Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T15/00—3D [Three Dimensional] image rendering
  • G06T15/50—Lighting effects
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T15/00—3D [Three Dimensional] image rendering
  • G06T15/50—Lighting effects
  • G06T15/506—Illumination models

Definitions

• 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").
• 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.
• 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.
• 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.
• the quantity of light diffused by the medium also varies according to 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.
• 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 heterogeneous participating medium.
• WO2009 / 003143 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.
• 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.
• the invention aims to overcome at least one of these disadvantages of the prior art.
• 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 basis from representative density values for a set of elements of the heterogeneous participating medium located along at least one direction of light emission by a light source, and estimating the amount of light diffused by the heterogeneous participating medium, according to at least one direction of light scattering, from the estimated projection coefficients.
• the elements of the heterogeneous participating medium are points or particles.
• the estimation of the projection coefficients is independent of the wavelength of the light emitted by the light source.
• the projection coefficients are estimated by taking into account a predetermined scale factor ⁇ .
• the projection coefficients are estimated using a ray sampling method, the heterogeneous participating medium being composed of points.
• the projection coefficients are estimated using a particle addition method, the heterogeneous participating medium being composed of particles.
• the method comprises a step of estimating values representative of the reduction in luminous intensity from the estimated projection coefficients.
• the estimation of the quantity of light diffused by said medium is carried out by discretizing the heterogeneous participating medium along the at least one diffusion direction.
• the estimation of the amount of light diffused by the heterogeneous participating medium is carried out using a ray sampling method.
• the estimation of the amount of light diffused by the heterogeneous participating medium is carried out using a particle addition method.
• the projection coefficients are stored in a projection texture.
• 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
• FIG. 5 illustrates a method for estimating the quantity of scattered light, according to one particular embodiment of the invention.
• FIG. 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.
• 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 is heterogeneous, that is to say that the physical characteristics of the medium, such as the density of the particles composing, for example, vary from one point to another in the medium.
• the participating medium is composed of small particles that interact with the light
• the incident light that is to say received from the light source 1 1 in a direction ⁇ ⁇ 1 10 is not only absorbed but is also broadcast.
• the light In an isotropic scattering participating medium, the light is diffused uniformly in all directions.
• 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 direction of incidence ⁇ ⁇ 1 10 and the diffusion direction ⁇ 0 ⁇ * 120 of the light.
• the amount of light diffused into an element (equated with a point or to a particle defined by a center and a radius of influence, a particle advantageously grouping together a set of points having the same properties M 13 of the medium 10 in the diffusion direction out 120 is calculated by the following equation:
• the amount of light scattered by an element M 13 of the medium reaching the eye of a spectator 12 located at a point C of the space in the out direction 120, that is to say the amount of light scattered by the element 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 out in the direction of the spectator 12, is then:
• D (M) or D (s) is the density of the medium in a given element, the density varying from one element to another since the medium is heterogeneous,
• ⁇ ( ⁇ , ⁇ 0 ⁇ , ⁇ ) is the phase function describing how the light coming from the direction of incidence ⁇ , ⁇ is diffused in the direction of diffusion ⁇ 0 ⁇ * to the element M,
• ⁇ L ri (M, cj0in) is the light intensity reduced to the element M coming from the direction of incidence ⁇ , ⁇ 1 and represents the amount of incident light arriving at the element 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 ⁇ , ⁇ 1 10, and is:
• Equation 2 makes it possible to calculate the amount of light diffused by an element M and reaching the eye of a spectator 12 located on the direction ⁇ 0 ⁇ .
• To calculate the quantity of light received by a spectator looking in the direction ⁇ 0 ⁇ it is then necessary to sum all the contributions of all the middle elements located on the axis or t, that is to say say the elements situated on the segment PM max , P and M max being the two points of intersection between the medium 10 and the direction or t 120.
• This total scattered luminance arriving at P 15 from the direction or t 120 due to the diffusion simple is then:
• This total scattered luminance is obtained by integrating the contributions of all the elements located between P and M max on a radius having added 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 5M size intervals and we obtain the following equation:
• the heterogeneous participating medium 10 is a three-dimensional element, shown in two dimensions in FIG. 1 for the sake of clarity.
• the heterogeneous participating medium 10 is formed of a multitude of points, a density value being associated with each point.
• the density values are advantageously stored in a texture called density texture.
• the heterogeneous participating medium 10 is formed
• FIG. 2 illustrates a method for estimating the attenuation of light from a light source 1 1 in the heterogeneous participating medium 10, and more particularly the application of the method of ray sampling to estimate the attenuation of light in the medium 10, according to a particular embodiment of the invention.
• 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.
• 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 Atti_ (M) 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,
• ⁇ is the extinction coefficient of the medium, corresponding to the sum of the medium diffusion coefficient a s and the absorption coefficient of the medium
• t (s) o t . D (s), which is to say that it is the density which varies from one point to another of the middle O.
• the density is constant from one point to another and this is the coefficient extinction that varies from one point to another or from one particle to another.
• each function f (x) (for example the function representative of the density) of a functional space can be represented as a linear combination of basic functions: where ⁇ is the j 'th coefficient of the basis function B j defined by:
• 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 medium 10 is discretized in a series of intervals 201, 202, 20i, 20i + 1, 20n of size S 5.
• the density also varies from one point to another, the density being equal to D in K and D, as a function of the position of the point M, on the radius of incidence ⁇ ⁇ 1 10.
• To calculate the j 'th coefficient ⁇ at the point M for example from the equation 14, it calculates the sum of the contributions of points K, M and M2 which are associated density values D; D 2 , DM.
• the variable xi of equation 14 corresponds to the distance between K 14 and the point considered along the radius of incidence considered (for example M 2 or M when calculating q at point M).
• the set of projection coefficients of the basic functions 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 map of shadow (of the English "shadow map”).
• the calculated coefficients are representative of the density (or density variation) along the emission direction associated with each element (so-called texel) of the projection texture.
• a graphical representation of the density variation in a given direction 1 10 is made possible by using these basic function coefficients, as shown in FIG. 2.
• Att L (M) The estimate of Att L (M) is fast because the projection coefficients cj have been previously estimated (and advantageously stored in a projection texture). It is then easy to find Lri (M) since L r i (M) is equal to the product of Att L (M) by the quantity of light emitted by the light source 11 along the direction of emission of the light. light. Lri (M) is thus equivalent to Att L (M) by a factor.
• the extinction coefficient of the medium t is dependent on the wavelength of the light emitted by the light source, it is necessary to calculate a set of basis function coefficients for each elementary component of the light, example the components R, G and B (of the English “Red, Green, Blue” or in French “Red, green, blue”), each component R, G and B having a particular wavelength or the components R, G, B and Y (from "Red, Green, Blue, Yellow” or "Red, Green, Blue, Yellow”).
• the estimation of the basic function coefficients is performed independently of the wavelength of the light emitted by the light source according to an advantageous variant of the invention. To do this, the term has been removed from equation 7 which becomes:
• the coefficient t is out of the equation 18 is taken into account when estimating the amount of light emitted by the point M as will be explained in reference to FIG 3.
• a scaling factor ⁇ is introduced into equation 15 or into equation 18.
• This scaling factor ⁇ allows advantageously to reduce the influence of the density in the equations 15 or 18 and in particular makes it possible to reduce, or even eliminate, the ringing artifacts, or Gibbs effects, due to the transformation of the reduced intensity of light in the functional space, for example in the Fourier space. Equations 15 and 18 then become according to this variant:
• the scale factor ⁇ 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.
• the operations described above are repeated for each illumination direction (or direction of incidence or light ray) starting from the light source 1 1 and passing through the medium 10.
• the coefficients of Basic function representative of the density as the crossing of the medium is stored in the projection texture.
• the projection texture then comprises all the projection coefficients representative of the density in the medium.
• 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.
• the ray sampling method is implemented according to a non-limiting embodiment of the invention.
• 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.
• 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 cjo out is carried out and after discretization of the segment PM is obtained in a multitude of elements 5 S :
• Equation 12 represents the amount of light emitted by a point M and received by a spectator. The term is calculated using the equation To obtain the total quantity of light received by a spectator situated at a point C looking in the direction ⁇ 0 ⁇ * 120, it suffices to sum the elementary light quantities emitted by the set of points M, ranging from P to M max . We obtain for this:
• 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.
• equation 12 According to the variant in which the coefficients Cj are calculated independently of the extinction coefficient a t , equation 12 becomes:
• equation 12 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:
• microprocessor 41 or CPU
• a graphics card 42 comprising:
• Random access memory type GRAM Graphical Random Access Memory
• I / O devices 44 such as for example a keyboard, a mouse, a webcam; and
• 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.
• 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.
• 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).
• the microprocessor 41 loads and executes the instructions of the program contained in the RAM 47.
• the random access memory 47 comprises in particular:
• parameters 471 representative of the heterogeneous participating medium 10 for example density parameters, light absorption coefficients, light scattering coefficients, scale factor ⁇ ).
• the algorithms implementing the steps of the method specific to the invention and described hereinafter are stored in the GRAM memory 47 of the graphics card 42 associated with the device 4 implementing these steps.
• the graphics processors 420 of the graphics card 42 loads these parameters into GRAM 421 and executes the instructions of these algorithms in the form of firmware of the "shader" type using the High Level Shader Language (HLSL), the GLSL (OpenGL Shading language), or the English language. OpenGL shaders ”) for example.
• HLSL High Level Shader Language
• GLSL OpenGL Shading language
• English language OpenGL shaders
• the memory GRAM 421 comprises in particular:
• projection coefficients 421 1 representative of the density at each point of the medium 10 or associated with each particle of the medium 10;
• values 4213 representative of the quantity of light diffused by the medium 10 along one or more observation directions.
• part of the RAM 47 is allocated by the
• the power supply 48 is external to the device
• FIG. 5 illustrates a method of estimating the scattering of light in a heterogeneous participating medium implemented in a device 4, according to a first example of non-limiting implementation that is particularly advantageous for the invention.
• the various parameters of the device 4 are updated.
• the representative parameters of the heterogeneous participating medium are initialized in some way.
• projection coefficients of a basic function are estimated, these projection coefficients being representative of the density whose values vary in the heterogeneous participating medium 10.
• the function has t (s) representative of the density variations in the medium 10 is projected and shown in a functional space of basic functions, for example using a Fourier transform or a discrete cosine transform. From the density values associated with the elements (ie at the points or particles) of the medium 10, a set of projection coefficients is calculated for a direction of emission of the light 1 10, or more precisely for the line segment corresponding to the intersection of a light beam 1 10, coming from a light source 1 1, with the medium 10.
• the line segment is advantageously divided spatially into a multitude of elementary pieces of the same length or of different lengths and the projection coefficients representative of the density are calculated for a point of each elementary piece of the segment.
• the method used to discretize the line segment and to estimate the projection coefficients is the so-called ray-marching algorithm method.
• the associated projection coefficients are obtained by summing density-dependent values associated with each point situated between the intersection point K 14 of the medium 10 and the radius of incidence 1 10 and the point M considered and the distance between the intersection point K 14 and the point corresponding to the discretization of this piece of segment. Then a value representative of the reduction of the luminous intensity at point M 13 is calculated from the estimated projection coefficients. Similarly, a value representative of the reduction in light intensity is calculated for each discretized point of the medium 10 along the radius 1 10 from the associated projection coefficients.
• projection coefficients are estimated for a given particle of the segment [KL] using a method called particle addition (particle blending).
• particle addition particle blending
• density-dependent values associated with particles located between the intersection point K and the particle of interest and dependent on the distance between the particles situated between K and the particle in question are added to each other.
• the projection coefficients representative of the density are estimated for any point of the medium 10 or for any particle of the medium 10.
• the estimated projection coefficients are recorded and stored in a projection texture 30.
• a storage space of the projection texture is allocated for storing the estimated projection coefficients for each incident light beam from the light source 1 1.
• the projection texture advantageously comprises all the projection coefficients of the medium 10, that is to say a set of projection coefficients for each point or each particle of the medium 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 viewer, the projection coefficients representative of the density being available at any time and immediately for use in the equations for estimating the light intensity reduction values.
• the amount of light scattered by the medium 10 in a transmission direction 120 is estimated using the projection coefficients estimated previously.
• 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.
• equation 24 is applied using the projection coefficients previously estimated.
• 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 estimation of the quantity of light diffused by said medium is carried out using a particle addition method.
• the total quantity of light received by a spectator located at a point C looking in the direction ⁇ 0 ⁇ * 120 is equal to the sum of the elementary light quantities emitted by all the particles located on the path ⁇ 0 ⁇ * between P to M max .
• This variant has the advantage of being able to sum the quantities of light emitted by the particles in any order and not necessarily progressing from P to M max by summing the values of quantities of lights emitted in this order.
• the order of taking into account the amount of light emitted by each particle is arbitrary and is advantageously taken care of directly by the rendering pipeline of the graphics card.
• 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.
• the invention is not limited to the embodiments described above.
• 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, the reduction of luminous intensity in the directions of incidence and emission, of the quantity of light diffused is not no longer 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 CPU type microprocessor.
• the basic functions used for estimating the projection coefficients are discrete cosine transform functions.
• the basic functions used are conventional Fourier functions or the Legendre polynomials or the Chebyshev polynomials.
• 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 according to an observation direction ⁇ 0 ⁇ .
• the luminance value calculated for display by each of the pixels of the image is recalculated to suit the viewer's different points of view.
• 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.

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