Documentation for PCAP - A Screen Capture Utility.
2/28/91

What it does ...

PCAP saves the image on your computer monitor to a hard disk file. The file is 
actually a DOS program (in COM format instead of EXE). When you run the 
program, the image will be displayed on your screen. PCAP is a memory resident 
program so it will steal some of your computers RAM but not much (only about 
4K). It will save text screens as well as CGA, EGA, MCGA and VGA graphics.

Why you might want to use it ...

About the only thing I do with it is save PRODIGY screens like the weather 
maps. Actually, I have my PC setup to dial in to PRODIGY everyday and grab the 
weather maps automatically so that they are waiting for me when I get to work 
(OK, so I'm a nerd). I also use PAN, a program automation (animation?) utility 
from PC-Mag, to do this. I'm including the PAN script file (minus my PASSWORD) 
that I use in case you want to try this. You'll have to get PAN somewhere 
else, though.  

Loading PCAP ...

Just run PCAP. The first time you do, it'll try to create a directory on drive 
C:, namely "C:\PCAPT". This is the directory where all the images will be 
saved.

Saving an image ...

Press ALT-P when you want to save the image on the screen. PCAP will sound 1 
long beep followed by a series of short beeps followed by another long beep. 
The last beep means that the image is saved. The length of time it takes to 
save the image will depend on if it is text (very fast), low (fast) or high 
(slower) resoultion graphics and on the speed of your computer. 

* You should not try to do anything else but wait while the screen is saving *

Displaying an image ...

All the images will be stored in files in the C:\PCAPT directory. They will 
have names *like* 0C13445D and no extensions. These names are given by DOS as 
the result of a "create unique file" call. Before you can display an image, 
you must rename the file to <name>.COM where '<name>' stands for anything you 
want. You might want to make it descriptive like MAP.COM or NEWS.COM. If you 
just want to put the 'COM' extension on all the image files in the directory, 
you can use the DOS RENAME command like this:

                           REN C:\PCAPT\*. *.COM

After you've but a COM extension (and optionally changed the file name) simply  
type in the file name at the DOS prompt to see the image. It will remain on 
the screen until any key is pressed.

Things you should know ...

I wrote this program a long time ago before I did any multitasking on my PC. 
Consequently, I didn't pay much attention to trying to safeguard against 
problems that can happen when TSR's and multitasking combine. About the only 
thing I did do was to set a flag so that you can't accidently press the ALT-P 
key combo too long and call the PCAP code again before it's done. This would 
certainly screw up something. I use PCAP with Windows now and everything works 
fine although I am careful to let it finish saving the screen before I do 
anything. The only strange thing I've run across is that after saving a 
Prodigy screen, the blinking box that acts as a cursor does not blink anymore 
and the mouse won't work until a key is pressed. After any key is pressed 
everything returns to normal.

PCAP won't work with the Hercules graphics modes or the 256 color MCGA nad VGA  
modes. I never had time to do these. PCAP will work with most text modes and 
all the CGA, EGA, MCGA and VGA graphics modes (with the exception of the 256 
color modes). Once an image file is created, the graphics and text mode is 
saved along with the image. The image can only be displayed on a system that 
is capable of using the saved graphics or text mode. For instance, if you save 
a VGA mode graphics image on one PC and try to display it on another PC with a 
CGA card, you'll see a blank screen. The converse will work, however, because 
the VGA supports all the standard graphics and text modes of the CGA.

If you use PCAP with an EGA, sometimes the colors that the image file displays 
will be different that those at the time of capture. This is because the EGA 
has write only registers and there is no foolproof way of saving the palette. 
I have included a file called PRODEGA.DBG that will help you get the right 
colors for PROGIGY when using an EGA. It is the input to the DOS DEBUG program 
that pokes the right values for the palette registers into the image file. To 
use it you'll need DEBUG. For an example, say your image file is a PRODIGY 
weather map and has been renamed to "MAP.COM" and the colors are way off when 
you display it because you have an EGA. If you have DEBUG, run it against 
"MAP.COM" and redirect the input to "PRODEGA.DBG". The command looks like:

                      DEBUG MAP.COM < PRODEGA.DBG

Try running MAP.COM again.                      

Format of image files ...

The image file consists of the saved mode information, the code to display the 
image and the image data itself. The image data is run-length coded. I'll send 
the format to anybody who asks (see the end of this file for my e-mail 
address). It's very simple to decompact. It works very well on PRODIGY 
screens, even in the high resolution (640x480x16) mode. I get about 3.75:1 
compaction on the maps and these are about the most complex screens in 
PRODIGY. However, there is the potential that an image file will be greater 
than 64K, especially in the high res VGA mode. If this happens then the image 
file won't display because COM files are limited to a 64K image on disk. I've 
never had this happen with PRODIGY screens but it does happen on some very 
complex images from other programs.

The end ...

I know this is a very sparse program. There are many, many, features that can  
be added to enhance it. If you have any suggestions, comment or ideas, you can 
sent them to me on Prodigy or Internet.

- Tom Cervenka, Prodigy: CMGB18A, Internet: ctct100@uicvmc.uiuc.aiss.edu