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 "image.ps". 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.,: "lpr image.ps" on a Unix system, or "$ print/queue=pslaser image.ps" 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 "stat" paper have been accepted for publication without re-photography required.
IMPOST produces a hardcopy of an image by representing pixel intensities as an 8-bit halftone gray scale. The optional keyword "FOUR" 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 "linear" 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:
With the FOUR option, the scale factor is 15 instead of 255, and INTENS(r,c) will take values from 0 to 15.
In this simple linear mapping, pixels with IMG(r,c) <= 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 > ZERO+SPAN are set black, otherwise, the color scale "wraps" 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 "negative") 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 "normal" or "positive" 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 "Histogram Equalization", 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 "FLAT" histogram mapping described above (which enhances fainter regions) and the "LOG" 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 "object" 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.
Labeling the Output
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.
Advanced Page Control
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:
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 "Encapsulated PostScript" 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.
A Note About the Name:
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, & Durkee. This firm represents the Minnesota Mining & Manufacturing Company (3M) in trademark and other intellectual property rights matters. It seems after coming across a web version of the Prospero manual (a VISTA derivative written at OSU that incorporates POSTIT because it was written by Rick Pogge), they deemed our use of the word POSTIT as an infringment of the trademark of 3M for its famous yellow sticky notes, aka Post-It(tm) Notes that might potentially confuse people encountering the web version of our help pages.
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.