This paper presents efficient image-based rendering techniques used in the context of an architectural walkthrough system. Portals (doors and windows) are rendered by warping layered depth images (LDIs). In a preprocessing phase, for every portal, a number of pre-rendered images are combined into a LDI. The resulting LDI stores, exactly once, all surfaces visible in at least one of the images used in the construction, so most of the exposure errors are efficiently eliminated. The LDI can be warped in the McMillan occlusion compatible ordering. A substantial increase in performance is obtained by warping in parallel. Our parallelization scheme achieves good load balancing, scales with the number of processors, and preserves the occlusion compatible ordering. A fast, conservative reference-image-space clipping algorithm also reduces the warping effort.