SYNOPSIS

vconvert [-repn repn] [-option ...] [infile] [outfile]

DESCRIPTION

vconvert converts Vista images to a specified pixel representation. The images are read from an input file, converted to the representation specified by the -repn command-line option, then written to an output file.

vconvert knows several ways of mapping input pixels to output pixels. You can choose the mapping you want by means of a -map command line option. If you don't specify that option, however, then you'll get a default mapping that is usually appropriate. The default mapping is one that retains an image's appearance as accurately as possible --- i.e., a particular shade of gray in the input image will be mapped to nearly the same shade in the output image. Alternatively, you can use -map to specify some other mapping that may change the brightness or contrast of an image while converting it to its new representation.

The following describes, in some detail, the various mappings supported by vconvert. Here the full range of pixel values capable of being represented in the source image is denoted [Smin,Smax]; the actual range of pixel values present in the source image is denoted [Amin,Amax]; and the full range of pixel values capable of being represented in the destination image is denoted [Dmin,Dmax]. (For float and double pixels, the ranges [Smin,Smax] and [Dmin,Dmax] are taken to be [-1,+1].) Define Sabs = max ( |Smin|, |Smax| ), and similarly define Aabs. Then the various mappings supported by vconvert are:

range

This is vconvert's default mapping, which preserves the appearance of an image. It maps [-Sabs,Sabs] to [-Dmax,Dmax]. For example, a ubyte image is converted to an sbyte image by dividing each pixel value by 2.

copy

This copies pixels unchanged without scaling them according to their new representation. It maps [-Sabs,Sabs] to [-Sabs,Sabs]. For example, a ubyte image is converted to an sbyte image by copying pixel values that are less than or equal to 127, and setting to 127 those that are greater than 127.

opt1

This enhances the contrast of an image while increasing all intensity levels by a constant multiple. It maps [-Aabs,Aabs] to [-Dmax,Dmax].

opt2

This enhances the contrast of an image. It not only increases intensity levels by a constant multiple, it also shifts the range of intensity levels by subtracting a constant term. The exact mapping depends on the values of Amin and Amax:

when Amin = Amax < 0

each destination pixel is Dmin.

when Amin = Amax = 0

each destination pixel is 0.

when 0 < Amin = Amax

each destination pixel is Dmax.

when Amin < Amax < 0

it maps [Amin,Amax] to [Dmin,-e].

when Amin < Amax = 0

it maps [Amin,0] to [Dmin,0].

when Amin < 0 < Amax

it maps [-Aabs,Aabs] to [-Dmax,Dmax].

when 0 = Amin < Amax

it maps [0,Amax] to [0,Dmax].

when 0 < Amin < Amax

it maps [Amin,Amax] to [e,Dmax].

Here, e is 1 / (Amax - Amin + 1) if destination pixels are float or double, and it is 1 otherwise.

opt3

This enhances the contrast of an image but, unlike opt1 and opt2 it doesn't necessarily preserve the signs of pixel values. The exact mapping depends on the values of Amin and Amax:

when Amin = Amax < 0

each destination pixel is Dmin.

when Amin = Amax = 0

each destination pixel is 0.

when 0 < Amin = Amax

each destination pixel is Dmax.

when Amin < Amax

it maps [Amin,Amax] to [Dmin,Dmax].

linear

This is an arbitrary mapping of the form:

        destination-pixel := source-pixel * a + b

where the values a and b are specified by command line options.

When floating point pixel values are converted to an integer representation each is rounded to the nearest integer after being mapped. Also, if a destination pixel value falls outside the range [Dmin,Dmax], it is set to Dmin or Dmax and a warning message is issued.

COMMAND LINE OPTIONS

vconvert accepts the following options:

-help

Prints a message describing options.

-in infile

Specifies the Vista data file containing the images to be converted.

-out outfile

Specifies where the converted images should be written as a Vista data file.

-repn bit | ubyte | sbyte | short | long | float | double

Specifies the pixel representation to which images should be converted. Default: ubyte.

-map range | copy | opt1 | opt2 | op3 | linear

Specifies the mapping of input pixel values to output pixel values. Default: range.

-a a and -b b

Specify the values a and b to be used when -map linear is the chosen mapping.

Input and output files can be specified on the command line or allowed to default to the standard input and output streams.

AUTHOR

Art Pope <[email protected]>