Many lighting methods used in computer graphics such as indirect illumination can have very high computational costs and need to be approximated for real-time applications. These costs can be reduced by means of upsampling techniques which tend to introduce artifacts and affect the visual quality of the rendered image. This paper suggests a versatile approach for accelerating the rendering of screen space methods while maintaining the visual quality. This is achieved by exploiting the low frequency nature of many of these illumination methods and the geometrical continuity of the scene. First the screen space is dynamically divided into separate sub-images, then the illumination is rendered for each sub-image in an adequate resolution and finally the sub-images are put together in order to compose the final image. Therefore we identify edges in the scene and generate masks precisely specifying which part of the image is included in which sub-image. The masks therefore determine which part of the image is rendered in which resolution. A step wise upsampling and merging process then allows optically soft transitions between the different resolution levels. For this paper, the introduced multi-resolution rendering method was implemented and tested on three commonly used lighting methods. These are screen space ambient occlusion, soft shadow mapping and screen space global illumination.
@cite_20: Global illumination provides a visual richness not achievable with the direct illumination models used by most interactive applications. To generate global effects, numerous approximations attempt to reduce global illumination costs to levels feasible in interactive contexts. One such approximation, reflective shadow maps, samples a shadow map to identify secondary light sources whose contributions are splatted into eye space. This splatting introduces significant overdraw that is usually reduced by artificially shrinking each splat's radius of influence. This paper introduces a new multiresolution approach for interactively splatting indirect illumination. Instead of reducing GPU fill rate by reducing splat size, we reduce fill rate by rendering splats into a multiresolution buffer. This takes advantage of the low-frequency nature of diffuse and glossy indirect lighting, allowing rendering of indirect contributions at low resolution where lighting changes slowly and at high-resolution near discontinuities. Because this multiresolution rendering occurs on a per-splat basis, we can significantly reduce fill rate without arbitrarily clipping splat contributions below a given threshold-those regions simply are rendered at a coarse resolution.
Nichols and Wyman @cite_20 describe a real-time technique for rendering indirect illumination using multi-resolution splatting. They use min-max mipmaps to find the discontinuities in the geometry. Using these discontinuities, the image space is hierarchically divided into smaller squares, so that areas with higher-frequency components obtain a finer resolution. After the image is completely split into such splats' of an appropriate size, the indirect illumination is rendered in all resolutions and the layers are then combined by upsampling to produce the final image. Our technique differs from the algorithm presented by Nichols and Wyman among other things in the method used to decide which resolution to render in. We can apply more flexible filters depending on the situation, while their approach using min-max mipmaps can only find geometric discontinuities. We also use a different approach to combine the final images that prevents visible artifacts. Finally, our technique is not only specialized for indirect illumination using Reflective Shadow Maps, but can also be applied and optimized for various lighting effects due to its high flexibility.