At present, a variety of global illumination algorithms can greatly improve the realism of 3D rendered images, and are widely used in the field of lighting engineering. However, since global illumination requires a lot of computation, these algorithms are generally limited to non-real-time lighting calculations and output still images in applications that require real-time rendering (such as virtual scene browsing), in order to achieve realistic lighting effects, The static output of global illumination is often used as the background scene, which can improve the accuracy and visual effect of real-time rendering to some extent. However, real-time applications lack good interactivity due to the use of a large number of pre-rendered objects that cannot be changed. On the other hand, some lighting algorithms l1~2 in recent years can process some direct illumination caused by complex light sources at near real-time speed, effectively improving the accuracy of illumination calculation, but still lack real-time processing of indirect illumination. Some global illumination effects have also been extensively studied: He-drich et al. separate the light source from the rendering process for preprocessing. * For the sake of brevity, x and will be omitted from the following discussion without confusing. The angle of the parameter is %. For a, we can derive a result and store the result in the Lumigraph; Soler et al. propose some effective methods for calculating the shadow, which can be used to calculate the shadow between objects and within the object separately; Debevec Radiance Map is used to implement ambient lighting. These methods can be applied to some real-time lighting calculations in specific situations, but are still difficult to handle in environments with complex lighting.
The method herein uses two volume data to calculate direct illumination and indirect illumination in real time: the data corresponding to the direct illumination is called the irradiance distribution, and is used to record the irradiance generated by the light source in a specific space; corresponding to indirect illumination The data is called a nuclear distribution and is used to record mutual reflection data between objects in the scene.
2 irradiance distribution function and irradiance reconstruction First, a simple calculation of several physical quantities gives a calculation method for irradiance reconstruction.
Radiance is a basic physical quantity in light measurement. It represents the density of light energy in a certain direction through a certain fixed point in unit solid angle. The unit is W/(sr*m3) light in three-dimensional space. The distribution can be completely traversed by the radiance distribution function L(x, which gives the radiance in any direction through any point X in space. Sometimes, a point on the surface of the object is received in unit time. The light energy density is more useful. The physical quantity is called the irradiance, and the unit is W/m2. The irradiance E corresponds to the integral of the incident radiance L projected on the bottom surface of the unit hemispherical surface, where Q(w) is the bottom direction. For the unit hemisphere of w, *0 is the angle between w and the direction of the incident radiance, and the integral element is the differential solid angle in the direction. We generalize the function E from the solid surface to the entire space and call it the irradiance distribution. Function. Like L, E is also a five-dimensional function, but the function has two useful properties. The final observation is easily obtained by calculation of E, which has a very good convolution in the continuity. Fa Lashi The shadow is generated. In this, Soler et al. pointed out that the illumination calculation of the occluded object can be approximately decomposed into the product of two independent functions, namely the illumination function G and the visibility function V. The shape of the shadow can be approximately written as V. The product of the projected image of the light source and the projected image of the occlusion has a method of quickly calculating the shadow, and the final lighting effect can be synthesized according to the following flow.
Stepl. Calculate the direct illumination generated by the light source, using the irradiance distribution to calculate in real-time calculations; StepZ projection shadows (using the convolution method described above) combined with the results of direct illumination, * Step3. In Stepl, Step2 Calculate the indirect diffusion based on the calculation results (see Section 4, Step 4. Every time an active object (including a dynamic light source) changes, the weight 4 uses the nuclear distribution to calculate the indirect diffusion in real time for some lighting design applications. Shooting is a very important part of lighting, especially in indoor lighting environments. Most scene lighting comes from indirect diffusion. This form of lighting is sometimes called colorbleeding effect. In the plane of concern Under the assumption of the Lambert model, the HL, K direction parameter w and the definition of the superposition K* can be omitted. In the rendering equation, the superposition Kn(x, j) has a significant physical meaning: starting from point x, The n-step propagation rate of the light energy attenuation reaching point j. In particular, if K and K are discretized (that is, a series of sampling points are taken for x and J), the matrix form is written, and the relationship between the two is pre-solution. R is defined as the function that characterizes the proportion of light energy attenuation from point x through any possible path to point j. Note that R always converges for rendering equations with practical physical meaning. With pre-solving After that, the rendering equation has a solution of the following form. It can be seen from the form of this solution that R is independent of the distribution Le of the light source and is only related to the relative position between the surfaces of the object. This shows that we can separate the light source and calculate the space between the objects separately. Mutual reflection information. In the process of preprocessing, calculating the mutual reflection information in the scene is actually calculating the Kn term 4.2 kernel distribution of n>2 in R and using the S point c on the surface of the L9 object in the direction of the product. Ubg uses its own global illumination, and the pre-resolving kernel can be faster. Although equations (6) and (7) give a method for calculating the pre-solving kernel, the calculation method of this large matrix has more redundant information and calculation. The amount is calculated enormously: Stepl. Use the spatial division method (such as octree) to divide the scene space, and set the spatial node containing the surface of the object as the sampling point. The irradiance proposed in dianceMap does not improve efficiency, but provides the ability to change illumination for static scenes.
5 Experimental data and results In a specific implementation, the scene to be rendered is divided into two parts: one is the active light source and the object, and a reusable irradiance distribution is required for each such object during preprocessing; Part of it is a relatively static environment. It is necessary to calculate its nuclear distribution in advance and create a single irradiance distribution for interacting with the active object and showing the results of rendering using this method. The middle wall and the painting are stationary objects, the chairs, plants and light sources are moving objects, and the diffusion between the walls is shown in Table 1. The test results for the medium scene are given. It can be seen that the main bottleneck of the calculation is the generation of shadows, where the shadow map uses a resolution of 128x 128, where most of the computation time is consumed in computing the Fourier transform.
Table 1 Partial test results Polygon number Number of vertices processed (no shadow) / s display rate (no shadow) fps display rate (shaded) fps directly used in the preprocessing process to calculate the irradiance distribution when sampling accuracy At 40x40x20, the calculation time varies from a few seconds to a dozen seconds depending on the complexity of each light source and object. The kernel distribution is generated by the rendering software RADIANCE. The calculation time of the middle scene is about 40 minutes. In the rendering stage, if the overhead of other graphics operations is not considered, the calculation of the irradiance part can reach 580,000 to 610,000 vertices per second, and calculate the indirect diffusion. The shot can reach 310,000 to 330,000 vertices per second. In general, this processing speed can better meet the requirements of generating various types of rendering lighting in real time, such as dynamically generating Lightmap. The above results were done on a personal computer with a 1GHz processor.
6 Conclusions This article describes how to use volume rendering for fast global illumination. The method generates two volume data-irradiance distributions and a nuclear distribution in the pre-processing process to record the irradiance distribution and the indirect diffusion information. Through these two kinds of volume data, direct and indirect lighting effects brought by various types of light sources can be adjusted and displayed in real time during rendering. This approach has great practical value for many engineering applications that require fast, precise illumination, such as various lighting design software, as well as real-time applications that require more realistic image output, such as virtual reality and entertainment software.
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