Real-Time Rendering of Light-Scattering Media

• US 20090006044A1
• Filed: 06/26/2007
• Published: 01/01/2009
• Est. Priority Date: 06/26/2007
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Zhou, Kun, Shum, Heung-Yeung, Guo, Baining, Lin, Stephen Ssu-te, Ren, Zhong
• Granted Patent: US 8,009,168 B2
• Time in Patent Office: Days
• US Class Current: 703/2
• CPC Class Codes: G06T 15/55 Radiosity

Abstract

A real-time algorithm for rendering of an inhomogeneous scattering media such as smoke under dynamic low-frequency environment lighting is described. An input media animation is represented as a sequence of density fields, each of which is decompressed into a weighted sum of a set of radial basis functions (RBFs) and an optional residual field. Source radiances from single and optionally multiple scattering are directly computed at only the RBF centers and then approximated at other points in the volume using an RBF-based interpolation. Using the computed source radiances, a ray marching technique using slice-based integration of radiance along each viewing ray is performed to render the final image. During the ray marching process, the residual field may be compensated back into the radiance integral to generate images of higher detail.

Claims

1. A method for rendering inhomogeneous scattering medium, the method comprising:

(a) receiving a RBF model density field which is expressed by a weighted sum of a set of radial basis functions (RBFs) each having a RBF center, the RBF model density field representing at least approximately a density field of an inhomogeneous scattering medium defined in a volume space;

(b) computing a value of source radiance at each RBF center;

(c) approximating values of the source radiance at other points in the volume space by interpolating from the values of the source radiance at the RBF centers;

(d) computing an effective exitant radiance at point x in the volume space at least partially based on the source radiance; and

(e) rendering an image of the inhomogeneous scattering medium based on the effective exitant radiance.

2. The method as recited in claim 1, wherein the RBF model density field is an approximation of the density field of the inhomogeneous scattering medium, and the step (d) comprises:

compensating the effective exitant radiance by taking into account a contribution of a residual density field representing a difference between the density field and the RBF model density field.

3. The method as recited in claim 2, further comprising:

zeroing entries of the residual density field at points where the residual density field has a value at or below a given threshold.

4. The method as recited in claim 2, further comprising:

compressing the residual density field.

5. The method as recited in claim 2, further comprising:

compressing the residual density field by performing a spatial hashing on values of the residual density field.

6. The method as recited in claim 1, wherein the inhomogeneous scattering medium comprises smoke.

7. The method as recited in claim 1, wherein the inhomogeneous scattering medium is a part of a video or animation comprising a sequence of renderings of images rendered at time intervals, and wherein at least steps (d) and (e) are performed in real time at each time interval.

8. The method as recited in claim 1, wherein the inhomogeneous scattering medium is a part of a video or animation rendered at ten frames per second or faster, and wherein at least the steps (d) and (e) are performed in real time for each frame.

9. The method as recited in claim 1, wherein the step (b) comprises computing a single scattering term of the source radiance at the RBF center.

10. The method as recited in claim 9, wherein computing the single scattering term comprises computing a spherical harmonic product and a convolution term[(L_in*{tilde over (τ

)}(c_l))*p], wherein L_in is a vector representing a spherical harmonic projection of incident radiance L_in, {tilde over (τ

)}(c_l) is transmittance vector associated with RBF center c_l, and p is a vector representing a spherical harmonic projection of phase function p.

11. The method as recited in claim 9, wherein computing the single scattering term comprises:

computing for each RBF B_h an accumulative optical depth through the RBF B_h; and

computing a transmittance vector {tilde over (τ

)}(c_l) associated with RBF center c_l from the computed accumulative optical depth.

12. The method as recited in claim 1, wherein the step (b) comprises computing a single scattering term and a multiple scattering term of the source radiance.

13. The method as recited in claim 1, wherein the step (b) comprises:

computing a n^th order scattering term (n≧

1); and

computing a (n+1)^th order scattering term based on the previously computed n^th order scattering term.

14. The method as recited in claim 13, wherein a first order, second order . . . , n^th order, and (n+1)^th scattering terms are computed reiteratively, until the last scattering term has a magnitude below a user-specified threshold, or a user-specified number of iterations has been reached.

15. The method as recited in claim 1, wherein the step (d) comprising:

decomposing the volume space into N(≧

2) slices of a user-controllable thickness Δ

u stacking along a current view direction; and

calculating slide by slide a discrete integral of the source radiance arriving at the point x along a current view direction.

16. A method for rendering an inhomogeneous scattering medium, the method comprising:

providing a RBF model density field which is expressed by a weighted sum of a set of radial basis functions (RBFs) each having a RBF center;

providing a residual density field, which compensates a difference between an overall density field of the inhomogeneous scattering medium and the RBF model density field;

computing a model source radiance in the volume space for the RBF model density field;

computing an effective exitant radiance of the overall density field at point x in the volume space based on the model source radiance by taking into account a contribution of the residual density field; and

rendering an image of the inhomogeneous scattering medium based on the effective exitant radiance.

17. The method as recited in claim 16, wherein computing the model source radiance comprises:

computing a value of source radiance at each RBF center; and

approximating values of the source radiance at other points in the volume space by interpolating from the values of the source radiance at the RBF centers.

18. One or more computer readable medium having stored thereupon a plurality of instructions that, when executed by one or more processors, causes the processor(s) to:

(a) compute a value of source radiance of a density field of an inhomogeneous scattering medium at each of a plurality of radial basis function (RBF) centers, wherein the density field is at least approximately represented by a weighted sum of a set of radial basis functions (RBFs) each having one of the plurality of RBF centers;

(b) approximate values of the source radiance at other points in the volume space by interpolating from the values of the source radiance at the RBF centers;

(c) compute an effective exitant radiance at point x in the volume space at least partially based on the source radiance; and

(d) render an image of the inhomogenous scattering medium based on the effective exitant radiance.

19. The computer readable medium as recited in claim 18, wherein the RBF model density field is an approximation of the density field of the inhomogeneous scattering medium, and the step (c) comprises:

compensating the effective exitant radiance by taking into account a contribution of a residual density field defined by a difference between the density field and the RBF model density field.

20. The computer readable medium as recited in claim 18, wherein the processor(s) include a central processing unit (CPU) and a graphics processing unit (GPU), and at least the steps (c) and (d) are performed by the GPU at each rendering in real time.