Classifications

• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T15/00—Three-dimensional [3D] 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 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.
• 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.
• 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 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.
• 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 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.
• the estimation of the values representative of the reduction of luminous intensity is carried out by discretization of the medium.
• the estimation of the values representative of the reduction of luminous intensity is carried out using the method of sampling of radius.
• 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.
• the estimation of the quantity of light diffused by the medium is carried out using the method of sampling of radius.
• 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
• FIGS. 5 and 6 illustrate a method of estimating the amount of scattered light, according to two particular embodiments 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 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.
• 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 ⁇ in 110
• 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 incidence direction ⁇ in 1 10 and the diffusion direction ⁇ out 120 of the light.
• the amount of light scattered at a point M 13 of the medium 10 in the diffusion direction ⁇ 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 ⁇ ⁇ ⁇ in the direction of the spectator 12, is then:
• 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
• the incidence direction ⁇ ⁇ 1 10 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 ⁇ in 1 10, and is:
• ⁇ represents the scattered luminance attenuation due to absorption and scattering along the path from P15 to M
• 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 ⁇ out. To calculate the amount of light received by a viewer looking in the ⁇ out direction . it is then necessary to sum all the contributions of all the points of the middle located on the axis ⁇ 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 ⁇ out 120. This total scattered luminance arriving at P 15 from the direction ⁇ out 120 due to the simple diffusion is then:
• This total scattered luminance is obtained by integrating the contributions of all the points situated between P and M max on a radius having ⁇ 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 ⁇ M 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.
• 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.
• 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.
• 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 .
• 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
• a t is the extinction coefficient of the medium, corresponding to the sum of the medium diffusion coefficient ⁇ ⁇ 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.
• 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.
• 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 ⁇ in 1 10. the following equation:
• equation 7 can thus be represented equivalently by:
• the extinction coefficient of the medium ⁇ t 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 ").
• 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 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.
• a scaling factor ⁇ is introduced into equation 7 or in equation T.
• This scale factor ⁇ 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 ⁇ 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 11 and passing through the medium 10.
• 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.
• 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.
• 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 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 ⁇ out is performed and after discretization of the segment PM is obtained in a multitude of elements.
• 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:
• 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 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:
• a random access memory of type G RAM (of the English “Graphical Random Access Memory”) 421;
• a random access memory Random Access Memory 47
• I / O devices English “Input / Output" 44, such as for example a keyboard, a mouse, a webcam
• 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 below are stored in the memory G RAM 47 of the graphics card 42 associated with the device 4 implementing these steps.
• 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.
• HLSL High Levei Shader Language
• GLSL OpenGL Shading language
• English language OpenGL shader language
• RAM RAM 421 comprises in particular:
• values 4213 representative of the quantity of light diffused by the medium 10 along one or more observation directions.
• 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 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.
• the power supply 48 is external to the device
• 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.
• 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 reduction of the luminous intensity in the heterogeneous participating medium 10.
• 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.
• 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.
• 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.
• ray-marching algorithm ray-marching algorithm
• a projection coefficient of a basic function is estimated by applying the equation 9 described in FIG. Figure 2.
• the projection coefficients are estimated for all the points forming the medium 10.
• 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.
• 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 12 is applied using the projection coefficient estimated previously.
• 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 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.
• 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.
• the various parameters of the device 4 are updated.
• the representative parameters of the heterogeneous participating medium are initialized in some way.
• Step 61 projection coefficients are estimated in the same manner as that described with respect to step 51 of the .
• Step 61 is therefore not detailed again here.
• the previously 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 ray. light incident from the light source 11.
• 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.
• step 63 the quantity of scattered light is estimated in the same manner as that described with regard to step 52 of FIG. 5.
• the invention is not limited to the embodiments previously described.
• 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.
• the basic functions used for estimating the projection coefficients are conventional Fourier functions.
• the basic functions used are Legendre polynomials or 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.
• 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.
• 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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• 2010
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