An Improved Scan-Line Algorithm for Rendering Arbitrary Portals

Portal-based rendering as one of the visibility determination methods is widely used in large-scale indoor scene. The main idea behind portal-based rendering is that when a given portal is visible, the cell which is behind the portal is visible, and otherwise it is invisible. In order to simplify the complexity of visibility determination, they always define portal as a planar convex polygon and the usefulness of portals for scene composition is limited. A more useful portal is a general portal which can be an arbitrary polygon or be thick and connect any two independent cells, and we call it arbitrary portal. In this paper, we provide an improved scan-line algorithm, which is based on portal texture for rendering arbitrary portal. First, we simplify an arbitrary portal to rectangle portal. Then, we use ray-casting method to sample the visible area of rectangle portal and get the corresponding portal texture. Thirdly, an improved scan-line algorithm is created to render arbitrary portal correctly by using portal texture. Our focus is to discuss the details of the improved scan-line algorithm. We have devised some tests to verify the correctness of our algorithm. The testing results showed that our system could run smoothly on the general personal computer and render the arbitrary portals correctly.