The variables ro, go and bo specify the red, green and blue output values, respectively. Alternatively, the single variable lo can be used to specify a brightness value for black and white output. The predefined functions ri(n), gi(n) and bi(n) give the red, green and blue input values for picture n. To access a pixel that is nearby the current one, these functions also accept optional x and y offsets. For example, ri(3,-2,1) would return the red component of the pixel from picture 3 that is left 2 and up 1 from the current position. Although x offsets may be as large as width of the picture, y offsets are limited to a small window (+/- 8 pixels) due to efficiency considerations. However, it is not usually necessary to worry about this problem -- if the requested offset is not available, the next best pixel is returned instead.
For additional convenience, the function li(n) is defined as the input brightness for picture n. This function also accepts x and y offsets.
The constant nfiles gives the number of input files present. The variables x and y give the current output pixel location for use in spatially dependent functions, the constants xmax and ymax give the input resolution, and the constants xres and yres give the output resolution (usually the same, but see below). The constant functions re(n), ge(n), be(n), and le(n) give the exposure values for picture n. Finally, the functions Ox(n), Oy(n) and Oz(n) return the ray origin in world coordinates for the current pixel in picture n, and Dx(n), Dy(n) and Dz(n) return the normalized ray direction. (Note that the ray origin and direction are not computable from all picture files.)
The -w option can be used to suppress warning messages about invalid calculations. The -o option indicates that original pixel values are to be used for the next picture, undoing any previous exposure changes or color correction.
The -x and -y options can be used to specify the desired output resolution, xres and yres, and can be expressions involving other constants such as xmax and ymax. The constants xres and yres may also be specified in a file or expression. The default output resolution is the same as the input resolution.
The -x and -y options must be present if there are no input files, when the definitions of ro, go and bo will be used to compute each output pixel. This is useful for producing simple test pictures for various purposes. (Theoretically, one could write a complete renderer using just the functional language...)
The standard input can be specified with a hyphen ('-'). A command that produces a RADIANCE picture can be given in place of a file by preceeding it with an exclamation point ('!').
pcomb -e `ro=ri(1)-ri(2);go=gi(1)-gi(2);bo=bi(1)-bi(2)' pic1 pic2 > diff
Or, more efficiently:
pcomb pic1 -s -1 pic2 > diff
To precompute the gamma correction for a picture:
pcomb -e `ro=ri(1)^.4;go=gi(1)^.4;bo=bi(1)^.4' pic > pic.gam
To perform some special filtering:
pcomb -f myfilt.cal -x xmax/2 -y ymax/2 input.pic > filtered.pic
To make a picture of a dot:
pcomb -x 100 -y 100 -e `ro=b;go=b;bo=b;b=if((x-50)^2+(y50)^2-25^2,0,1)' > dot
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