Smaller than Pixels: Rendering Millions of Stars in Real-Time

Kurzfassung

Many applications need to display realistic stars. However, rendering stars with their correct luminance is surprisingly difficult: Usually, stars are so far away from the observer, that they appear smaller than a single pixel. As one can not visualize objects smaller than a pixel, one has to either distribute a star's luminance over an entire pixel or draw some kind of proxy geometry for the star. We also have to consider that pixels at the edge of the screen cover a smaller portion of the observer's field of view than pixels in the centre. Hence, single-pixel stars at the edge of the screen have to be drawn proportionally brighter than those in the centre. This is especially important for virtual-reality or dome renderings, where the field of view is large. In this paper, we compare different rendering techniques for stars and show how to compute their luminance based on the solid angle covered by their geometric proxies. This includes point-based stars, and various types of camera-aligned billboards. In addition, we present a software rasterizer which outperforms these classic rendering techniques in almost all cases. Furthermore, we show how a perception-based glare filter can be used to efficiently distribute a star's luminance to neighbouring pixels. Our implementation is part of the open-source space-visualization software CosmoScout VR.