{"responseHeader":{"status":0,"QTime":6,"params":{"q":"{!q.op=AND}id:\"95867\"","hl":"true","hl.simple.post":"","hl.fragsize":"5000","fq":"!embargo_tdt:[NOW TO *]","hl.fl":"ocr_t","hl.method":"unified","wt":"json","hl.simple.pre":""}},"response":{"numFound":1,"start":0,"docs":[{"modified_tdt":"2015-11-02T00:00:00Z","thumb_s":"/03/23/0323ad5f2ae227042081dc4e3356fbeb89c65e60.jpg","oldid_t":"compsci 3685","setname_s":"ir_computersa","file_s":"/34/5b/345b98aceda7b3683bed1e61343f650f1de51945.pdf","title_t":"Page 35","ocr_t":"21 4.2 Projection Volume Rendering A volume rendering based on a projection method has b en d v lop d by u [3 ]. The quality of the images produced by using our method is as good as th quality of images produced by using ray-casting methods. However, the difficulties n ount red in ray-casting methods do not exist in our method. Our method is composed of the following steps: At first, the connectivities of cells are calculated by using the method presented in Chapter 3. Then the z-coordinates of vertices in the eye coordinate system are computed according to the viewing position. The visibility orders of cells are decided by using the topological sorting algorithm proposed in [49]. After the visibility orders of cells are determined, cells are rendered one by one in a front-to-rear order. When a cell is rendered, all its four faces are projected onto the image plane by using the standard scan-line algorithm. During the scan-line operation, the function values and the z-coordinates of the vertices are interpolated along the face edges and along the scanlines. Since the cell is a convex polyhedron, all the pixels in the projection area of the cell will be covered twice by the faces of the cell, one by the front faces and the other by the rear faces. Therefore two z-coordinates and two function values are calculated at each pixel inside the area covered by the projection. In the view point of ray-casting methods, these two z-coordinates are the z-coordinates of the points where a ray enters and leaves the cell. They specify a ray segment truncated by the boundaries of the cell. After the faces of a cell have been projected, based on the type of images to be produced, different rendering techniques are used to compute the effects contributed by the ray segment on the pixel. 4.2.1 Semitransparent Cloud Images If a semitransparent cloud image is the target image, some sample points are uniformly selected on the ray segment of a pixel. Function values are interpolated in these points, and are converted into colors and opacities. The colors and the opacities of function values are retrieved from look-up tables. Then the colors are multiplied with the opacities to produce the effective colors at the sample points. The colors of the ray segment are computed by accumulating the effective colors of the sample points based on the compositing formula in Equation 4.1. Finally, the colors of the ray segment are further com posited with the colors of the pixel. An example of the projection method is depicted in Figure 4.2.","restricted_i":0,"id":95867,"created_tdt":"2015-11-02T00:00:00Z","format_t":"application/pdf","parent_i":95941,"_version_":1642982571745738753}]},"highlighting":{"95867":{"ocr_t":[]}}}