CN104200509A - Photon mapping accelerating method based on point cache - Google Patents

Abstract

The invention discloses a photon mapping accelerating method based on point cache. The method includes the following steps: 1 photon tracking which includes emitting a certain number of photons to a scene from a light source, then tracking the moving track of the photons, recording the information of the photons in collision with an object and storing the information into a photon picture; 2 preprocessing which includes pre-computing the irradiance, storing the irradiance into the point cache, continuing computing a tinter after the irradiance is computed and storing obtained color values in the point cache mode; 3 rendering which includes conducting rendering according to a light tracking algorithm and emitting tracking reflection, refraction and diffuse reflection light at the intersection point position of light and the surface of the object according to the object surface attribute. A final gathering algorithm emits N diffuse reflection light, the diffuse reflection light intersects with the object on the scene, the color values of N sampling points in the point cache closest to the intersection point are returned, the color values are subjected to mean value computing to serve as the indirect illumination color value, and finally the indirect illumination color value plus the direct illumination color value is the final image.

Description

A kind of Photon Mapping accelerated method based on point cache

Technical field

The present invention relates to graphics Realistic Rendering field, be specifically related to a kind of Photon Mapping global illumination accelerated method based on point cache.

Technical background

Rendering is the major technique of animation film braking.Current Rendering mainly contains two kinds of developing direction, and a kind of is play up (the Photorealistic Rendering) that pursues real photo level picture quality; A kind of is non-truly play up (NPR) that pursues special artistic effect.Realistic Rendering is the effect that 3-dimensional work is pursued mostly at present, and Research Points mainly concentrates on authenticity.Local light is according to referring to the direct irradiation effect of light source to body surface, and global illumination refers to the radiation response between object and object, comprises the impact on object by the light of object reflection, refraction or scattering in scene.

Playing up of sense of reality effect must depend on global illumination.Global illumination plays vital effect in computer graphics.A lot of methods constantly propose to be used for simulating global illumination, such as the whole bag of tricks based on Monte Carlo ray trace, radiosity method, Photon Mapping and the Rendering based on point.

Biggest advantage of light track algorithm [An improved illumination model for shaded display] is from screen space emission of light, and in scene space, follows the tracks of light.The intersection point of playing up light and object, returns to color, and chief ray returns to screen space by color and forms image.Light with object collision in, can reflect according to body surface attribute refraction, or the behavior such as scattering and absorption; According to body surface attribute, continue to follow the trail of light.Traditional ray tracing technology can show reflected refraction effect preferably, also can generate the shade that validity is higher, but traditional ray tracking technology is not suitable for diffuse reflection surface.Owing to need to launch up to a hundred sampling light at diffuse reflection surface, carry out balance high frequency noise.Because every light is all independently followed the trail of and plays up, so the point in scene may repeatedly intersect and repeatedly calculate with light, caused the shortcoming in ray tracing performance simultaneously.

Photon Mapping method [A practical guide to global illumination using ray tracing and photon mapping] is to be suggested on the basis of biggest advantage of light track algorithm, and realizes a part of algorithm according to ray trace.The method is the algorithm of two steps, and the first step builds photon figure by forward ray trace, and second step is played up whole scene by the information in photon figure.Its core concept is from light source, to start to follow the trail of the transmission of luminous energy, each is transmitted to pilot process and record, and finally according to projection or reverse ray tracing, collects these information, to reach the object of playing up.Because intersecting of middle each light and scene goes on record, so the method has been avoided the problem of double counting in reverse ray tracing.The method can be simulated a lot of global illumination effects, such as look excessive phenomenon and caustic, but because the randomness of photo emissions function causes the inhomogeneous of distribution of photons to cause direct visual photon figure in Photon Mapping calculates often to occur noise, in conjunction with final aggregation algorithms, can effectively address these problems.When introducing final aggregation algorithms, also can when calculating diffuse surface, launch many light, affect counting yield.

Chinese patent (application number: CN 201010138101, patent name: proposed a kind of by the method for Photon Mapping and ray tracing combination a kind of global illumination method based on Photon Mapping), the method is based on traditional Photon Mapping method, need to be by large-scale photo emissions guarantee rendering accuracy, a large amount of photons is followed the tracks of to calculate and is had larger system execution time and storage space expense, so this patented method counting yield efficiency is lower.

Christensen proposes to utilize the algorithm that the mode of precomputation irradiance is accelerated final gathering a kind of in article [Efficient simulation of light transport in scences with participating media using photon maps], this algorithm is saved in the irradiance value of precomputation in photon figure, thereby the stage that makes finally to assemble can directly be read the irradiance in photon figure, and needn't to photon figure, carry out again the calculating of irradiance, thereby the speed of Photon Mapping algorithm integral body has been improved.This algorithm is provided with very large dependence to parameter, when photon number is too much, calculates irradiance also seem unnecessary and need not in each photon position.

Summary of the invention

The present invention is directed to the counting yield of existing Photon Mapping method slow and accelerate the large problem of buffer structure committed memory.A kind of Photon Mapping accelerated method based on point cache is provided, utilizes the thought of the global illumination algorithm based on point, generate the texture cache based on sampled point, as accelerating buffer memory, the emergent radiation degree that storage photon calculates; This method can guarantee the distribution of sampled point in cache file when guaranteeing that buffer memory accelerates, and avoids the too much consumption of unnecessary calculating and internal memory.

In order to realize the Photon Mapping method based on point cache, the present invention adopts following technical scheme:

In photo emissions step with in the middle of playing up step, add the sampling buffer memory stage based on putting.This stage, in middle render process, direct visual photon figure information, replaced the direct illumination surface information in the global illumination technology based on point, generates point cache data with reference to the global illumination technology based on point.In final rendering process, utilize point cache data to carry out the color of compute ray intersection point, eliminate the double counting of repeatedly searching photon figure at same intersection point place, accelerate rendering efficiency.

A Photon Mapping accelerated method based on point cache, comprises the steps:

Step 1. photon tracking phase; In from light source to scene, launch the photon of some, then follow the tracks of the movement locus of these photons, the information recording of the photon bumping with object is got off and is saved in photon figure;

Step 2. pretreatment stage; Precomputation irradiance, and be saved in point cache; And after having calculated irradiance, continue to calculate tinter; Form by the color value obtaining with point cache saves;

Step 3. is played up the stage; According to Ray Tracing Algorithm, play up, at the intersection point place of light and body surface, according to body surface attribute, reflection is followed the tracks of in transmitting, refraction, irreflexive light; Final aggregation algorithms transmitting N bar diffuse light, every diffuse light and object scene intersect, return from the color value of sampled point in the nearest point cache of intersection point, by the color value process mean value computation of sampled point in N point cache, the average obtaining is that indirect light shines color value, is finally added and obtains final image with direct illumination color value again.

Described step (1) comprises the steps:

Step (1.1): launch at random the photon of some from light source, determine at random photon direction, and follow the trail of photon light according to the mode of following the trail of light in biggest advantage of light track algorithm in scene;

Step (1.2): photon is reflected, absorbs or directly enter interior of articles through surface at collided body surface; When photon collision is preserved photon information in photon figure during at diffuse reflection surface.

Described step (2) comprises the steps:

Step (2.1): the order generating according to pixel is chosen sampled point;

Step (2.2): search the individual photon of k (k >=1) of its arest neighbors in overall photon figure at sampling point position x place, and utilize this k photon this sampled point to be carried out to the calculating of irradiance, computing formula is as shown in formula (1);

$\widehat{L_r}(x,w)=\frac{2f_r(x,w_i,w)}{\mathrm{\πr}{\left(x\right)}^2}\underset{i=1}{\overset{k}{\mathrm{\Σ}}}(1-\frac{{(x-x_i)}^T(x-x_i)}{r{\left(x\right)}^2}){\mathrm{\Φ}}_i,$ Formula (1)

In formula (1) the irradiance calculating at sampling point position x place, w is the incident ray direction of sampled point x, w _ibe the direction of i photon, x _ibe the position of i photon, Φ _ibe i photon energy, r (x) is from sampled point x photon farthest and the distance of x in k photon.F _rbe bidirectional reflectance distribution function, represent the proportion between incident irradiance and outgoing irradiance.

Step (2.3) a: parameter using the irradiance value calculating as diffuse reflection material tinter, participates in calculating; Finally calculate the color of body surface up-sampling point;

Step (2.4): obtain after the color value of this sampled point precomputation, then the position of this sampled point, surface normal and its color value are saved in point cache file.

The diffuse reflection material tinter of describing in described step (2.3) is used for calculating color of object surface, comprises the radiancy information of calculating body surface and the surface color information of calculating object; The radiancy information of body surface is calculated by photon in Photon Mapping algorithm, as shown in step (2.2); The colouring information of body surface is obtained by the radiancy product of superficial makings color and body surface.

Described step (3) comprises the steps:

Step (3.1): play up scene according to ray trace flow process, ask friendship from screen space emission of light and scene, according to body surface shader computations intersection point color; According to the description of tinter, at diffuse reflection surface, will perform step (3.2) and step (3.3);

Step (3.2): launch the final sampling ray of assembling of N bar at diffuse reflection surface at random in the hemisphere in space, colored spots place from colored spots, and calculate the intersection point of every ray and scene;

Step (3.3): utilize the coordinate information of intersection point to search for its nearest neighbor point in point cache file, the color value that obtains nearest neighbor point preservation returns; And this value is decayed according to the length of sampling ray from colored spots to intersection point, after N color value after decay carries out mean value computation as the indirect illumination value of colored spots;

Step (3.4): according to the description of tinter, by the direct illumination value at colored spots place and illumination value addition indirectly, as the rreturn value of light, say that color returns to screen space, composition diagram picture.

Beneficial effect of the present invention:

1. the present invention proposes the Photon Mapping accelerated method based on point cache first, the radiancy information that buffer structure is preserved the diffuse surface of photon calculating of having sampled.In the final rendering stage, can utilize the radiancy information of buffer memory directly to calculate, avoid a little repeatedly inquiring about photon figure same playing up.Accelerated rendering efficiency.

2. the present invention has adopted the buffer structure based on point, and sampling point distributions is more even, when gauging surface radiancy cache information, can only in sample point, calculate, and is not subject to the constraint of distribution of photons.Avoided causing because distribution of photons is inhomogeneous simultaneously in the few surperficial error of calculation of distribution of photons with in the many surperficial calculating wastes of distribution of photons.The cache information point that reasonably distributed, has also accelerated the computing velocity of cache information simultaneously.

3. the point cache that the present invention adopts can significantly reduce storage.The generation step of point cache is as the process of independently playing up for a time, Samples selecting number relevant with the factor such as the size of scene, resolution, and irrelevant with the number of photon.So the storage size of point cache is also only relevant with scene size with image resolution ratio, in the situation that photo emissions number is a lot, be much smaller than the size of photon figure.

Accompanying drawing explanation

Fig. 1 is the Photon Mapping accelerating algorithm overview flow chart based on point cache.

Fig. 2 .a is that the overall photon figure of BOX scene shows expression.

Fig. 2 .b is the point cache design sketch of BOX scene.

Fig. 2 .c is the rendering effect figure of BOX scene.

Embodiment

Below in conjunction with playing up example and accompanying drawing, the invention will be further described.

Playing up algorithm for one, is the process of picture that a three-dimensional scenic is converted into.Before rendering task starts, the expression by a three-dimensional scenic in modeling software is converted into the representation that render engine can read and identify.Scene has comprised video camera, solid, light source, material, the information such as pinup picture texture.User submits a contextual data file bag that comprises complete information to, and render engine can be identified the video camera described in these files, solid, the information such as light source.After whole contextual data is read in, start to play up algorithm, painted and output image.

BOX scene is one and plays up example.In this scene, comprised a box, in box, there is light source, clear glass ball and camera.Render engine, after having read whole contextual data, starts to play up.Detailed process is as follows:

Photon Mapping accelerated method based on point cache, comprises the steps:

Step 1. photon tracking phase; In from light source to scene, launch the photon of some, then follow the tracks of the movement locus of these photons, the information recording of the photon bumping with object is got off and is saved in photon figure; Specific as follows:

Photon is followed the tracks of, and launches the photon of some in from the light source on box top to scene, then in scene, picture tracking light, follows the trail of the movement locus of photon.The information recording of the photon bumping with box solid is got off and is saved in overall photon figure, and photon is absorbed on collided box surface or diffuse reflection occurs, and the photon that diffuse reflection behavior occurs is continuous tracked at scene relaying.The photon crossing with glass bead reflects on glass bead surface, and at photon, through glass bead and box surface collision, these photons are also stored in photon figure.With maximum diffuse reflection number of times and maximum number of specular reflections, finish photon tracing process, complete photon map generalization.Fig. 2 .a is that the overall photon figure of BOX scene shows expression.The size of overall situation photon figure is 65.3MB.

Step 2. pretreatment stage; Start independently to play up a flow process, precomputation irradiance, and be saved in point cache.The algorithm of utilization based on point cache, when playing up, according to playing up flow process, in scene according to pixel rendering sequential sampling, Photon Mapping accelerating algorithm will be searched the individual photon of k (k=500) of its arest neighbors in overall photon figure in sample point, and utilize these 500 photons this sampled point to be carried out to the calculating of irradiance, proceed afterwards the calculating of tinter, the color value obtaining is saved in the position of this sampled point, surface normal and its color value in point cache file with the form of point cache, and is not saved in photon figure; Concrete steps enter:

Step (2.1): the order generating according to pixel is chosen sampled point.

Step (2.2): search the individual photon of k (k=500) of its arest neighbors in overall photon figure at sampling point position x place, and utilize these 500 photons this sampled point to be carried out to the calculating of irradiance, computing formula is as shown in above-mentioned formula (1).

Step (2.3) a: parameter using the irradiance value calculating as diffuse reflection material tinter, participates in calculating; Finally calculate the color of body surface up-sampling point.

Step (2.4): obtain after the color value of this sampled point precomputation, then the position of this sampled point, surface normal and its color value are saved in point cache file.

The computation process of the diffuse reflection material tinter of describing in step (2.3) is used for calculating color of object surface, and step comprises the radiancy information of calculating body surface and the surface color information of calculating object.The radiancy information of body surface is calculated by photon in Photon Mapping algorithm, as shown in step (2.2).The colouring information of body surface is obtained by the radiancy product of superficial makings color and body surface.

Fig. 2 .b represents point cache design sketch, owing to not preserving the illumination information at glass bead place in overall photon figure, so glass bead is black herein.In point cache file, on body surface, be that the sampled point of various discrete forms.The size of point cache file is that 12.3M is big or small much smaller than photon figure, and the sampling resolution of point cache is 800*600 herein, according to experiment, shows, resolution is reduced to 80*60, plays up error also in visual range.

Step 3. is played up the stage; According to Ray Tracing Algorithm, play up, at the intersection point place of light and body surface, according to body surface attribute, reflection is followed the tracks of in transmitting, refraction, diffuse light; Final aggregation algorithms transmitting N (N=128) bar diffuse light, every diffuse light and object scene intersect, return from the color value of sampled point in the nearest point cache of intersection point, by the color value process mean value computation of sampled point in 128 point caches, it is shone to color value as indirect light, finally be added and obtain final image with direct illumination color value again, specific as follows:

Step (3.1): play up scene according to ray trace flow process, ask friendship from screen space emission of light and scene, according to body surface shader computations intersection point color; According to the description of tinter, at diffuse reflection surface, will perform step (3.2) and step (3.3);

Step (3.2): launch 128 final sampling rays of assembling at diffuse reflection surface at random in the hemisphere in space, colored spots place from colored spots, and calculate the intersection point of every ray and scene;

Step (3.3): utilize the coordinate information of intersection point to search for its nearest neighbor point in point cache file, the color value that obtains nearest neighbor point preservation returns; And this value is decayed according to the length of sampling ray from colored spots to intersection point, after 128 color values after decay carry out mean value computation as the indirect illumination value of colored spots;

Step (3.4): according to the description of tinter, by the direct illumination value at colored spots place and illumination value addition indirectly, as the rreturn value of light, say that color returns to screen space, composition diagram picture.

Claims (6)

1. the Photon Mapping accelerated method based on point cache, is characterized in that, comprises the steps:

Step 1. photon tracking phase; In from light source to scene, launch the photon of some, then follow the tracks of the movement locus of these photons, the information recording of the photon bumping with object is got off and is saved in photon figure;

Step 2. pretreatment stage; Precomputation irradiance, and be saved in point cache; And after having calculated irradiance, continue to calculate tinter; Form by the color value obtaining with point cache saves;

Step 3. is played up the stage; According to Ray Tracing Algorithm, play up, at the intersection point place of light and body surface, according to body surface attribute, reflection is followed the tracks of in transmitting, refraction, irreflexive light; Final aggregation algorithms transmitting N bar diffuse light, every diffuse light and object scene intersect, return from the color value of sampled point in the nearest point cache of intersection point, by the color value process mean value computation of sampled point in N point cache, the average obtaining is that indirect light shines color value, is finally added and obtains final image with direct illumination color value again.

2. the Photon Mapping accelerated method based on point cache as claimed in claim 1, is characterized in that, described step (1) comprises the steps:

Step (1.1): launch at random the photon of some from light source, determine at random photon direction, and follow the trail of photon light according to the mode of following the trail of light in biggest advantage of light track algorithm in scene;

Step (1.2): photon is reflected, absorbs or directly enter interior of articles through surface at collided body surface; When photon collision is preserved photon information in photon figure during at diffuse reflection surface.

3. the Photon Mapping accelerated method based on point cache as claimed in claim 1, is characterized in that, described step (2) comprises the steps:

Described step (2) comprises the steps:

Step (2.1): the order generating according to pixel is chosen sampled point;

Step (2.2): search the individual photon of k (k >=1) of its arest neighbors in overall photon figure at sampling point position x place, and utilize this k photon this sampled point to be carried out to the calculating of irradiance;

Step (2.3) a: parameter using the irradiance value calculating as diffuse reflection material tinter, participates in calculating; Finally calculate the color of body surface up-sampling point;

Step (2.4): obtain after the color value of this sampled point precomputation, then the position of this sampled point, surface normal and its color value are saved in point cache file.

4. the Photon Mapping accelerated method based on point cache as claimed in claim 3, is characterized in that, the computing formula of described step (2.2) is as follows:

Computing formula is as shown in formula (1);

$\widehat{L_r}(x,w)=\frac{2f_r(x,w_i,w)}{\mathrm{\πr}{\left(x\right)}^2}\underset{i=1}{\overset{k}{\mathrm{\Σ}}}(1-\frac{{(x-x_i)}^T(x-x_i)}{r{\left(x\right)}^2}){\mathrm{\Φ}}_i,$ Formula (1)

In formula (1) the irradiance calculating at sampling point position x place, w is the incident ray direction of sampled point x, w _ibe the direction of i photon, x _ibe the position of i photon, Φ _ibe i photon energy, r (x) is from sampled point x photon farthest and the distance of x in k photon.F _rbe bidirectional reflectance distribution function, represent the proportion between incident irradiance and outgoing irradiance.

5. the Photon Mapping accelerated method based on point cache as claimed in claim 4, it is characterized in that, the diffuse reflection material tinter of describing in described step (2.3) is used for calculating color of object surface, comprises the radiancy information of calculating body surface and the surface color information of calculating object; The radiancy information of body surface is calculated by photon in Photon Mapping algorithm, as shown in step (2.2); The colouring information of body surface is obtained by the radiancy product of superficial makings color and body surface.

6. the Photon Mapping accelerated method based on point cache as claimed in claim 1, is characterized in that, described step (3) comprises the steps:

Step (3.1): play up scene according to ray trace flow process, ask friendship from screen space emission of light and scene, according to body surface shader computations intersection point color; According to the description of tinter, at diffuse reflection surface, will perform step (3.2) and step (3.3);

Step (3.2): launch the final sampling ray of assembling of N bar at diffuse reflection surface at random in the hemisphere in space, colored spots place from colored spots, and calculate the intersection point of every ray and scene;

Step (3.3): utilize the coordinate information of intersection point to search for its nearest neighbor point in point cache file, the color value that obtains nearest neighbor point preservation returns; And this value is decayed according to the length of sampling ray from colored spots to intersection point, after N color value after decay carries out mean value computation as the indirect illumination value of colored spots;

Step (3.4): according to the description of tinter, by the direct illumination value at colored spots place and illumination value addition indirectly, as the rreturn value of light, say that color returns to screen space, composition diagram picture.