IMPOST makes a hardcopy print of an image in PostScript format. It is designed to be used just like the TV command, with very similar keywords. You can include extra information on the picture with the TITLE, AXES, COMMENT, INFO and related keywords. These are discussed below.
IMPOST creates a PostScript file on disk. This file will be located in the current directory (either where you started VISTA or under Unix versions of VISTA, the last directory you CD'd to). By default this file will take the name ;SPMquot;image.ps;SPMquot;. You can specify your own name for the file by using the FILE= or OUT= keywords. Currently, this file is not automatically printed. From outside of VISTA (or from a spawned subprocess or C-shell), submit this to your printer as you would submit any other PostScript file (e.g.,: ;SPMquot;lpr image.ps;SPMquot; on a Unix system, or ;SPMquot;$ print/queue=pslaser image.ps;SPMquot; on a VMS system).
Multiple copies can be made using the COPIES=n keyword. The POSITIVE keyword makes white images on a black background; the default is to make black images on a white background.
Output from IMPOST, if prepared with care, is of the highest quality. IMPOST generated prints made on good quality paper or special ;SPMquot;stat;SPMquot; paper have been accepted for publication without re-photography required.
<#2871#>Intensity Scaling<#2871#>
IMPOST produces a hardcopy of an image by representing pixel intensities as an 8-bit halftone gray scale. The optional keyword ;SPMquot;FOUR;SPMquot; will use a 4-bit (16-level) gray scale instead. Although an 8-bit gray scale has 256 levels from white to black, on standard 300 dpi laser printers, only 32 levels are distinguishable. The output is a PostScript file which may be sent to any standard PostScript hardcopy device.
The simplest intensity scaling is through use of the Z= and L= keywords
like in the TV command. These set the zero point (Z=ZERO) and intensity
span (L=SPAN) of a ;SPMquot;linear;SPMquot; mapping from the actual image intensity in each
pixel, IMG(r,c), to a displayed gray scale value between 0 and 255,
INTENS(r,c), following the formula:
In this simple linear mapping, pixels with IMG(r,c) ;SPMlt;= ZERO will set
INTENS(r,c)=0 (white) and pixels with IMG(r,c)=ZERO+SPAN will set
INTENS(r,c)=255 (black). If the CLIP keyword is used, then all values of
IMG ;SPMgt; ZERO+SPAN are set black, otherwise, the color scale ;SPMquot;wraps;SPMquot; and above
IMG=ZERO+SPAN, INTENS starts again at 0. This continues modulo (255/SPAN)
until all pixel intensity values are mapped.
The default mapping is an inverse (or ;SPMquot;negative;SPMquot;) scale, where the image is
drawn as black against a white background. If the optional keyword
POSITIVE is given, then the intensities are mapped in a more ;SPMquot;normal;SPMquot; or
;SPMquot;positive;SPMquot; scale in which the image is displayed as white against a black
background, though the results with most laser printers is less than
optimal.
The second scaling option is to map image intensities into an 8-bit gray
scale using the distribution of pixel intensities in the image to provide a
scaling in which all gray levels are equally present. Such a scaling is
called a ;SPMquot;Histogram Equalization;SPMquot;, and is used to bring up faint features
without sacrificing more high-intensity features in the same image. This
is done using the HIST keyword. Histogram equalization is a common
technique in image processing, most notably it is used for making the
high-contrast images that came from the Voyager and Viking space probes.
Note that a simple one-to-one mapping of pixel intensities and gray level
is lost, and such mapping should be used with caution.
If the Z= and L= keywords are used with HIST, then Z= is taken to be the
lowest intensity value to be considered, and L= tells hist to ignore all
intensities above Z+L when determining the histogram mapping. The CLIP
keyword is implicit with any histogram scaling option -- no rollover is
permitted.
Additional histogram scaling options are available via the HIST=LOG and
HIST=SQRT keywords. HIST=LOG will map intensities by equalizing over
logarithmic intervals. This is useful when there is a very large range of
intensities in the image, and you wish to emphasize the brighter regions
without washing out fainter features. The HIST=SQRT option will map
intensities by equalizing over square-root intervals. This represents a
middle ground between the ;SPMquot;FLAT;SPMquot; histogram mapping described above (which
enhances fainter regions) and the ;SPMquot;LOG;SPMquot; histogram mapping.
Some experimentation to determine the best option to use will undoubtably
be required. Images which are mostly background sky will usually not
benefit from any form of histogram equalization, while images of extended
objects in which the frame is more-or-less filled with ;SPMquot;object;SPMquot; will
usually benefit. By setting the zero point (Z=) to be above the image sky
level, you can have the advantages of the histogram equalization scalings
while still showing surrounding objects.
<#2875#>Labeling the Output<#2875#>
By default, IMPOST will only draw a box around the image and display a bar
with the gray scale intensity range (0-255 or 0-15) in a box below the
image. You can add axes and labels using a variety of keywords provided.
<#2889#>Advanced Page Control<#2889#>
A number of optional keywords are provided which affect the physical size
of the image as it appears on the page, its orientation, the number of
copies printed etc. These options may be used if IMPOST prints are
intended for use in papers or dissertations, where precise specification of
margins and the location and orientation of the image on the page are
essential. These keywords are described below:
<#2898#>Advanced Notes:<#2898#>
IMPOST uses the LickMongo PostScript drivers to make the axes and labels.
As such, while PostScript instructions generated by this program may be
used to produce the image hardcopy on any PostScript device via an
appropriate server, they do not fully conform to to the ;SPMquot;Encapsulated
PostScript;SPMquot; structuring conventions described in the 'PostScript Language
Program Design' Manual (Adobe Systems Incorporated 1988), or following the
Version 2.0 EPS Specifications (Adobe Systems Inc. January 1989). In
future releases of LickMongo, this should be corrected (it is an artifact
of the fact that the LickMongo PostScript drivers were written well before
the standards were published). Even so, with minor modifications, it
should be possible to import images created with IMPOST into other
encapsulated PostScript files (like those generated by TeX).
Images created with IMPOST have been displayed successfully on the X11/NeWS
window system, on displayed PostScript devices, and transmitted
electronically via both email and tcp/ip with no difficulties.
'PostScript' is a registered trademark of Adobe Systems Incorporated.
<#2899#>A Note About the Name:<#2899#>
IMPOST is the command formerly known as POSTIT until 1997 Nov 21, when the
author (Rick Pogge) was made aware of a letter to his department chairman
from the law firm of Arnold, White,
While we disagree that anyone could possibly confuse the ubquitous yellow
sticky notes manufactured by 3M with an obscure function embedded within an
even more obscure advanced astronomical image processing package, the
current legal climate of excessive litigiousness that serves only to
trivialize intellectual property rights requires that we change the name.
With the FOUR option, the scale factor is 15 instead of 255, and
INTENS(r,c) will take values from 0 to 15.