Screen Compactor

Screen Compactor





R. Swailes presents a memory saving program


for the Spectrum which is designed to help


adventure writers to draw pictures more efficiently.





The original purpose of this compactor program was to


enable adventure game writers to draw pictures quickly


without wasting too much memory. Some adventure games draw


pictures for each location but take a long time about it.


This program is a compromise between slow pictures and


minimum waste of memory and instant pictures but with con-


siderable waste of memory.


The average saving of memory is about 40 per cent, though


for simple line drawings it will be much higher. For very


detailed pictures completely filling the screen there will


be a minimal saving.





Begins at pixel one





The Spectrum screen $ save begins at pixel one and saves


its value - 1=On 0=Off. The first eight pixels, for example


0,0,1,1,1,1,0,0 are represented by binary 00111100 or 60


decimal. At the first screen location in memory (16384), if


Peeked, would be seen the value 60. The save continues


through all locations saving the pixel values in blocks of


eight. There are 49152 pixels - 49152/8 = 6144 bytes or 6K


of memory.


The compactor program relies on the fact that most pic-


tures have more background than picture. Where background =


eight pixels off Bin 00000000 = decimal 0.


Even if saving a page of alpha numerics economies can be


made because the top line of any 8 by 8 character is always


blank, and since the screen scan does the top row of the


first eight lines together - chapter 24 display file -


these can be compressed. The bottom lines of each character


are usually also blank - except lower case letters with


tails - and similar savings can be made here.


The machine code save "picture to code" looks at the


first screen location (16384) to see if any of the eight


pixels are on - giving a value greater than zero. If so it


saves the value as normal. Should it be zero - after sto-


ring the 0 - it checks the next location to see if that is


also zero and continues until a lit pixel is found. The


program then saves the number of consecutive zeros. For


example.


Screen$ Code:


64, 2, 0 0 0 0 0 0 0 0 0 12, 19 = 13 bytes


Compacted Data:


64, 2, _0,_9,_0_, 12, 19 = 7 bytes


0 indicates start of a loop of zeros.


9,0 Length of loop (low order byte first)


0x256 + 9 = 9 Loop=9)


Thus 0, 0, 24 24x256 + 0 = 6144 bytes = 6K


is a screen full of zeros in 3 bytes. To convert the code


back to a picture the reverse applies.


Type in listing 1 and Run, then type in Data 1. If all


goes well then save to tape the machine code for picture


saving.


Amend listing 1 with listing 2 and Run, then type in


Data 2. This will also save to tape if it is correct. This


is the machine code for picture printing.





Clear the machine





Clear the machine and type in the Demonstration Program


and Run. It will load up the Data 1 and Data 2 Code from


tape. It then expects a Screen $ to load in.


* If you haven't a Screen $ handy on tape then delete line


90 and insert 90 List 120. Following the program and your


Screen $ - or listing - will be saved in code - in memory.


The length of the code and saving can be seen. Press any


key and the Screen $ instantly reappears.


A list of Pokes - table 1 - is supplied in case you don't


want to save a whole screen. Just amend the started data in


listings 1 and 2 and this will enable you to save either


the Top 1/2, Top 2/3, the whole screen or the whole and


Attribute File.


You can draw and save several pictures one after the


other; just Poke 23728-9 as lines 50, 60, 70 in the main


program - with the position in memory where the picture


code is to be stored. Save the picture with Rand USR 25000


and then Peek 23728-9 - line 120 in the main program - to


find the length of the picture code in memory and work out


the next picture code start position and repeat.





For all Spectrums





To recall any picture Poke 23728-9 - lines 150,160 in the


main program - where V=start position of picture code in


memory then Rand USR 25130.


The program was written for all Spectrums, and the


machine code is fully relocatable. For 48K machines instead


of typing in numbers 25 etc. type in 35xxx or 45xxx.


Your picture code can be saved to tape once stored in


memory. See chapter 20, Spectrum manual.