Grabbing Images from Amiga Screens
 

This function is available in the Project menu, and as a stand alone
program for users with less than 1.5 megabytes of memory. It is the heart
of image transmission from the AVT system.

The AVT system can grab video from any Amiga screen - at least, all the
standard screens. This includes:

    320x200     2 color
    320x200     4 color
    320x200     8 color
    320x200     16 color
    320x200     32 color
    
    320x400     2 color
    320x400     4 color
    320x400     8 color
    320x400     16 color
    320x400     32 color
    
    640x200     2 color
    640x200     4 color
    640x200     8 color
    640x200     16 color
    
    640x400     2 color
    640x400     4 color
    640x400     8 color
    640x400     16 color
    
    320x200     Hold-and-Modify (HAM)
    320x400     Hold-and Modify

This means that you can grab your workbench screen and send that puppy
right on out... You can even send the control panel of the AVT system, if
you like - it IS on it's own screen.

You'll notice that there are two grab entries in the project menu;
One is "Grab Screen" and one is "Framed Grab".

Grab Screen attempts to take the ENTIRE image on the screen being grabbed
and make it fit in the resolution you were in when you selected Grab Screen.

Framed Grab takes the source HAM screen pixel-by-pixel and captures it into
the resolution you are using (128x128 or 256x256) regardless of the
resolution of the source screen.

If you use the GrabScreen program supplied instead of the menu function,
simply follow the prompts. You can grab in high or low resolution, from
HAM or normal screens, and if a HAM screen, the option to grab pixel for
pixel is also offered there.

You can run the GrabScreen program from either the CLI or the WorkBench.

Let's say you are in high resolution (256x240). If you do a regular grab of
a HAM screen, the program takes the 320 by 200 and compresses it into the
256x240. As you might well imagine, some information is lost - there
just aren't enough pixels, even in standard hi-res SSTV, to represent
all the data on a screen as dense as a standard HAM screen.

In Framed Grab, The program will take the FIRST 256 pixels from the left
side of the HAM screen, and map those directly into the SSTV image.
This means that image information on the right is lost - but the information
that WAS taken is 100% accurate - pixel-for-pixel. If you use the AVT modes,
you'll find that you can send sections of your HAM screens all the way
around the world, and the recipient will be able to read the text you
might have typed in there - in any font at all.

Of course, you can use the 94 second mode - that is 320 pixels wide, and
you WILL get all the pixels.

AVT System SSTV resolutions
 
In order to make the most effective use of the Grab facility, you should
have a good understanding of what the AVT considers each SSTV mode. This is
because the AVT really only has three internal modes:

    128x128 b&w or color
    256x240 b&w or color
    320x200 color only

These modes "cover" the other SSTV modes - that is, particular SSTV modes
map into one or another of these resolutions. This is which SSTV modes that
the AVT supports which match which resolutions (sure is a lot of "whiches!"):

    128x128
    8.5 second B&W
    8.5 second frame sequential color
    8.0 second Robot B&W
    8.0 second frame sequential Robot color
    12 second Robot "composite color"
    12 second Robot B&W
    24 second AVT color
    24 second Volker-Wrasse line sequential red-enhanced sync color
    24 second "composite" robot color
    25.5 Line sequential color
    
    256x240
    17 second B&W (Mode a - nominally 128x256)
    17 second Frame sequential color (a)
    17 second B&W (Mode b - nominally 256x128)
    17 second Frame sequential color (b)
    24 second Robot B&W
    24 second Robot frame sequential color
    36 second Robot B&W
    34 second Std hi-res B&W
    34 second Frame sequential hi-res color
    36 second Robot "composite" color
    48 second Volker-Wrasse line sequential red-enhanced syn color
    51 second line sequential color
    72 second Robot "composite" color
    90 second AVT color
    96 second Volker-Wrasse line sequential red-enhanced sync color
    102 second line sequential color
    
    320x200
    94 second AVT color

The thing to keep in mind about the Grab operations is that when you are in
one of the above modes, the Grab goes into one of the three resolutions
named above - and which one it goes into simply depends on which
sstv mode you're in - which one of the three classes of modes broken out
in the above table.

Here is an example: You are currently in 24 second AVT mode and you do
a Grab. That measn you grabbed in 128x128 - because the 24 second AVT
mode is under the 128x128 classigfication.

After a while, you'll know just off the top of your head - in the meantime,
refer to this table as required.