Use of bidirectional texture functions

Abstract

The present disclosure relates to the use of bidirectional texture functions (BTFs) of objects, in particular physical car paint samples, in the design and development of cars and other everyday objects. The present disclosure also relates to corresponding computer systems.

Description

Technical Field

[0001] This disclosure relates to the use of bidirectional texture functions (BTFs) for objects (particularly physical automotive paint samples) in the design and development of automobiles and other everyday objects. This disclosure also relates to corresponding computer systems. Background Technology

[0002] Current automotive paint color design processes are based on physical samples of automotive paint applied to the most common small flat surfaces. Working solely with physical samples has several drawbacks. Paint samples are both expensive and time-consuming. Furthermore, due to cost considerations, it is difficult to infer from small samples what the coating will look like under different three-dimensional shapes (e.g., car bodies) or different lighting settings what the coating will look like. Automotive paints are often chosen for their gonioapparent effect, particularly those caused by interference and / or metallic pigments, such as metallic flake pigments or special effect flake pigments, such as pearlescent flake pigments.

[0003] Using a digital model of a car's paint appearance, a computer can generate an image of the car's paint applied to any shape under arbitrary lighting conditions. A bidirectional texture function (BTF) represents such a digital model that can also capture the spatial variations in the appearance of the car's paint, such as shimmer. Based on the computer-generated image of the car's paint applied to an object, the characteristics of the car's paint color can be virtually evaluated.

[0004] BTF is a representation of the texture appearance as a function of viewing and lighting directions (i.e., viewing and lighting angles). It is an image-based representation because the geometry of the object surface to be considered is unknown and unmeasured. BTF is typically captured by imaging the surface at hemispherical sampling points in possible viewing and lighting directions. BTF measurements are collections of images. BTF is a 6-dimensional function (Dana, Kristin J., Bram van Ginneken, Shree K. Nayar, and Jan J. Koenderink. 'Reflectance and Texture of Real-World Surfaces'. ACM Transactions on Graphics 18, no. 1 (January 1999): 1–34. https: / / doi.org / 10.1145 / 300776.300778).

[0005] Golla, Tim et al., “An Efficient Statistical Data Representation for Real-Time Rendering of Metallic Effect Car Paints”, in J. Barbic et al. (Eds.): EuroVR, LNCS 10700, pp. 51-68, 2017, disclose a representation of metallic car paint based on the statistical properties of calculated real-world samples and suitable for real-time rendering. This representation enables the generation of BTFs at arbitrary resolutions.

[0006] Rump, Martin et al., “Photo-realistic Rendering of Metallic Car Paint from Image-Based Measurements”, Computer Graphics Forum, Vol. 27, pp. 527-536, Wiley Online Library (2008) specifically discloses a measurement and rendering architecture designed for metallic car paint and based on BTF measurements. Rendering is performed using BRDF and BTF rendering techniques. Summary of the Invention

[0007] This disclosure relates to the use of bidirectional texture functions (BTFs) for objects (particularly physical automotive paint samples) in the design and development of automobiles and other everyday objects.

[0008] Uses of bidirectional texture functions for objects having the features of the independent claims, methods for using bidirectional texture functions, and computer systems are provided respectively. Further features and embodiments of the claimed uses, methods, and systems are described in the dependent claims and the specification.

[0009] According to this disclosure, a two-way texture function (BTF) of a material (e.g., automotive paint) is used to render a representation of an object that accurately reproduces the object's optical appearance, particularly its color and color effects, under a given object lighting.

[0010] In the context of this disclosure, the term "rendering" is used to describe the automated process of generating realistic images of objects by means of computer programs.

[0011] To accurately reflect the texture of an object, BTF includes a spatial texture image table that depends on lighting and viewing angle and orientation.

[0012] In one embodiment, BTF is used for OEM (Original Equipment Manufacturer) vehicle component design to design vehicle components that match the optical appearance of adjacent components in the finished vehicle and avoid unsolvable color matching problems, such as those between the front apron and the hood of a vehicle. BTF allows for accurate reproduction of the optical appearance of the finished vehicle.

[0013] In another embodiment, BTF is used for automotive color design to simulate the optical appearance of finished vehicles, including the complex color appearance and optical effects of finished vehicles; thereby helping to develop colors best suited to the complex shape of the vehicle body and shortening development time.

[0014] Vehicle representations obtained using BTF can also be used to simulate and visualize colors on vehicles, particularly effect coatings, in car configurator scenarios. This helps customers perceive differences between colors within the same color group (e.g., between standard and premium colors, and between interference colors (ICs)). Representations obtained using BTF also allow for realistic visualizations of vehicles, including colors and effects, in car sales platform scenarios.

[0015] Objects rendered using BTF functions are not limited to vehicles or vehicle parts. This representation can also be used in other areas, such as games, furniture, clothing, and packaging materials.

[0016] In an exemplary embodiment, BTF is used for furniture design to design a piece of furniture. BTF allows for the accurate reproduction of the optical appearance of the finished product, including the complex color appearance and optical effects of the finished product.

[0017] In another exemplary embodiment, BTF is used for clothing design to design a garment. BTF allows for the accurate reproduction of the optical appearance of the finished garment, including the complex color appearance and optical effects of the finished garment.

[0018] This disclosure also provides a method for designing a vehicle component that matches the optical appearance of an adjacent component in a finished vehicle, the method involving simulating and visualizing the finished vehicle using BTF.

[0019] This disclosure also provides a method for developing automotive coatings, which involves using BTF to simulate and visualize the optical appearance of a finished vehicle.

[0020] This disclosure also provides a method for designing a piece of furniture, which involves simulating and visualizing a piece of furniture using BTF.

[0021] This disclosure also provides a method for designing a garment, which involves simulating and visualizing a garment using BTF.

[0022] In this disclosure, the BTF used is a specific BTF that is generated through a process including at least the following steps:

[0023] - Measure the initial BTF of the object using a camera-based measurement device.

[0024] - Using a spectrophotometer, spectral reflectance data of an object are captured for a pre-given number, i.e., a finite number, of different measurement geometries.

[0025] - Adapt the initial BTF to the captured spectral reflectance data to obtain an optimized BTF.

[0026] To improve color accuracy, it is recommended to acquire an initial BTF (Brightness Spectrum Transformation) of the object (particularly a physical automotive paint sample) using a camera-based measurement device in the first step. Then, in the second step, a second spectral measurement is performed on the same sample using a spectrophotometer, particularly a handheld spectrophotometer. This yields additional, more accurate spectral reflectance data for a small (e.g., <25) measurement geometry. The initial BTF is then enhanced using this more accurate but sparser spectral reflectance data. The result is a BTF that captures the appearance of color and spatial variations (such as the shimmer of an automotive paint sample) with sufficient accuracy.

[0027] According to one embodiment, a camera-based measurement device creates multiple images (photographs) of an object / sample at different viewing angles, different illumination angles, different illumination colors, and / or different exposure times, thereby providing multiple measurement data that take into account various combinations of illumination angle, viewing angle, illumination color, and / or exposure time. The camera-based measurement device can be a commercially available measurement device, such as, for example, X-Rite. A small plate with the sprayed automotive paint sample and clear coat is inserted into the measuring device, and the measurement process begins. An initial BTF is obtained from the measurement and subsequent post-processing.

[0028] In post-processing, images / photographs with different illumination colors and exposure times but equal illumination angles and viewing angles are combined to form an image with high dynamic range. Furthermore, perspective correction is applied to the photographs on the sample. Based on the data obtained from the photographs and post-processing, the parameters of the initial BTF are determined.

[0029] To obtain an optimized BTF, the initial BTF is adapted to the captured spectral reflectance data. This involves segmenting the initial BTF into different terms, each containing a set of parameters. The parameters for each term can then be optimized separately using the captured spectral reflectance data.

[0030] Therefore, the initial BTF is segmented into two main terms: the first is a uniform bidirectional reflectance distribution function (BRDF) describing the reflectivity of an object (e.g., a car paint sample) that depends solely on the measurement geometry, and the second is a texture function that explains the spatially varying appearance of the object, i.e., adding a view- and lighting-dependent texture image. The texture image stored in the model has the following properties: on average, the sum of the intensities in each of the RGB channels of all pixels is zero. From a distance, the overall color impression of the car paint is determined not by the color at a single point but by the average color over a larger area. Due to these properties, it is assumed that the average color of larger areas of the texture image is zero or close to zero. This allows the texture image to be overlaid without altering the overall color. This also means that the texture image can be ignored when optimizing the BTF.

[0031] For the representation of BTF, the color model first introduced by Rump et al. (Rump, Martin, Ralf Sarlette, and Reinhard Klein. "Efficient Resampling, Compression and Rendering of Metallic and Pearlescent Paint." In Vision, Modeling, and Visualization, 11–18, 2009) and also proposed by Golla, Tim et al. ("An Efficient Statistical Data Representation for Real-Time Rendering of Metallic Effect Car Paints", in J. Barbic et al. (Eds.): EuroVR, LNCS 10700, pp. 51-68, 2017) is used.

[0032]

[0033] in

[0034] x: Surface coordinates of the sample / object

[0035] Lighting and viewing direction at the base coating of the sample

[0036] Color chart depending on lighting and viewing direction

[0037] a: Albedo or diffuse reflectance

[0038] The k-th Cook-Torrance lobe corresponds to the commonly used BRDF used to describe the gloss of microfaceted surfaces.

[0039] S k Weight of the k-th Cook-Torrance lobe

[0040] a k Parameters of the Beckman distribution of the k-th Cook-Torrance lobe

[0041] F 0,k Fresnel reflectance of the k-th Cook-Torrance lobe

[0042] Spatial texture image table depending on lighting and viewing direction

[0043] Typically, the bidirectional reflectance distribution function (BRDF) is a function of four real variables that defines how light is reflected at an opaque surface. This function takes the direction of light incidence as an example. and launch direction And return along Outgoing reflected radiation and from direction The ratio of incident irradiance to the surface. A BRDF (Brilliant Light Fiber Data Set) is a collection of photometric data for any material (in this case, an object, i.e., a paint sample), describing the photometric reflectance and scattering characteristics of the material (object) as a function of the illumination angle and the reflected scattering angle. A BRDF describes the spectral and spatial reflectance and scattering characteristics of an object (especially angle-varying materials comprised of the object) and provides a description of the material's appearance; many other appearance properties, such as gloss, haze, and color, can be easily derived from a BRDF.

[0044] Typically, a BRDF consists of three color coordinates that are functions of scattering geometry. When processing a BRDF, a specific light source and color system (such as CIELAB) must be specified and included in any data.

[0045] As can be seen from equation (1), the first term (i.e., BRDF) is divided into the first sub-term corresponding to the color table. and the second sub-term corresponding to the intensity function By optimizing the parameters of the color table in the first optimization step while keeping the parameters of the intensity function unchanged, and by optimizing the parameters of the intensity function in the second optimization step while keeping the parameters of the color table unchanged, the parameters of the initial BTF are optimized to minimize the color difference between the spectral reflectance data and the initial BTF.

[0046] Spectral reflectance data (i.e., spectral reflectance profiles) are acquired only for a limited number of measurement geometries. Each such measurement geometry is defined by a specific illumination angle / direction and a specific viewing angle / direction. Spectral reflectance measurements are performed, for example, by a handheld spectrophotometer, such as the Byk-5000, which has six measurement geometries (fixed illumination angle and viewing / measurement angles of -15°, 15°, 25°, 45°, 75°, and 110°). X-Rite MA- with twelve measurement geometries (two illumination angles and six measurement angles) Or X-Rite MA (Two illumination angles and up to eleven measurement angles) are used to perform this. The spectral reflectance data obtained from these measurement devices are more accurate than the color information obtained from camera-based measurement devices.

[0047] According to another embodiment, in order to optimize the color table for each spectrometry in the first optimization step, a first CIEL*a*b* value is calculated from spectral reflectance data (curve), and a second CIEL*a*b* value is calculated from the initial BTF. A correction vector in the a* and b* coordinates is calculated by subtracting the second CIEL*a*b* value from the first CIEL*a*b* value. The correction vector is then interpolated and extrapolated component-wise to obtain the full range of viewing and illumination angles stored in the color table. The interpolated correction vector is applied to the initial BTF CIEL*a*b* value of each spectral measurement geometry stored in the color table, and the corrected BTF CIEL*a*b* value is transformed into normalized linear sRGB coordinates (such that their sum is, for example, equal to 3) and finally stored in the color table.

[0048] Multilevel B-spline interpolation algorithms (see Lee, Seungyong, George Wolberg and Sung Yong Shin, “Scattered data interpolation with multilevel B-splines”. IEEE Transactions on Visualization and Computer Graphics 3, Nr. 3 (1997): 228–244) can be used for component-wise interpolation and extrapolation of correction vectors.

[0049] According to another embodiment, in order to optimize the parameters of the intensity function in the second optimization step, a cost function is defined based on the sum of color differences of all spectral reflectance measurement geometries. The cost function C(α,S,F0,a) is defined in all reflectance measurement geometries according to the following equation:

[0050]

[0051] in

[0052] G: The set of measurement geometries available for spectral reflectance data

[0053] g: A measurement geometry in the set of measurement geometry

[0054] ΔE(f Test ,f Ref ): Measurement at color f Test and f Ref Weighted color difference formula for the difference between them

[0055] Reference color derived from spectral measurements

[0056] For a given lighting and viewing direction, the test color is calculated from the initial BTF.

[0057] α = (α1, α2, α3): The parameter vectors of the Beckman distribution of the three Cook-Torrance lobes.

[0058] S = (S1, S2, S3): Weight vectors of the three Cook-Torrance lobes

[0059] F0=(F 0,1 ,F 0,2 ,F 0,3 ): Fresnel reflection vectors of the three Cook-Torrance lobes

[0060] P(α,S,F0,a): Penalty function

[0061] As shown in equation (2), the cost function can be supplemented by a penalty function designed to take into account specific constraints, which preferably include keeping parameter values ​​within a valid range.

[0062] To calculate the color difference, the initial BTF is evaluated at different spectral reflectance measurement geometries, and the resulting CIEL*a*b* values ​​are compared with the CIEL*a*b* values ​​of spectral reflectance measurements using a weighted color difference formula (such as the formula defined in DIN6157 / 2, for example). The parameters of the intensity function are optimized using a nonlinear optimization method (such as the Nelder-Mead-Downhill-Simplex method, for example) to minimize the cost function.

[0063] When optimizing the color table, a correction vector is determined for each spectral reflectance measurement geometry of the spectrophotometer. The correction vectors result in the differences in reflected radiance and spectral reflectance data in the RGB channels of the BRDF portion of the initial BTF for the same geometry. The calculation of the correction vectors is performed in the CIEL*a*b* color space. The resulting correction vectors are then interpolated component-wise across the entire parameter range of the color table.

[0064] According to another embodiment, the first and second optimization steps are repeated / iterated to further improve the accuracy of the optimized BTF. The number of iterations can be specified and predefined. It has been found that three iterations can produce reliably good results.

[0065] Optimized BTF has been found to be more accurate than the initial BTF obtained directly from camera-based devices. This applies not only to a small (limited) number) of spectral reflectance geometries that provide additional spectral reflectance data, but also to a full range of lighting and viewing directions.

[0066] Two-way texture functions (BTFs) for system-generated objects can be used, including the following:

[0067] - A camera-based measurement device configured as the initial BTF of the object being measured.

[0068] - A spectrophotometer configured to capture spectral reflectance data from a predetermined number of objects with different measurement geometries.

[0069] - A computing device, which is communicatively connected to a camera-based measurement device and a spectrophotometer, and is configured to receive the initial BTF and captured spectral reflectance data of the object via the respective communication connections, and to adapt the initial BTF to the captured reflectance data to obtain an optimized BTF.

[0070] The system may further include a database configured to store an initial BTF, spectral reflectance data for a pre-given number of different measurement geometries, and an optimized BTF. A computing device can communicatively connect to the database to acquire the initial BTF and spectral reflectance data for the objects for a pre-given number of different measurement geometries and to store the optimized BTF. This means that the initial BTF obtained from the camera-based measurement device and the spectral reflectance data captured by the spectrophotometer can be stored in the database before the computing device acquires the initial BTF and spectral reflectance data to adapt the initial BTF to the captured reflectance data, thus obtaining the optimized BTF. In this scenario, the camera-based measurement device and the spectrophotometer are also communicatively connected to the database. Therefore, the communication connection between the computing device and the camera-based measurement device, and the communication connection between the computing device and the spectrophotometer, can both be direct connections or indirect connections via the database. Each communication connection can be wired or wireless. Each suitable communication technology can be used. The computing device, the camera-based measurement device, and the spectrophotometer may each include one or more communication interfaces for communicating with each other. This communication can be performed using wired data transmission protocols such as Fiber Distributed Data Interface (FDDI), Digital Subscriber Line (DSL), Ethernet, Asynchronous Transfer Mode (ATM), or any other wired transmission protocol. Alternatively, communication can be conducted wirelessly via a wireless communication network using any of a variety of protocols, such as General Packet Radio Service (GPRS), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access (CDMA), Long Term Evolution (LTE), Wireless Universal Serial Bus (USB), and / or any other wireless protocol. The corresponding communication can be a combination of wireless and wired communication.

[0071] The computing device may include one or more input units or be able to communicate with one or more input units, such as touch screens, audio inputs, motion inputs, mice, keypads, etc. Furthermore, the computing device may include one or more output units or be able to communicate with one or more output units, such as audio outputs, video outputs, screen / display outputs, etc.

[0072] This disclosure also relates to computer systems, including:

[0073] -Computer unit;

[0074] - A computer-readable program having program code stored in a non-transitory computer-readable storage medium, wherein when the program is executed on a computer unit, the program code causes the computer unit to:

[0075] A representation of the object is generated using the object's bidirectional texture function (BTF), which accurately reproduces the object's optical appearance.

[0076] This representation is generated using a rendering engine.

[0077] In one embodiment, the representation of the object is generated using a 3D rendering engine.

[0078] In another embodiment, the representation of the object is generated using a real-time rendering engine. The 3D (real-time) rendering engine simulates light propagation in a virtual 3D light scene, taking into account the optical (reflective) properties of the materials present in the scene. A mathematical graphical model describes the optical properties of the materials.

[0079] In one embodiment, the computer system includes a specific program called a shader for interpreting the BTF. In another embodiment, the system includes an importer and shaders for a rendering engine.

[0080] An importer is a software application that reads a data file or metadata information in one format and converts it to another format using a special algorithm (such as a filter). The importer itself is usually not a complete program but rather an extension of another program implemented as a plugin. When implemented in this way, the importer reads data from a file and converts it to the native format of the managed application. The importer's role is to read information about the BTF from a file and provide it to the shader. If the shader uses a restricted BTF model, the importer must translate the parameters of the full BTF model into the restricted model. This is done by optimizing the parameters of the restricted model so that the reflectivity of a set of measurement geometries is as similar as possible to the reflectivity of the full BTF model. In one embodiment of a computer system, the importer is configured to read the BTF from a file and translate the BTF parameters into parameters of the texture functions used by the shader.

[0081] The importer reads information from the BTF and provides it to specific shaders in the rendering engine. If the shader cannot interpret the full graphics model contained in the BTF and uses a simplified model, the importer needs to translate the information in the BTF.

[0082] Shaders are a type of computer program originally used for shading in 3D scenes (producing appropriate levels of lightness and color in a rendered image), but now they perform a variety of specialized functions in various areas within the category of computer graphics effects. Beyond simple lighting models, more complex uses of shaders include: altering the hue, saturation, brightness (HSL / HSV), or contrast of an image; producing blur, halos, volumetric lighting, normal mapping (for depth effects), bokeh, cel shading, split toning, bump mapping, distortion, chroma keying (for so-called "blue screen / green screen" effects), edge and motion detection, and psychedelic effects.

[0083] Shaders describe the characteristics of vertices or pixels. Vertex shaders describe the properties of vertices (position, texture coordinates, color, etc.), while pixel shaders describe the characteristics of pixels (color, z-depth, and alpha value). The vertex shader is called for each vertex in the primitive (possibly after tessellation); thus, one vertex is inside, and a (updated) vertex is outside. Each vertex is then rendered as a series of pixels on the surface (memory block) that will eventually be sent to the screen.

[0084] In the scenario described in this disclosure, fragment shaders are primarily used. Only special code for vertex shaders is required, as vertex shaders must prepare some input data for fragment shaders.

[0085] Fragment shaders calculate the color and other properties of each "fragment": a rendering unit that affects at most a single output pixel. The simplest type of fragment shader outputs a color value for a single screen pixel; more complex shaders with multiple inputs / outputs are also possible. Fragment shaders range from simply always outputting the same color, to applying lighting values, to performing bump mapping, shadows, specular highlights, translucency, and other effects. They can vary the fragment's depth (for Z-buffering) or output more than one color if multiple render targets are active.

[0086] The vertex shader runs once for each vertex provided to the graphics processor. Its purpose is to transform the 3D position of each vertex in virtual space into its 2D coordinates on the screen (and a depth value in the Z-buffer). Vertex shaders can manipulate properties such as position, color, and texture coordinates, but cannot create new vertices.

[0087] In one embodiment, the computer system provides plugins for at least one rendering software application (such as V-ray or LuxCoreRender) and / or at least one game engine (such as Unreal Engine or Unity).

[0088] In one embodiment, the computer system provides a plugin for Unity. Unity is a cross-platform game engine developed by Unity Technologies. This engine can be used to create 3D, 2D, virtual reality, and augmented reality games, as well as simulations and other experiences. The engine has been adopted by industries beyond video games, such as film, automotive, architecture, engineering, and construction.

[0089] A computer system may include one or more output units or be able to communicate with one or more output units, such as video output, screen / display output, artificial reality (AR) or virtual reality (VR) output, etc.

[0090] Embodiments of the present invention can be used with or incorporated into a computer system, which may be a standalone unit or include one or more remote terminals or devices communicating with a central computer located, for example, in the cloud, via a network such as, for example, the Internet or an intranet. Therefore, the computing devices and related components described herein may be part of a local computer system or a remote computer or online system, or a combination thereof. The databases and software described herein may be stored in the computer's internal memory or in a non-transitory computer-readable medium.

[0091] Other aspects of the invention will be realized and obtained through the elements and combinations particularly described in the appended claims. It should be understood that this description is exemplary and explanatory only and does not limit the invention as described.

Claims

1. The use of a bidirectional texture function (BTF) for an object in the design and development of automobiles, vehicle parts, a piece of furniture, a piece of clothing, or packaging materials, characterized in that... The BTF has been generated by a method that includes at least the following steps: - Measure the initial BTF of the object using a camera-based measurement device; - Use a spectrophotometer to capture the spectral reflectance data of the object for a pre-given number of different measurement geometries; - By using the formula (1) of the initial BTF: （1） Segmented into the first item Second item The first item is further divided into a first sub-item corresponding to a color table that depends on the lighting and viewing direction. and the second sub-term corresponding to the intensity function. The optimization process minimizes the color difference between the captured spectral reflectance data and the initial BTF by optimizing the parameters of the first sub-item in the first optimization step while keeping the parameters of the second sub-item unchanged, and by optimizing the parameters of the second sub-item in the second optimization step while keeping the parameters of the first sub-item unchanged, thereby obtaining an optimized BTF that adapts the initial BTF to the captured spectral reflectance data. in, Let be the surface coordinates of the object. The lighting and viewing / observation direction for the base coating of the object. : For color charts that depend on lighting and viewing direction, Albedo or diffuse reflectance Let k be the k-th Cook-Torrance lobe, which corresponds to the bidirectional reflectance distribution function (BRDF) describing the gloss of the microfacet surface. The weights of the k-th Cook-Torrance lobe are... Let be the parameters of the Beckman distribution of the k-th Cook-Torrance lobe. Let be the Fresnel reflectance of the k-th Cook-Torrance lobe. A spatial texture image table that depends on lighting and viewing direction.

2. The use according to claim 1, wherein, The BTF is used to design vehicle components that match the optical appearance of components adjacent to the vehicle component in the finished vehicle.

3. The use according to claim 1, wherein, The BTF is used in the car configurator.

4. The use according to claim 1 or 3, wherein, The BTF is used in the scenario of an automobile sales platform.

5. A computer system, comprising: - Computer unit; - A computer-readable program having program code stored in a non-transitory computer-readable storage medium, wherein when the program is executed on the computer unit, the program code causes the computer unit to: A representation of the object is generated using the object's bidirectional texture function (BTF), which accurately reproduces the object's optical appearance at a given illumination. The representation is generated using a rendering engine. The BTF has been generated by a method including at least the following steps: The initial BTF of the object was measured using a camera-based measurement device. The spectral reflectance data of the object are captured using a spectrophotometer for a pre-given number of different measurement geometries; By using the formula (1) of the initial BTF: （1） Segmented into the first item Second item The first item is further divided into a first sub-item corresponding to a color table that depends on the lighting and viewing direction. and the second sub-term corresponding to the intensity function. The optimization process involves minimizing the color difference between the captured spectral reflectance data and the initial BTF by optimizing the parameters of the first sub-item in a first optimization step while keeping the parameters of the second sub-item unchanged, and by optimizing the parameters of the second sub-item in a second optimization step while keeping the parameters of the first sub-item unchanged, thereby obtaining an optimized BTF that adapts the initial BTF to the captured spectral reflectance data. in, Let be the surface coordinates of the object. The lighting and viewing / observation direction for the base coating of the object. For color charts that depend on lighting and viewing direction, Albedo or diffuse reflectance Let k be the k-th Cook-Torrance lobe, which corresponds to the bidirectional reflectance distribution function (BRDF) describing the gloss of the microfacet surface. The weights of the k-th Cook-Torrance lobe are... Let be the parameters of the Beckman distribution of the k-th Cook-Torrance lobe. Let be the Fresnel reflectance of the k-th Cook-Torrance lobe. For spatial texture image tables that depend on lighting and viewing direction, And the object is a car, a vehicle part, a piece of furniture, a piece of clothing, or packaging material.

6. The computer system according to claim 5, wherein, The rendering engine is a 3D rendering engine.

7. The computer system according to claim 5 or 6, wherein, The rendering engine is a real-time rendering engine.

8. The computer system of claim 5 or 6, further comprising a shader for the rendering engine.

9. The computer system according to claim 8, wherein, The shaders include fragment shaders and vertex shaders.

10. The computer system of claim 8, further comprising an importer for the shader.

11. The computer system according to claim 10, wherein, The importer is configured to read the BTF from a file and translate the parameters of the BTF into parameters of the texture function used by the shader.

Images