PCOMB(1)

NAME
pcomb - combine RADIANCE pictures

SYNOPSIS
pcomb [ -h ][ -w ][ -x xres ][ -y yres ][ -f file ][ -e expr ] [ [ -o ][ -s factor ][ -c r g b ] input .. ]

DESCRIPTION
Pcomb combines equal-sized RADIANCE pictures and sends the result to the standard output. By default, the result is just a linear combination of the input pictures multiplied by -s and -c coefficients, but an arbitrary mapping can be assigned with the -e and -f options. The variable and function definitions in each -f source file are read and compiled from the RADIANCE library where it is found. Negative coefficients and functions are allowed, and pcomb will produce color values of zero where they would be negative.

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 (+/- 32 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, and WE gives the white efficacy (lumens/brightness) for pixel values, which may be used with the -o option or the le(n) values to convert to absolute photometric units (see below). 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, and pa(n) gives the corresponding pixel aspect ratio. Finally, for pictures with stored view parameters, 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. In addition, the function T(n) returns the distance from the origin to the aft clipping plane (or zero if there is no aft plane), and the function S(n) returns the solid angle of the current pixel in steradians (always zero for parallel views). If the current pixel is outside the view region, T(n) will return a negative value, and S(n) will return zero. The first input picture with a view is assumed to correspond to the view of the output picture, which is written into the header.

The -h option may be used to reduce the information header size, which can grow disproportionately after multiple runs of pcomb and/or pcompos(1). 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 ('!').

EXAMPLES
To produce a picture showing the difference between pic1 and pic2:

    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' inp.hdr > gam.hdr

To perform some special filtering:

    pcomb -f myfilt.cal -x xmax/2 -y ymax/2 input.hdr > filtered.hdr

To make a picture of a dot:

    pcomb -x 100 -y 100 -e 'ro=b;go=b;bo=b;b=if((x-50)^2+(y-50)^2-25^2,0,1)' > dot

ENVIRONMENT
RAYPATH    the directories to check for auxiliary files.

AUTHOR
Greg Ward

SEE ALSO
getinfo(1), icalc(1), pcompos(1), pfilt(1), rpict(1)