Method of forming a perspective rendering from a voxel space
View Patent ↗A method for forming a high spatial resolution perspective rendering from a low spatial resolution voxel space is disclosed. The method comprises steps of: a) initializing a virtual window of predetermined resolution pixels, and placing the virtual window in or near the voxel space; b) sparsely ray-casting a plurality of vectors from a predetermined vantage-point through the virtual window into the voxel space; and c) calculating a visualization-value at a series of positions along each vector. In a position ordering of steps from the vantage-point to the pixel, an accumulated transparency-value threshold is calculated. Values of proximate voxels are interpolated into an interpolated voxel value for each position. The interpolated voxel values are then transformed into a derived visualization-value and transparency value.
1. A method of forming a perspective rendering from a voxel data set, comprising:
(a) raycasting at least one ray from a predetermined location into a voxel space, by sampling points along said ray in a space defined by said voxel data set;
(b) accumulating the effect of opacity along a path of the ray, using opacity values at said sampling points, into a ray storage value;
(c) associating points along the cast ray with material classes;
(d) determining if a ray passes from a point in a first material class to a point in a second material class;
(e) if the ray is determined to pass between material classes, accumulating a lighting contribution of a surface, associated with a boundary between the two classes, into said ray storage value;
(f) repeating at least (a), (b), (d), and (e) for a plurality of cast rays; and
(g) forming a perspective rendering from said ray storage values.
2. A method according to claim 1 , comprising determining the location of said boundary in (e) during said ray casting.
3. A method according to claim 2 , wherein said boundary is set to be at a position between said two points of different classes.
4. A method according to claim 2 , wherein said boundary is determined by examining at least one additional sampling point between the two points of different classes.
5. A method according to claim 4 , comprising repeating examining sampling points between points of different classes, until a desired precision of boundary determination is achieved.
6. A method according to claim 1 , comprising calculating said lighting contribution of the surface during said ray casting.
7. A method according to claim 6 , wherein calculating a lighting contribution of the surface comprises determining a normal to said boundary at a point on the boundary.
8. A method according to claim 1 , comprising:
providing an index array indicating for at least some of the voxels of the voxel data set, if a class-boundary does not pass near the voxel.
9. A method according to claim 8 , comprising:
avoiding said determining in (d) if a sampled point has a negative indication in said index array.
10. A method according to claim 8 , comprising:
reusing an opacity value from a previous sampled point, in accumulating the effect of opacity along the ray path, if a sampled point has a negative indication in said index array. indication in said index array.
11. A method according to claim 8 , wherein said index array is generated by setting a value indicating a lack of a boundary for all voxels that are surrounded by voxels in a same class.
12. A method according to claim 1 , wherein said associated lighting contribution of the surface comprises a surface lighting calculation of said boundary.
13. A method according to claim 1 , comprising stopping said ray casting if said accumulated opacity is over a threshold.
14. A method according to claim 1 , wherein said sampling points are separated by a step size and wherein said step size is dependent on the opacity value at the sampling points.
15. A method according to claim 14 , wherein said step size is always smaller than a voxel cross-section along the path of the cast ray.
16. A method according to claim 14 , wherein said step size is dependent on an opacity at a currently sampled point.
17. A method according to claim 14 , wherein said step size is dependent on the opacities of neighboring voxels to the currently sampled point.
18. A method according to claim 1 , comprising providing a definition of voxel value intervals for each class, prior to said ray casting.
19. A method according to claim 1 , wherein the opacity value of at least some of the sampling points is determined by:
interpolating between voxels near said point; and
transforming said interpolated voxel value into an opacity value for said point.
20. A method according to claim 19 , wherein said interpolation is dependent on a distance between said sampled point and the predetermined location from which the ray casting is performed.
21. A method according to claim 20 , wherein said interpolation varies between a cubic interpolation for nearby points and a linear interpolation for far points.
22. A method according to claim 1 , wherein said predetermined location is within the voxel space.
23. A method according to claim 1 , wherein said voxel data set comprises a medical imaging data set.
24. A method according to claim 1 , wherein (f) comprises:
sparsely casting rays; and
determining if to cast at least one additional ray between cast rays.
25. A method according to claim 24 , wherein determining if to cast said at least one additional ray comprises determining if said neighboring rays to said additional ray are statistically homogeneous.
26. A method according to claim 25 , wherein statistical homogeneity is determined with respect to the ray storage values.
27. A method according to claim 25 , wherein statistical homogeneity is determined with respect to depth factors associated with the ray.
28. A method according to claim 1 , wherein (g) comprises interpolating between stored values of cast rays.
29. A method according to claim 1 , wherein (f) comprises progressively increasing the density of raycasting.
30. A method according to claim 29 , wherein the progressively cast rays are cast in parallel.
31. A method according to claim 29 , wherein additional cast rays are cast to progressively generate nested levels of resolution in the formed image.
32. A method according to claim 1 , comprising rendering said formed perspective rendering on a display.
33. A method according to claim 1 , comprising defining a window in or near the voxel space through which to cast said rays.
34. A method according to claim 33 , wherein said window is perpendicular to a provided orientation vector.
35. A method according to claim 33 , wherein said window is flat and rectangular.
36. A method according to claim 33 , wherein said window is curved.
37. A method according to claim 33 , wherein said window is defined by pixels in a uniformly spaced rectangular grid.
38. A method according to claim 33 , wherein said window is defined by pixels using coordinates which are one of circular coordinates, elliptical coordinates and another conic projection of coordinates.
39. A method according to claim 1 , wherein said accumulation of opacity comprises updating a storage value CT as follows: CT=CT*T^step_size, where T is a transparency value corresponding to the opacity value.
40. A method according to claim 1 , wherein said rays are cast in parallel.
41. A method according to claim 1 , wherein the voxel data set is generated by one of CT (Computerized Tomography), MRI (Magnetic Resonance Imaging), Ultrasound, a geophysical survey, a meteorological survey, a scientific simulation, an animation model having more than two dimensions and a set of simultaneous equations.
42. A method according to claim 1 , wherein each voxel in the voxel data set has associated therewith a visual representation value and comprising:
determining a visualization value associated with a sampled point from the voxel associated visual representation values; and
accumulating said point associated visualization value into said stored value.
43. A method according to claim 42 , wherein said visual representation value is a gray scale value.
44. A method according to claim 42 , wherein said visual representation value is a color value.
45. A method according to claim 42 , wherein accumulating said point associated visualization values comprises selectively accumulating values based on front surface detection.
46. A method according to claim 42 , wherein said point associated visualization value comprises a volume lighting value.
47. A method according to claim 42 , wherein said point associated visualization value comprises a surface lighting value.
48. A method according to claim 1 , wherein advancing along a ray is coordinated with an opacification process.
49. Apparatus for forming a perspective rendering from a voxel space including:
a memory for storing a voxel data set;
a computer processor performing the following:
(a) raycast at least one ray from a predetermined location into the voxel space sampling points along said ray in a space defined by said voxel data set;
(b) accumulate the effect of opacity along the ray path, using opacity values at said sampling points, into a ray storage value;
(c) associate points along the cast ray with material classes;
(d) determine if a ray passes from a point in a first material class to a point in a second material class:
(e) if the ray is determined to pass between classes accumulate a lighting contribution of a surface, associated with a boundary between the two classes, into said ray storage value;
(f) repeat at least (a) (b) (d) and (e) for a plurality of cast rays; and
(g) form a perspective rendering from said ray storage values; and
a second memory for storing said formed perspective rendering.
50. A method according to claim 1 , comprising providing at least one association between two different material classes and a lighting contribution of a surface associated with the boundary.
51. A method according to claim 1 , wherein determining if a ray passes from a point in a first material class to a point in a second material class, comprises determining using opacity values of the points.
52. A method according to claim 1 , wherein the material classes belong to a plurality of possible material classes.