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Test Your Knowledge in Computers #825
Difficulty: Medium
The term API seems to appear for the first time in the article of Ira W. Cotton, Data structures and techniques for remote computer graphics, published in 1968.
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vwrays(1) [bsd man page]

VWRAYS(1)						      General Commands Manual							 VWRAYS(1)

vwrays - compute rays for a given picture or view SYNOPSIS
vwrays [ -i -u -f{a|f|d} | -d ] { view opts .. | picture [zbuf] } DESCRIPTION
Vwrays takes a picture or view specification and computes the ray origin and direction corresponding to each pixel in the image. This information may then be passed to rtrace(1) to perform other calculations. If a given pixel has no corresponding ray (because it is out- side the legal view boundaries), then six zero values are sent instead. The -i option may be used to specify desired pixel positions on the standard input rather than generating all the pixels for a given view. If the -u option is also given, output will be unbuffered. The -f option may be used to set the record format to something other than the default ASCII. Using raw float or double records for exam- ple can reduce the time requirements of transferring and interpreting information in rtrace. View options may be any combination of standard view parameters described in the rpict(1) manual page, including input from a view file with the -vf option. Additionally, the target X and Y dimensions may be specified with -x and -y options, and the pixel aspect ratio may be given with -pa. The default dimensions are 512x512, with a pixel aspect ratio of 1.0. Just as in rpict, the X or the Y dimension will be reduced if necessary to best match the specified pixel aspect ratio, unless this ratio is set to zero. The -pj option may be used to jitter samples. The default value of 0 turns off ray jittering. If the -d option is given, then vwrays just prints the computed image dimensions, which are based on the view aspect and the pixel aspect ratio just described. The -ld switch will also be printed, with -ld+ if the view file has an aft clipping plane, and -ld- otherwise. This is useful for passing options to the rtrace command line. (See below.) If the view contains an aft clipping plane (-va option), then the magnitudes of the ray directions will equal the maximum distance for each pixel, which will be interpreted correctly by rtrace with the -ld+ option. Note that this option should not be given unless there is an aft clipping plane, since the ray direction vectors will be normalized otherwise, which would produce a uniform clipping distance of 1. If a picture is given on the command line rather than a set of view options, then the view and image dimensions are taken from the picture file, and the reported ray origins and directions will match the center of each pixel in the picture (plus optional jitter). If a depth buffer file is given as well, then vwrays computes the intersection point of each pixel ray (equal to the ray origin plus the depth times the ray direction), and reports this instead of the ray origin. The reported ray direction will also be reversed. The inter- pretation of this data is an image of origins and directions for light rays leaving the scene surfaces to strike each pixel. EXAMPLES
To compute the ray intersection points and returned directions corresponding to a picture and its depth buffer: vwrays scene_v2.hdr scene_v2.zbf > scene_v2.pts To determine what the dimensions of a given view would be: vwrays -d -vf myview.vf -x 2048 -y 2048 To generate a RADIANCE picture using rtrace instead of rpict: vwrays -ff -vf view1.vf -x 1024 -y 1024 | rtrace `vwrays -d -vf view1.vf -x 1024 -y 1024` -ffc scene.oct > view1.hdr AUTHOR
This work was supported by Silicon Graphics, Inc. BUGS
Although vwrays can reproduce any pixel ordering (i.e., any image orientation) when given a rendered picture, it will only produce standard scanline-ordered rays when given a set of view parameters. SEE ALSO
rcalc(1), rpict(1), rtcontrib(1), rtrace(1) RADIANCE
1/15/99 VWRAYS(1)

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