SCREEN COMPACT

by Angelo Colucci

from Your Sinclair #28 (April 1988)

[files SCOMPACT.*]



I know what you're thinking (call me Uri!). You're thinking "screen

compacting - boring! Easey-peasey-japaneasy!" I thought that as well

until I sighted Angelo Colucci's program and frankly, I was amazed. Not

only is Angelo's program the smallest, most efficient, and user-

friendly of the lot I received, but also has possibly the most visually

stunning demo you'll ever see on the Spectrum ever! Wow!



METHOD



Angelo's screen compact program is self-contained. Type in the Basic

prog and it'll tell you what to do. Type in the demo program, run it,

and leave it for about two hours, and then be prepared to be as amazed

as I was.



[The CODE file for the animation has already been generated and saved]

[in SCOMPACT.TAP . To view the animation:                            ]

[LOAD "DEMO" : CLEAR 25999: LOAD "FRAMES" CODE 26000 : GO TO 1000    ]

[JimG                                                                ]



HOW DOES IT DO IT?



This routine will compact most screens into under 50 percent as large. 

It does this by firstly searching for a byte that is not used in the

screen and using it as a flag. Then, the routine will scan the screen

and search for 4 to 255 repeats of the same byte and it'll then compress

this repetition into 3 bytes:

	1st byte = flag byte

	2nd byte = number of repeats

	3rd byte = value of repetition

This is clever and you can see its effect in the incredible demo which

should take up 122880 bytes but only takes 38620 bytes which is 60

percent smaller! Brilliant!



BASIC PROGRAM



This program contains the machine code and instructions on use. Type it

in and save it with SAVE "LOADER" LINE 10.



DEMO PROGRAM



This is the amazing bit. Type it in, SAVE it with SAVE "DEMO" LINE 10

[DM had a habit of always saying to "SAVE ... LINE 10", whether or not

LINE 10 existed in the listing or was the correct line from which to

auto-run. In this case it should be 2000. JimG]. Then leave it for an

hour or two. The effect is stunning.