Screen Scrunger

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\H11\H07\H10\H00 SCREEN SCRUNGER





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Calling all those of you who are contemplating adding graphics


to your adventure programs. Chris Wood has devised a couple of


screen compressor programs that'll "scrunge" those screens and


save those precious bytes.


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What we're talking about here is a two-in-one


"screen scrunger" - or screen compressor, if you


prefer - that's fast, relocatable and short


(less than 50 bytes each!). Using either of


these routines, or a combination of both, you


can "scrunge" most screens down to at least half


their original size and, with a built-in relo-


catable screen scrunger saved as a standard with


each picture, you can have between 10 and 15


screens stored in memory at any one time!


Adventure writers only require the top third


or so of each screen saved, which is why the


program included does this ... and even lets you


move bits of the screen around. The only


restriction on saving parts of the screen is


that attributes have to be compressed separ-


ately; this is because the program can only


handle continuous chunks of memory.





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ON-SCREEN STORAGE


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At this stage, there's no need to go into too


much detail about how the Spectrum's screen is


laid out - but if you've got a couple of days


spare sometime ... Suffice to say that what you


see on-screen is no more than an area of memory


that we can see and, as far as the Spectrum's


concerned, it's nothing but a jumble of binary


figures.


In the meantime, here's some information on


how the scrunger routines work ... and why you


need two of them! Take a look at a few SCREEN$


pictures and you'll see there are quite a few


large areas of the same colour - from which


you'd surmise that the same value is popping up


in memory quite often. You'd be right!


Thus, our first routine dives through the


screen memory counting the number of times a


particular value pops up until a different value


is found. Once another value has been found, the


Spectrum resets its internal counter, stores the


data





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10 POKE 23658,8


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Line 10 Turns the Caps Lock feature on


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20 PAPER 7: INK 0: CLS


30 PRINT " HOW MUCH DO YOU WA


NT SAVED?"


40 PRINT ''" 1. PART OF SCREEN


$ "''" 2. ALL OF SCREEN$ "


50 IF INKEY$="2" THEN POKE 30


009,64: POKE 30048,91: POKE 3005


6,64: POKE 30065,91: POKE 40010,


64: POKE 40046,91: POKE 40054,64


: POKE 40063,91: GO TO 500


60 IF INKEY$<>"1" THEN GO TO


50


70 CLS : PRINT ''" INPUT LIMI


TS"


80 PRINT ''"64.TOP OF INK"''"7


2.MIDDLE OF INK"''"80.BOTTOM OF


INK"''"88.TOP OF ATTRIBUTES"''"8


9.MIDDLE OF ATTRIBUTES"''"90.BOT


TOM OF ATTRIBUTES"''"91.END OF S


CREEN"


90 INPUT "FROM? ";start: INPUT


"END? ";end


95 POKE 30009,start: POKE 4001


0,start: POKE 30048,end: POKE 40


046,end


100 INPUT "PARTS TO GO TO SAME


PLACE?";a$: IF CODE a$=CODE "Y"


THEN GO TO 110


105 INPUT "TO GO TO? ";dest


106 LET diff=dest-start: LET en


d=end+diff: LET start=start+diff


110 POKE 30056,start: POKE 4005


4,start: POKE 30065,end: POKE 40


063,end


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Lines 20-110 Provide menu options for the screen scrunger


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500 GO SUB 990: LET l=USR 30000


: LET m=USR 40000: LET l=l-30054


: LET m=m-40052


510 CLS : PRINT "EXTRACTING 0 A


ND 255 TAKES "; PAPER 6;l''; PAP


ER 7;"SQUASHING ALL DATA TAKES "


; PAPER 6;m


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Lines 500-510 Find out how much space is saved by each


routine, and then print up each option.


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520 PRINT ''"SAVE FIRST OR SECO


ND?"


530 INPUT "SAVE? (1 OR 2)?";a


540 IF a=1 THEN CLS : RANDOMIZ


E USR 30054: SAVE "SQUASHED SCRE


EN$ "CODE 30054,l: RUN


550 IF a=2 THEN CLS : RANDOMIZ


E USR 40052: SAVE "SQUASHED SCRE


EN$ "CODE 40052,m: RUN


560 GO TO 530


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Lines 520-560 Save the "scrunged" screen


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990 POKE 65370,64: POKE 65373,1


96: RANDOMIZE USR 65368: RETURN


995 POKE 65370,196: POKE 65373,


64: RANDOMIZE USR 65368: RETURN


999 STOP


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Lines 990-999 Contain the screen handling subroutines


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1000 RESTORE 1010: LET ch=0: FOR


n=0 TO 81: READ a: LET ch=ch+a:


POKE 40000+n,a: POKE 49000+n,a:


NEXT n


1001 IF ch<>8217 THEN PRINT "ER


ROR IN DATA IN LINE 1010": STOP


1005 LET ch=0: FOR n=0 TO 97: RE


AD a: LET ch=ch+a: POKE 30000+n,


a: POKE 39000+n,a: NEXT n


1006 IF ch<>9171 THEN PRINT "ER


ROR IN DATA IN LINE 1020": STOP


1008 FOR n=65368 TO 65379: READ


a: POKE n,a: NEXT n: CLEAR 29999


: PRINT "LOADING": LOAD ""SCREEN


$ : GO SUB 995: GO TO 1


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Lines 1000-1008 Provide the initialisation routines


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1009 REM SQUASH ALL CODE


1010 DATA 197,209,33,85,0,25,235


,221,33,0,64,14,1,221,126,0,221,


190,1,32,8,121,60,167,40,3,79,24


,10,221,126,0,18,19,121,18,19,14


,1,221,35,221,229,225,124,254,91


,32,220,213,193,201,17,0,64,221,


33,33,0,221,9,122,254,91,200,221


,70,1,221,126,0,18,19,16,252,221


,35,221,35,24,236,0


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Lines 1009-1010 Contain the data for the first screen scrunger


routine


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1019 REM EXTRACT 0 AND 255 CODE


1020 DATA 197,209,33,100,0,25,23


5,33,0,64,14,1,126,167,40,9,254,


255,40,5,237,160,3,24,21,71,121,


60,35,40,7,126,184,32,3,12,24,24


3,120,18,19,121,18,19,14,1,124,2


54,91,32,217,213,193,201,17,0,64


,221,33,46,0,221,9,122,254,91,20


0,221,126,0,167,40,10,254,255,40


,6,18,19,221,35,24,236,221,70,1,


18,19,16,252,221,35,221,35,24,22


3,0,0


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Lines 1019-1020 Contain the data for the second screen scrunger


routine


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1030 DATA 17,0,176,33,0,64,1,0,2


7,237,176,201


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Line 1030 Provides the save routine for the SCREEN$





The "screen scrunger" program should be initialised with "RUN


1000" to set up the data lines 1009-1030. Once the data has been


initialised, the program flow is directed to the menu of options


offered by the scrunger.


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and carries on looking through the rest of the


screen memory. For example, the following data


"1,1,1,1,1,1,2,2,2,2,3,3,3,3" in screen memory


would be scrunged and stored as "1*6,2*4,3*4".


As the routine allocates a single byte to


store the internal counter, if there's more than


256 occurrences of the same value the counter


resets itself and the next value is treated


anew. For example, a row of 400 twos becomes


"2*0,2*144" (where a zero value denotes '256').





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FOR MY NEXT TRICK ...


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The first routine will suit most screen-


scrunging requirements, but very complex screens


with a few repeated values can trip up the


routine. For example, a block of memory that


originally looks like "1,2,3,4" becomes


"1*1,2*1,3*1,4*1", and four bytes is transformed


into eight - not a good method if complex illus-


trations, such as the start-up screen in Lunar


Jetman, can grow to around 7.5K instead of the


usual 6.5K!


And this is where the second routine comes in.


Complex visual screens, such as Ultimate's,


require a routine that just takes all the free


bytes (that is, all the zeros in the background)


and just squashes up all the rest of the data.


Using this method, you can't help but make a


saving.


However, to make the second routine even more


clever, it's also programmed to take out 255s


that occur in solidly-filled blocks of the


screen. You'll be glad to know that the Lunar


Jetman screen mentioned earlier is scrunged


happily down to around 4.5K!


But don't start worrying which routines you're


going to have to use with each screen, as the


Basic program (the two routines are provided in


the data statements) tells you which "scrunger"


is the most efficient for each case.


To make use of the "scrunged" screens saved by


the program, call the machine code from the same


address at which you load the code; each picture


saved is accompanied by a screen decompressor


routine at its start. For example, if you want


to put your "scrunged" screen at address 40000,


you'd get it back on-screen by executing


"RANDOMIZE USR 40000". Happy scrunging, my


dearios!





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\H11\H07\H10\H00SQUEEZE ME!


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First of all, load up the "Screen Scrunger" and type RUN 1000.


Now, load the SCREEN$ - in this case, a view from Quicksilva's


Zombie Zombie - and it'll be stored in a high location in


memory. The program will then move on to the options screen ...





!Szombi1





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The options screen allows you to choose whether you want to


scrunge the whole SCREEN$ or just a portion of it. In this case,


the middle section was scrunged - by typing in '72' and '80' as


the parameters for the compression.





!Szombi2





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The program offers you the option of saving the compressed


SCREEN$ code - displaying the relevant section on-screen whilst


saving takes place. The "Screen Scrunger" itself will advise you


as to which routine is the most efficient for the SCREEN$ you're


scrunging.





!Szombi3





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CARTOON





!Scartoon





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--


from Your Spectrum #13 (Apr.1985)


--


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