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Show 61 Table 7.1. Summary of shading method execution times. Case Triangle Count Form Factors Matrix Solution Ray Tracing Boxes 1218 1:56 0:04 0:41 Vase 1170 2:54 0:02 2:32 Cylinder 1800 4:39 0:04 1:46 1596 primary sample rays were fired per triangle. Execution times are in hours and minutes on Vax 8600, with approximately 17 million bytes of physical memory available for program execution. 7.5 Image Dynamic Range Some test cases produced images with wide dynamic ranges which were difficult to display effectively. The problem resulted when visually interesting features were located in widely separated intensity ranges of a single image. Several attempts were made to alleviate the problem, including intensity scaling with truncation, and gamma correction in both the frame buffer look-up tables and the floating point image representation. None of these approaches achieved good results. Occasionally, image intensities were scaled up to bring out features in low intensity ranges. Pixel intensity values were truncated at a maximum value for display. An occasional side effect of the drastic intensity scaling and truncation was the production of aliasing artifacts. This was especially true when a narrow filter width (one pixel wide or less) was used for filtering intensity samples for a given pixel. To alleviate this problem, the dynamic range can be reduced in a different manne: r. Instead of truncating values above a specified maximum, a smooth curve can be used to map image values into a specified range. Alternatively, homomorphic image processing techniques [32] could be used for dynamic range compression and contrast enhancement. |
