the calculating radiance transfer data is calculated from a parameterized model of the radiance transfer over the surface of the modeled object, wherein the parameterized model defines the radiance transfer at the points over the surface of the modeled object as a function of at least one parameter varying according to location of the points over the surface of the modeled object, in combination with the function of at least the sum of one or more products of coefficients and circularly symmetric basis functions.

5. The computer graphics image rendering method of claim 1 wherein the calculating data of radiance transfer comprises:

calculating a radiance transfer vector at a sampled point; and

approximating the vector using zonal harmonics.

6. The computer graphics image rendering method of claim 5 wherein the approximating the radiance transfer vector comprises, iteratively performing the following up to a desired number of zonal harmonics lobes:

fitting a zonal harmonics lobe to a residual of the radiance transfer vector according to a best fit criterion, the residual initially being the spherical harmonics vector; and

subtracting the fitted zonal harmonics lobe from the current residual to produce a residual for a next iteration.

7. The computer graphics image rendering method of claim 1 wherein the modeled object is deformable, the method further comprising:

deforming the modeled object, thereby causing orientations of at least some of the sampled points on the modeled object to change to respective deformed coordinate frames;

for a sampled point on the modeled object whose orientation has changed to a respective deformed coordinate frame,

applying a rotation to the radiance transfer data of the sampled point from the rest coordinate frame to the respective deformed coordinate frame of the sampled point; and

evaluating radiance transfer of the sampled point from a lighting environment for a view direction based on the rotated radiance transfer data of the sampled point.

8. The computer graphics image rendering method of claim 7 wherein applying the rotation to the radiance transfer data comprises applying the rotation to axes of the circularly symmetric basis functions.

9. The computer graphics image rendering method of claim 1 wherein the modeled object is deformable, the method further comprising:

deforming the modeled object, thereby causing orientations of at least some of the sampled points on the modeled object to change to respective deformed coordinate frames;

for a sampled point on the modeled object whose orientation has changed to a respective deformed coordinate frame,

applying a rotation to the incident light from the lighting environment from the rest coordinate frame to the respective deformed coordinate frame of the sampled point; and

evaluating radiance transfer of the sampled point from the rotated incident light of the lighting environment for a view direction based on the radiance transfer data of the sampled point.

10. The computer graphics image rendering method of claim 1 wherein the radiance transfer data represents shadowing and light scattering of local surface features of the modeled object at the sampled point.

11. The computer graphics image rendering method of claim 10 wherein the radiance transfer data further represents translucence of the modeled object at the sampled point.

12. At least one data carrying medium having encoded thereon a software program executable on an image rendering system for a method of rendering graphics images of a modeled object based on data of local, deformable precomputed radiance transfer for sampled locations over a surface of the modeled object, the method comprising:

for a plurality of locations on the modeled object;

computing a rotation from a rest coordinate frame of the precomputed radiance transfer data to a deformed frame for the respective location;

applying the rotation to the precomputed radiance transfer data of the respective location;

based on the rotated radiance transfer data of the respective location, evaluating radiance transfer from a lighting environment for a view direction to yield a shading result for the respective location; and

producing an image of the modeled object with the locations on the modeled object shaded according to their respective shading result.

13. The computer-readable data carrying media of claim 12 wherein the radiance transfer data for a respective location on a diffuse surface of the modeled object is a matrix G_i* of coefficients in a zonal harmonics representation having lobe axes s_i*, and the evaluating comprises calculating the shading result according to the equation,

$r=\sum _{}^{}$

i = 1 N ⁢

y ⁢

 

⁢

†

⁡

( s i * ) ⁢

G i * ⁢

e ,

where e is a lighting vector representing incident lighting from the lighting environment.

14. The computer-readable data carrying media of claim 12 wherein the radiance transfer data for a respective location on a glossy surface of the modeled object is a BRDF-specialized transfer matrix M′

resulting from a singular value decomposition of coefficients in a zonal harmonics representation having lobe axes s_i* over a sampling of view and light directions, and the evaluating comprises calculating the shading result according to the equation, r(v)=u(v)M′

e, where u(v) is a view factor vector and e is a lighting vector representing incident lighting from the lighting environment.

15. A graphics image rendering system for rendering images of a deformable modeled object, comprising:

a lighting simulator operating to perform a lighting simulation of the modeled object to produce a set of radiance transfer matrices for a set of sampled locations over a surface of the modeled object representing radiance response including local effects to incident lighting at the respective sampled locations, where the radiance transfer matrices contain a set of coefficients for a representation of the radiance response at the respective sampled location as a sum of products of the coefficients multiplied by circularly symmetric basis functions;

a run-time image rendering engine operating to determine lighting from a lighting environment for each of a plurality of viewed locations on the modeled object in an image as a function of incident lighting from the lighting environment, the set of radiance transfer matrices, and rotations for the viewed locations from a rest coordinate frame to a deformed frame on the modeled object in a deformed position; and

a display driver operating to present the image of the modeled object in the lighting environment with the determined lighting on a display.

16. A graphics image rendering system of claim 15 wherein the circularly symmetric basis functions are zonal harmonics.

17. The graphics image rendering system of claim 15 wherein the lighting simulator operates to produce the radiance transfer matrices by calculating a radiance transfer vector at a viewed location, and approximating the vector using zonal harmonics.

18. The graphics image rendering system of claim 17 wherein the lighting simulator approximate the radiance transfer vector by iteratively performing the following up to a desired number of zonal harmonics lobes:

fitting a zonal harmonics lobe to a residual of the radiance transfer vector according to a best fit criterion, the residual initially being the spherical harmonics vector; and

subtracting the fitted zonal harmonics lobe from the current residual to produce a residual for a next iteration.

19. The graphics image rendering system of claim 15 wherein the run-time image rendering engine further operates to apply the rotations from the rest coordinate frame to the respective deformed coordinate frame for the viewed locations to the respective radiance transfer matrices of the viewed locations, and to determine lighting of the viewed locations by evaluating radiance transfer from the lighting environment for the view direction based on the respective rotated radiance transfer matrices of the viewed locations.

20. The graphics image rendering system of claim 15 wherein the run-time image rendering engine further operates to apply the rotations from the rest coordinate frame to the respective deformed coordinate frame for the viewed locations to the incident light from the lighting environment, and to determine lighting of the viewed locations by evaluating radiance transfer from rotated incident light from the lighting environment based on the respective radiance transfer matrices of the viewed locations.