A challenge in presenting augmenting information in outdoor augmented reality (AR) settings lies in the broad range of uncontrollable environmental conditions that may be present, specifically large-scale fluctuations in natural lighting and wide variations in likely backgrounds or objects in the scene. In this paper, we present a user-based study which examined the effects of outdoor background textures, changing outdoor illuminance values, and text drawing styles on user performance of a text identification task with an optical, see-through augmented reality system. We report significant effects for all of these variables, and discuss design guidelines and ideas for future work.

Splatting is a popular direct volume rendering algorithm. However, the algorithm does not correctly render cases where the volume sampling rate is higher than the image sampling rate (e.g. more than one voxel maps into a pixel). This situation arises with orthographic projections of high-resolution volumes, as well as with perspective projections of volumes of any resolution. The result is potentially severe spatial and temporal aliasing artifacts. Some volume ray-casting algorithms avoid these artifacts by employing reconstruction kernels which vary in width as the rays diverge. Unlike ray-casting algorithms, existing splatting algorithms do not have an equivalent mechanism for avoiding these artifacts. In this paper we propose such a mechanism, which delivers high-quality splatted images and has the potential for a very efficient hardware implementation.Keywords and Phrases: volume rendering, splatting, direct volume rendering, resampling, reconstruction, anti-aliasing, perspective projection. INTRODUCTIONIn the past several years, direct volume rendering has emerged as an important technology in the fields of computer graphics and scientific visualization, and splatting is one of several popular techniques for direct volume rendering. The majority of images produced through direct volume rendering have used orthographic projections, in part because such projections are useful in many of the application areas (such as biomedical and fluid flow visualization) which have initially motivated work in volume rendering. Perspective projections offer a viewpoint which more naturally correlates to the way we perceive the physical world, and perspective projections are essential when it is desirable to "fly through" the data -flight simulators are one example. A perspective projection of a volume dataset gives another perceptual cue which can be employed when comprehending spatial relationships.Any volume rendering algorithm which supports perspective projections has to deal with the problem of non-uniform sampling produced by diverging viewing rays. If not addressed this can result in potentially severe aliasing artifacts. Although other volume rendering approaches have dealt with this problem (e.g. raycasting [15] and shear-warp [5][6]), to date the problem has not been addressed in the context of splatting. In this paper, we present a modification to the splatting algorithm which prevents the aliasing that arises from this non-uniform sampling. The same type of resampling problems occur with an orthographic projection if the volume resolution is higher than the image resolution (e.g. if many voxels project into each pixel). Our modified splatting algorithm also avoids aliasing in this situation. In the next section we describe the splatting algorithm and related previous work, and then we give some advantages and disadvantages of splatting as compared to other volume rendering techniques. In Section 4 we describe our anti-aliasing technique and argue for its correctness. We follow this with implementation de...

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We present a new rendering technique, termed LOD-sprite rendering, which uses a combination of a level-of-detail (LOD) representation of the scene together with reusing image sprites (previously rendered images). Our primary application is accelerating terrain rendering. The LOD-sprite technique renders an initial frame using a high-resolution model of the scene geometry. It renders subsequent frames with a much lower-resolution model of the scene geometry and texture-maps each polygon with the image sprite from the initial high-resolution frame. As it renders these subsequent frames the technique measures the error associated with the divergence of the view position from the position where the initial frame was rendered. Once this error exceeds a user-defined threshold, the technique re-renders the scene from the high-resolution model. We have efficiently implemented the LOD-sprite technique with texture-mapping graphics hardware. Although to date we have only applied LOD-sprite to terrain rendering, it could easily be extended to other applications. We feel LOD-sprite holds particular promise for real-time rendering systems.

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