EP2502206A2

EP2502206A2 - Method for estimating light scattering

Info

Publication number: EP2502206A2
Application number: EP10773358A
Authority: EP
Inventors: Cyril Delalandre; Pascal Gautron; Jean-Eudes Marvie
Original assignee: Thomson Licensing SAS
Current assignee: Thomson Licensing SAS
Priority date: 2009-11-16
Filing date: 2010-11-09
Publication date: 2012-09-26
Also published as: WO2011057997A3; US20120232830A1; WO2011057997A2

Abstract

The invention relates to a method for estimating the amount of light scattered by a heterogeneous participating medium (10). In order to improve the rendering while minimizing the necessary calculation times thereof, the method includes the steps of: estimating projection coefficients within a function base on the basis of light intensity reduction values estimated for a set (13) of points of said medium that are located along at least one direction (110) for emission of light by means of a light source (11), and estimating the amount thereof of light scattered by said medium (10) in at least one scattering direction (120) for said light on the basis of said estimated projection coefficients.

Description

METHOD FOR ESTIMATING LIGHT DISTRIBUTION

1. Field of the invention

The invention relates to the field of synthetic image composition and more particularly to the field of simulating the scattering of light in a heterogeneous participating medium. The invention is also in the context of special effects for a composition in real time (of the English "live").

2. State of the art

According to the state of the art, different methods exist to simulate the diffusion of light in participating environments such as for example fog, smoke, dust or clouds. The participating media correspond to media composed of suspended particles that interact with the light to modify the path and the intensity in particular.

The participating media can be broken down into two parts, namely homogeneous media such as water and heterogeneous media, such as smoke or clouds. In the case of homogeneous participating media, it is possible to calculate in an analytical way the attenuation of the light emitted by a light source. Indeed, because of their homogeneous nature, these media have parameters such as the absorption coefficient of light or the scattering coefficient of light of constant value at any point in the medium. On the contrary, the absorption and scattering properties of light vary from one point to another in a heterogeneous participating medium. The calculations necessary to simulate the scattering of light in such a heterogeneous medium are then very expensive and it is thus not possible to calculate analytically and in real time the amount of light scattered by a heterogeneous participating medium. Moreover, since the medium is not diffuse (that is to say, since the diffusion of the medium is isotropic), the quantity of light diffused by the medium also varies as a function of the direction of diffusion of the light. that is, the direction in which a person looks at this environment. Calculations estimating the amount of light scattered must then be repeated for each direction of observation of the medium by a person to obtain a realistic rendering of the medium. To perform the real-time rendering of heterogeneous participating media, some methods perform the pre-calculation of certain parameters representative of the heterogeneous participating medium. While these methods are ideally suited for use in a post-production studio, for example, and provide good quality rendering, these methods are not suitable in the context of interactive design and real-time rendering of a participating environment. heterogeneous. Such a method is for example described in the patent application WO2009 / 003143 filed by Microsoft Corporation and published on December 31, 2008. The object of the invention WO2009 / 003143 object is a real-time software rendering a heterogeneous medium and describes a solution using radial basic functions. However, this solution can not be considered as a real-time rendering solution since certain pre-treatments must be applied offline (from the "offline") to the participating medium in order to calculate projection coefficients representing the environment that will be used for real time calculations of image synthesis.

With the emergence of games and interactive simulation applications, especially in three-dimensional (3D) applications, there is a need for real-time simulation methods that render realistic heterogeneous participant media.

3. Summary of the invention.

The invention aims to overcome at least one of these disadvantages of the prior art.

More particularly, the invention particularly aims to optimize the computation time required to compose a realistic real-time rendering of the light scattering in a heterogeneous participating medium.

The invention relates to a method for estimating the amount of light diffused by a heterogeneous participating medium, the method comprising the steps of:

estimating projection coefficients in a function base from estimated light intensity reduction values for a set of middle points along at least one direction of light emission by a light source, and estimating the quantity of light diffused by the medium, according to at least one direction of diffusion of the light, from the estimated projection coefficients.

Advantageously, the estimation of the projection coefficients is independent of the wavelength of the light emitted by the light source.

According to a specific characteristic, the projection coefficients are estimated by taking into account a predetermined scale factor Δ.

According to a particular characteristic, the method comprises a step of estimating, for each point of the medium, a value representative of the reduction of luminous intensity at a given point of the medium.

Advantageously, the estimation of the values representative of the reduction of luminous intensity is carried out by discretization of the medium.

According to a specific characteristic, the estimation of the values representative of the reduction of luminous intensity is carried out using the method of sampling of radius.

Advantageously, the estimate of the amount of light diffused by the medium is achieved by discretizing the medium along the at least one direction of diffusion.

According to a particular characteristic, the estimation of the quantity of light diffused by the medium is carried out using the method of sampling of radius.

According to another characteristic, the projection coefficients are stored in a projection texture.

4. List of figures.

The invention will be better understood, and other features and advantages will appear on reading the description which follows, the description referring to the appended drawings among which:

FIG. 1 schematically illustrates a heterogeneous, light-scattering participating medium, according to a particular embodiment of the invention;

FIG. 2 schematically illustrates a method for estimating the attenuation of light in a medium of FIG. 1, according to a particular embodiment of the invention; FIG. 3 schematically illustrates a method for estimating the quantity of light diffused by a medium of FIG. 1, according to one particular embodiment of the invention;

FIG. 4 illustrates a device implementing a method for estimating the quantity of scattered light, according to an example of a particular implementation of the invention;

~ Figures 5 and 6 illustrate a method of estimating the amount of scattered light, according to two particular embodiments of the invention.

5. Detailed description of embodiments of the invention.

Figure 1 illustrates a heterogeneous participating media (heterogeneous participant media), for example a cloud. A participating medium is a medium, composed of a multitude of particles in suspension, which absorbs, emits and / or diffuses light. In its simplest form, a participating medium absorbs only light, for example light received from a light source 11 such as the sun for example. This means that light passing through the medium 10 is attenuated, the attenuation depending on the density of the medium. The medium being heterogeneous, that is to say that the physical characteristics of the medium, such as the density of the particles composing it for example, vary from one point to another in the medium. Since the participating medium is composed of small particles that interact with the light, the incident light, that is, received from the light source 11 in a direction ω _in 110, is not only absorbed but is also diffused . In an isotropic scattering participating medium, the light is diffused uniformly in all directions. In an anisotropic scattering participating medium, such as the cloud 10 illustrated in FIG. 1, the scattering of the light depends on the angle between the incidence direction ω _in 1 10 and the diffusion direction ω _out 120 of the light. The amount of light scattered at a point M 13 of the medium 10 in the diffusion direction ω _out 120 is calculated by the following equation: The amount of light scattered by a point M 13 of the medium reaching the eye of a viewer 12 located at a point C of the space in the direction u) _0lrt 120, that is to say the amount of light scattered by the point M and attenuated by the medium 10 on the path MP, the point P being situated at the intersection of the middle 10 and the direction ω _ου ΐ in the direction of the spectator 12, is then:

for which :

• where _s is the diffusion coefficient of the medium,

· Where _a is the absorption coefficient of the medium,

• extinction coefficient of the medium,

D (M) is the density of the medium at a given point, the density varying from one point to another since the medium is heterogeneous,

• is the phase function describing how the light

from the incidence direction ω, _η is scattered in the diffusion direction ω _out at the point M,

• is the reduced luminous intensity at point M from

the incidence direction ω _ίη 1 10 and represents the amount of incident light arriving at the point M after attenuation due to the path of the light in the medium 10 on the segment KM, K being the point of intersection between the medium 10 and the radius of incidence ωin 1 10, and is:

· represents the scattered luminance attenuation due to absorption and scattering along the path from P15 to M

13.

Equation 2 makes it possible to calculate the quantity of light diffused by a point M and reaching the eye of a spectator 12 situated on the direction ω _out. To calculate the amount of light received by a viewer looking in the ω _out direction _. it is then necessary to sum all the contributions of all the points of the middle located on the axis ω _out. that is, the points on the PM _maX segment _! P and M _max being the two points of intersection between the medium 10 and the direction ω _out 120. This total scattered luminance arriving at P 15 from the direction ω _out 120 due to the simple diffusion is then:

In this case, it is assumed that the light traveling along the path C-P is not attenuated.

This total scattered luminance is obtained by integrating the contributions of all the points situated between P and M _max on a radius having ω _out as direction. Such an integral equation can not be solved analytically in the general case and even less so for real-time estimation of the amount of scattered light. The integral is evaluated numerically using the so-called ray sampling or ray-marching method. In this method, the integration domain is discretized into a multitude of size intervals δ _M and we obtain the following equation:

Advantageously, the heterogeneous participating medium 10 is a three-dimensional element, shown in two dimensions in FIG. 1 for the sake of clarity.

FIG. 2 illustrates a method for estimating the attenuation of light from a light source 11 in the heterogeneous participating medium 10, and more particularly the application of the radius sampling method to estimate the attenuation light in the medium 10, according to a particular embodiment of the invention. As has been described with regard to FIG. 1, the light diffused at a point M 13 by the medium 10 is a composition of the light attenuation received by the medium 10 of a light source 11 and the diffusion of this light. amount of attenuated light received by the medium 10. In a first step, with reference to FIG. 2, the term of the equation 1 representative of the attenuation of the light received from the light source 11 in the medium 10 is estimated . The representative term of the attenuation of the simple diffusion at a point M of the medium 10 is represented by the following equation, equivalent to the equation 3: where AÎÎLÎM) is the attenuation of the luminous intensity at the point M 13 and represents the amount of incident light arriving at the point M after attenuation,

D (s) is the density of the medium,

a _t is the extinction coefficient of the medium, corresponding to the sum of the medium diffusion coefficient σ _δ and the absorption coefficient of the medium

The medium 10 being heterogeneous, its light attenuation varies according to the point M considered. Such an integral type equation can not be solved analytically, the number of operations to be performed being too large, especially for a real-time resolution. In order to estimate the attenuation of the light at the point M, the integration domain situated on the direction of incidence 1 10 considered between the entry point K 14 of the light ray 1 10 in the medium 10 and a considered point of the middle 10 is discretized in a series of intervals 201, 202, 20i, 201 + 1, 20n of size _S 5. Moreover, the density also varies from one point to another, the density being equal to Di in K and D, as a function of the position of the point M, on the radius of incidence ω _in 1 10. the following equation:

Since each function of the functional space can be written as a linear combination of basic functions, a basic function being an element of a basis for a functional space, equation 7 can thus be represented equivalently by:

where q is the j ^'th coefficient of the base function Bj and wherein q is defined by the sum of i samples from 0 to _L N of Att (Xi) .bj (Xi), ie:

Xi corresponding to the position of a point M on the segment KL or equivalent to the distance KM _i on the line 1 10.

The extinction coefficient of the medium δt being dependent on the wavelength of the light emitted by the light source, it turns out necessary to calculate a set of basic function coefficients for each elementary component of the light, for example the components R, G and B (of the English "Red, Green, Biue" or in French "Red, green, blue" ), each component R, G and B having a particular wavelength or the components R, G, B and Y (of the English "Red, Green, Blue, Yellow" or in French "Red, green, blue , yellow "). To avoid these numerous calculations and thus accelerate the processing required for the estimation of these coefficients and also to minimize the storage space required for the storage of these basic function coefficients, the estimation of the basic function coefficients is performed independently of the wavelength of the light emitted by the light source. To do this, the term has been _removed from equation 7 which becomes:

Equation 9 then becomes:

equation 9 'being independent of the wavelength of the light emitted by the light source.

The coefficient o _t exited from the equation 9 'is taken into account when estimating the quantity of light emitted by the point M as will be explained with regard to FIG. 3, in particular in equation 12.

The set of basic function coefficients thus calculated is stored in a projection texture (of the English "projective texture map" or "projective texturing"), such a projection texture can be compared to a shadow map ( of the English "shadow map"). The coefficients calculated are representative of the attenuation function of the light along the emission direction associated with each element (called texel) of the projection texture. A graphical representation of the attenuation of light in a given direction 110 is made possible by using these basic function coefficients, as shown in FIG. 2.

According to one variant, a scaling factor Δ is introduced into equation 7 or in equation T. This scale factor Δ advantageously makes it possible to reduce the influence of density in equations 7 or T and makes it possible in particular to reduce, or even eliminate, ringing artefacts, or Gibbs effects, due to the transformation of the reduced intensity of light in the functional space, for example in the Fourier space. Equations 7 and 7 ^' then become according to this variant:

The scale factor Δ is advantageously parameterizable and determined by the user and is for example equal to twice the maximum density of the medium, or more than twice the maximum density, for example three or four times the maximum of density.

Advantageously, the operations described above are repeated for each illumination direction (or direction of incidence or light ray) starting from the light source 11 and passing through the medium 10. For each light beam, the function coefficients The basis of the attenuation of light as the medium passes through is stored in the projection texture. The projection texture then comprises all the projection coefficients representative of the attenuation of light in the medium. It is thus possible to represent an attenuation curve, such as the curve 20, for each direction of incidence of the light coming from the light source 1 1. Furthermore, the equation 9 can be solved in a reduced number iterations (for example 10, 50 or 100 iterations) and it is thus possible to calculate the reduced intensity in real time for a multitude of points of a medium 10.

FIG. 3 illustrates a method for estimating the simple scattering of light in the heterogeneous participating medium 10, more particularly the application of the radius sampling method for estimating this simple diffusion in the medium 10, and more generally a method estimation of light scattering by the medium 10 using the basic function coefficients calculated above, according to a particular embodiment of the invention. To calculate the simple scattering of light in the medium 10, the ray sampling method is implemented according to a non-limiting embodiment of the invention. In a first step, the attenuation factor of the light of a point M 13 of the medium 10 corresponding to the attenuation of the light on the path going from M 13 to P 15, is estimated by the following equation:

The density D (s) of an element s (that is to say, the point M _> considered, the position of the point M, ranging from P to M) of the line segment [PM] varying since the medium 10 is heterogeneous.

Since the medium is heterogeneous, equation 10 is very expensive in computing power and can not be calculated analytically. To overcome this problem, a sampling of the radius PM in the direction ω _out is performed and after discretization of the segment PM is obtained in a multitude of elements.

From the projection coefficients representative of the attenuation of light on the incident path of light from a light source 11 (estimated via equation 8 described with reference to FIG. 2) and stored in a texture of projection 30 and from equation 11, it is possible to estimate the global attenuation of light at a point M such that it is received by a spectator 12 (that is to say composition of the luminous attenuation in the medium 10 according to ω, η 110 and ω _out 120) analytically, the resources in terms of computational power required being much lower than those needed for an analytical resolution of the integral form equations. From equations 2, 8, 9 'and 11, it is then possible to estimate the amount of light emitted by a point M 13 of the middle and received by a spectator 12 looking in the direction ω _ουί . We obtain as follows:

is

where x represents the position of the point M considered on the segment [KL] or equivalently the distance from K to M along the direction ω, _π 1 10 and N _c represents the number of projection coefficients. Equation 12 represents the amount of light emitted by a point M and received by a spectator. To get the total amount of light received by a spectator located at a point C looking in the direction u) _out 120, it is sufficient to sum the elementary light quantities emitted by the set of points M, ranging from P to M _max . We obtain for this:

To obtain the total amount of light scattered by the medium 10 and received by the viewer 12, the estimates described above are repeated for all directions starting from the user and passing through the medium 10. The sum of the amounts of light received by the viewer in each observation direction provides the amount of light received from the medium 10 by the viewer 12.

According to the variant in which the coefficients Cj are calculated independently of the extinction coefficient,, equation 12 becomes:

According to the variant according to which a scale factor is introduced into the calculation of the attenuation of the light in M, equation 12 becomes: FIG. 4 schematically illustrates an example of a hardware embodiment of a device 4 adapted to the estimation of the quantity of light diffused by a heterogeneous participating medium 10. The device 4 corresponding, for example, to a personal computer PC, to a laptop ( from the English "laptop") or a game console.

The device 4 comprises the following elements, interconnected by an address and data bus 45 which also carries a clock signal:

a microprocessor 41 (or CPU);

a graphics card 42 comprising:

· Multiple GPUs 420

(or GPUs);

A random access memory of type G RAM (of the English "Graphical Random Access Memory") 421;

a non-volatile memory of ROM type (of the English "Read Only Memory") 46;

a random access memory (Random Access Memory) 47; one or more I / O devices (English "Input / Output") 44, such as for example a keyboard, a mouse, a webcam; and

- a power supply 48.

The device 4 also comprises a display screen type display device 43 connected directly to the graphics card 42 to display in particular the rendering of computed and compounded synthesis images in the graphics card, for example in real time. The use of a dedicated bus for connecting the display device 43 to the graphics card 42 has the advantage of having much higher data transmission rates and thus of reducing the latency for the display of data. 'images composed by the graphics card. According to one variant, the display device is external to the device 4. The device 4, for example the graphics card, comprises a connector adapted to transmit a display signal to an external display means such as for example an LCD screen or plasma, a video projector.

It is observed that the word "register" used in the description of the memories 42, 46 and 47 designates in each of the memories mentioned, as well a memory area of low capacity (a few binary data) that a memory area of large capacity (allowing storing an entire program or all or part of the representative data data calculated or display).

On power-up, the microprocessor 41 loads and executes the instructions of the program contained in the RAM 47.

The random access memory 47 comprises in particular:

in a register 430, the operating program of the microprocessor 41 charged at powering up the device 4;

parameters 471 representative of the heterogeneous participating medium 10 (for example density parameters, light absorption coefficients, light scattering coefficients, scale factor Δ).

The algorithms implementing the steps of the method specific to the invention and described below are stored in the memory G RAM 47 of the graphics card 42 associated with the device 4 implementing these steps. At power-up and once the parameters 470 representative of the medium loaded in RAM 47, the graphic processors 420 of the graphics card 42 loads these parameters into G RAM 421 and executes the instructions of these algorithms in the form of firmware of the "shader" type using the HLSL (High Levei Shader Language) language or in French "Programming language" shader " high-level "), the GLSL (OpenGL Shading language) or English language (" OpenGL shader language ") for example.

RAM RAM 421 comprises in particular:

in a register 4210, the representative parameters of the medium 10;

projection coefficients 4211 representative of the reduced light intensity at each point of the medium 10;

light intensity reduction values 4212 for each point of the medium 10;

values 4213 representative of the quantity of light diffused by the medium 10 along one or more observation directions.

According to a variant, a part of the RAM 47 is allocated by the CPU 41 to store the coefficients 4211 and values 4212 and 4213 if the available memory space in G RAM 421 is insufficient. This variant, however, results in longer latency times in the composition of an image comprising a representation of the medium 10 composed from the microprograms contained in the GPUs since the data must be transmitted from the graphics card to the random access memory 47 via the bus 45 whose transmission capacities are generally lower than those available in the graphics card for passing the data from GPUs to G RAM and vice versa.

According to another variant, the power supply 48 is external to the device

4. FIG. 5 illustrates a method for estimating the scattering of light in a heterogeneous participating medium implemented in a device 4, according to a first example of nonlimiting implementation that is particularly advantageous for the invention.

During an initialization step 50, the various parameters of the device 4 are updated. In particular, the representative parameters of the heterogeneous participating medium are initialized in some way. Then, during a step 51, projection coefficients of a basic function are estimated, these projection coefficients being representative of the reduction of the luminous intensity in the heterogeneous participating medium 10. To do this, a value of representative of the reduction of the luminous intensity is calculated for a set of representative points of a line segment corresponding to the intersection of a light ray 1 10, coming from a light source 11, with the medium 10. Thus for each point of the segment under consideration is calculated a value representative of the reduction of the luminous intensity, this value being minimal at the point of incidence K 14 of the radius 110 in the medium 10 and being larger and larger as and when as one sinks into the medium 10 to reach a maximum value at the point L which corresponds to the second point of intersection of the light ray 1 10 with the medium 10. The estimate of the values rep of the light intensity reduction is carried out according to any method known to those skilled in the art, for example by discretization of the line segment corresponding to the intersection of the light beam with the medium 10. To do this, the segment is by example divided spatially into a multitude of elementary pieces of the same length or different lengths and the reduction in light intensity is calculated for a point of each elementary piece of the segment. The values representative of the reduction of the luminous intensity of the other points of an elementary piece are then estimated by interpolation, for example from the values calculated for two points of two consecutive elementary pieces. Advantageously, the method used to discretize the line segment and to estimate the reduction in luminous intensity is the so-called ray-marching algorithm (ray-marching algorithm) as described with reference to FIG. FIG. 2. Then for each value representative of the reduction of the luminous intensity at a point of the medium 10 along the radius 1 10, a projection coefficient of a basic function is estimated by applying the equation 9 described in FIG. Figure 2. These projection coefficients are also representative of the reduction of the luminous intensity at points of the medium 10.

Advantageously, the projection coefficients are estimated for all the points forming the medium 10. To do this, the representative values of the reduction of the luminous intensity are estimated for a set of light rays coming from the source light, passing through the medium 10 and between the light rays 31 and 32 defining the medium 10. Each light segment passing through the medium 10 is discretized to deduce the light intensity reduction values, for example by radius sampling, then the Projection coefficients are deduced by applying equation 9.

Then, during a step 52, the amount of light scattered by the medium 10 in a transmission direction 120 is estimated using the projection coefficients estimated previously. To do this, the line segment corresponding to the intersection of the transmission direction 120 with the medium 120, that is to say the segment [PM _max ] is discretized spatially in a multitude of points or elementary pieces representative of this segment. For each point of this segment (respectively each elementary chunk), equation 12 is applied using the projection coefficient estimated previously. According to one variant, the ray sampling method is implemented to estimate the reduction in light intensity between a point of the segment considered and the point P located at the periphery of the medium 10 in the emission direction 120. The use of projection coefficients representative of the reduction of the luminous intensity according to an incident light ray makes it possible to simplify the calculations to be implemented while providing a realistic estimate of the reduction of the luminous intensity in a heterogeneous medium. No precalculation is then necessary to render the scattering of light in a heterogeneous participating medium, allowing real-time rendering of such media in interactive applications of the video game type for example in which the user is led to virtually move in a space comprising one or more heterogeneous participating media.

Advantageously, the quantity of light diffused by the medium 10 is estimated for several directions of emission. By summing these quantities of lights estimated for a plurality of transmission directions, the total amount of light diffused by the medium 10 and perceived by a spectator observing the medium 10 is obtained.

The steps 51 and 52 are advantageously reiterated as a spectator 12 moves around the medium 10, the image forming the rendering of the medium 10 being recomposed for each elementary movement of the spectator 12 around the medium 10. FIG. 6 illustrates a method for estimating the scattering of light in a heterogeneous participating medium implemented in a device 4, according to a second particularly advantageous nonlimiting implementation example of the invention.

During an initialization step 60, the various parameters of the device 4 are updated. In particular, the representative parameters of the heterogeneous participating medium are initialized in some way.

Then, during a step 61, projection coefficients are estimated in the same manner as that described with respect to step 51 of the _. Figure 5. Step 61 is therefore not detailed again here.

Then, during a step 62, the previously estimated projection coefficients are recorded and stored in a projection texture 30. Thus, a storage space of the projection texture is allocated for storing the estimated projection coefficients for each ray. light incident from the light source 11. There are as many storage spaces in the projection texture 30 as there are light rays from the source 11 and passing through the medium 10. The projection texture advantageously comprises the set of projection coefficients of the medium 10, that is to say a projection coefficient for each point of the middle 10. Such a storage of the projection coefficients offers the advantage of accelerating the estimation calculations of the quantity of light diffused by the medium 10 and perceived by a spectator, the projection coefficients representative of the reduction of the incident light intensity being available at any time immediately for use in equations 12 and 13.

Finally, during a step 63, the quantity of scattered light is estimated in the same manner as that described with regard to step 52 of FIG. 5. Of course, the invention is not limited to the embodiments previously described.

In particular, the invention is not limited to a method for estimating the amount of light diffused by a heterogeneous participating medium but also extends to any device implementing this method and in particular all the devices comprising at least one GPU. The implementation of the equations described with reference to FIGS. 1 to 3 for the estimation of the projection coefficients, intensity reduction luminous in the directions of incidence and emission, the amount of light scattered is not limited to an implementation in firmware type shader but also extends to an implementation in any type of program, for example programs executable by a microprocessor of the CPU type.

Advantageously, the basic functions used for estimating the projection coefficients are conventional Fourier functions. According to one variant, the basic functions used are Legendre polynomials or Chebyshev polynomials.

By way of example, the broadcasting method implemented in a device comprising a 3.6GHz Xeon® microprocessor and a nVidia geforce GTX280 graphics card makes it possible to compose the 20 frames per second rendering in real time for a heterogeneous participating medium. cloud type consisting of 4096 spheres. The use of the invention is however not limited to real-time use but also extends to any other use, for example for so-called postproduction processing in the recording studio for the rendering of computer-generated images. example. The implementation of the invention in postproduction offers the advantage of providing an excellent visual rendering in terms of realism in particular while reducing the calculation time required.

The invention also relates to a method for composing a two-dimensional or three-dimensional video image in which the amount of light scattered by a heterogeneous participating medium is calculated and the information representative of the luminance that results therefrom is used. for displaying the pixels of the image, each pixel corresponding to an observation direction in a viewing direction ω _out - The luminance value calculated for display by each pixel of the image is recalculated to fit to the different points of view of the viewer.

The present invention can be used in video game applications for example, whether by programs executable in a PC or portable computer or in specialized gaming consoles producing and displaying images in real time. The device 5 described with reference to FIG. 5 is advantageously provided with interaction means such as keyboard and / or joystick, other modes of introduction of commands such as, for example, voice recognition being also possible.

Claims

A method for estimating the amount of light scattered by a heterogeneous participating medium (10), characterized in that the method comprises the steps of:

- estimating (51, 62) projection coefficients in a functional space a function basis from estimated light intensity reduction values for a set of points of said medium along at least one direction of emission ( 110) by a light source (11), and

estimating (53, 64) the quantity of light diffused by said medium (10), according to at least one diffusion direction (120) of the light, from said estimated projection coefficients.

2. Method according to claim 1, characterized in that the estimation of said projection coefficients is independent of the wavelength of the light emitted by the light source (11).

3. Method according to one of claims 1 to 2, characterized in that the projection coefficients are estimated by taking into account a predetermined scale factor Δ.

4. Method according to one of claims 1 to 3, characterized in that it comprises an estimation step (61), for each point of said medium (10), a value representative of the reduction of light intensity at a given point of said medium.

5. Method according to one of claims 1 to 4, characterized in that said estimate of the values representative of the reduction of light intensity is achieved by discretization of said medium.

6. Method according to one of claims 1 to 5, characterized in that said estimate of the values representative of the reduction of light intensity is performed using the method of sampling radius.

7. Method according to one of claims 1 to 6, characterized in that said estimate of the amount of light diffused by said medium is achieved by discretizing said medium along the at least one direction of diffusion.

8. Method according to one of claims 1 to 7, characterized in that said estimate of the amount of light scattered by said medium is performed using the method of sampling radius.

9. Method according to one of claims 1 to 8, characterized in that said projection coefficients are stored in a projection texture (30).