A Research On Matting And Compositing Of Transparent Object And Its Shadow

It is still a hot research field in computer graphics that how to synthesize realistic results of transparent object under various environments and illuminations. Compared to the result of common opaque object, it is more complicate to render this result because of the special properties of transparent material, which may contain many different effects such as reflection and refraction effects, caustic shadow effects and specular results.In traditional computer graphics research, ray-tracing or photon mapping algorithms are always used to render this kind of result if we know the model and material of object. Actually this method has certain defects and limitations: firstly, it requires the precise model and property of object while 3-D scanning techniques can only obtain the accurate geometry shape and are not quite suitable to acquire material property and additional complicated appearances, especially complex specular property and reflection and refraction property; Secondly, consider the difference of manufacture craft and raw material, natural optic appearances inside the object are so complicated and can not be exactly simulated by existing rendering algorithms, so many artifacts are brought forth in the result images; lastly under the prerequisites of model and material property, it can not be wildly used in various practical applications because they use already known model and change the material property to verify the algorithms and render the results. For these reasons, the realistic result of real transparent object in arbitrary illuminations can not be well rendered by current algorithms.In view of many limitations of current techniques, we combine the environment matting method, image-based render method and image-based relighting algorithm and introduce a new render algorithm to synthesize the result of transparent object under arbitrary scene and light condition. Our main contributions include:(1) Based on the algorithm, we design a completed method and capture procedure, then manufacture an automatic light stage to acquire the basis images of our algorithm.(2) Using environment matting algorithm to synthesize the reflection and refraction effects of transparent object. Because of the difficulty to acquiring the model of real transparent object and the complexity of natural optic appearances inside the object, these effects can not be well simulated by current model-based render methods. In this paper, we use environment matting algorithm to avoid complicated modeling and rendering process, extract and composite matting data of object in radiance space and then obtain more accurate and realistic results. (3) Using image-based relighting algorithm to calculate accurate specular properties of transparent object under different light conditions. Highlight effects have close correlation with many factors include light condition, object model, self-occlusion and light refraction and so on. Consider the rendering complexity, many previous researches and rendering results always neglect the calculation process of specular effects. In this paper, we first extract the specular information of object under different illuminations in several basis images and then calculate specular effect of object under new arbitrary illumination by relighting algorithm.(4) Using shadow matting method to synthesize realistic caustic shadow effect of transparent object. Knowing precise object model, current techniques always do many researches to simulate light transportation process and then calculate this special kind of shadow. But their result are not accurate enough because of the nondetermi-nacy of light transportation. In this paper, according the shadow matting method, through capturing several images of transparent object and caustic shadow under certain illuminations we can extract the according shadow matting data after color space conversion and then synthesize the target caustic shadow of object under new arbitrary environment and illumination.In our algorithm, the environment matting data, specular property and shadow matting data are extracted accurately and treated as intermediate results. Therefore, using this method, we can directly edit these intermediate results and extend our method to generate many special effects easily. In this paper we have done some attempts in this aspect and generate some special effects described as follows:(1) By directly editing the environment matting data and shadow matting data,we can calculate the new matting data and specular information when the refractive index of transparent object changes and then synthesize the fade-in and fade-out effect of transparent object under arbitrary real environment and scene.(2) We first analyzing the shadow matting data and calculate the inner silhouette structure information of caustic shadow, then interpolate the caustic shadow to simulate the dynamic effect that shadow can vary according to changeable light position and direction, guaranteeing that the inner structure and texture of caustic shadow maintain consistently. Using this method, we can make up and solve the problem caused by lower capture resolution.