# Approximate Image-Space Multidirectional Occlusion Shading Model for Direct Volume Rendering
## 2 Related Work
[25] proposed a volume-based ambient occlusion model. They presented a view-independent approach by calculating ambient occlusion using normal and depth buffers with multiple volume textures. [PERSON] et al. [26] showed an enhanced visualization using a proxy image where a depth proxy was used to increase the visual cues. A directional occlusion shading model was proposed by [PERSON] et al. [2], who presented a model for ambient occlusion within the slice-based direct volume rendering. The model was designed with conical subsets by positioning a light in front of the camera. As an extension to this work, [PERSON] et al. [3] presented a multidirectional occlusion with elliptical kernels. However, these two models worked only with the slice-based direct volume rendering and could not be applied to other volume rendering systems, such as ray casting. Global illumination models, such as shadows [27], have been studied in many areas, including surface rendering and direct volume rendering. [PERSON] et al. [27] presented an efficient shadow rendering model with precomputed shadow maps for the direct volume rendering. Their approach required extra shadow textures to generate the final shadow volume rendering. [PERSON] et al. [28] employed an exponential shadow map to achieve fast shadow renderings with fewer artifacts. [PERSON] and [PERSON] [29] proposed a 4D light transport model to decouple the illumination computation from the isosurface generation. Recently, ambient volume scattering [30] has been proposed to simulate scattering on mesoscopic scales in direct volume rendering, and an advanced lighting design [31] for the global illumination in volume rendering was presented. However, these approaches do not interactively reflect the changes of volume rendering settings, such as the transfer function and lighting properties. [PERSON] and [PERSON] [32] extended their previous paper [31] to compute the light volume more interactively with CUDA and achieved faster global illumination. However, the light volume size affected the computation speed exponentially, and the technique did not support interactive hard shadows. Another important class of illumination techniques includes screen-space shading models [4; 5; 6; 7; 8; 9; 33] that have been widely used in the polygonal surface rendering and gaming industry. However, they have not been applied to volume rendering. [PERSON] et al. [7] presented unsharp masking of the depth buffer to enhance the perceptual quality of images. Simple approximations of the ambient occlusion in the screen-space were proposed in various polygonal surface renderings [4; 5; 6]. These approaches compute the occlusion based on the depth and normal by sampling test points inside a hemisphere centered at each pixel. The rendering performance was excellent, but there were some limitations, such as the poor quality due to the undersampling, data quantization, and missing values. In order to remedy the problems, they proposed randomized sampling and blurring techniques. [PERSON] et al. [8] introduced an interactive multiresolution image-space rendering model for the dynamic area lighting. [PERSON] and [PERSON] [33] applied dynamic height fields for the self-shadowing. [PERSON] et al. [9] presented a screen-space directional occlusion model generalized from the screen-space ambient occlusion. They accounted for the direction of the incoming light and included one bounce of indirect illumination with minor extra computing time. The occlusion was computed with the visible directions only, in contrast to the screen-space ambient occlusion, but it was still based on a hemisphere at each pixel.