RCS

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RCS (Reverse Computer Screen) - by Einar Saukas

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RCS is an utility to reorder bytes from ZX-Spectrum screens before compression.



Technically, the ZX-Spectrum screen can be divided in 4 parts:



* Bitmap sector 0: upper 1/3 screen (2048 bytes located from 16384 to 18431)

* Bitmap sector 1: middle 1/3 screen (2048 bytes located from 18432 to 20479)

* Bitmap sector 2: lower 1/3 screen (2048 bytes located from 20480 to 22527)

* Attribute area (768 bytes located from 22528 to 23295)



RCS reorders bytes within each bitmap sector, without affecting attributes. If

you apply RCS encoding before compression, the obtained compression ratio should

be at least 10% better than usual.





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USAGE

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To apply RCS encoding to a file, use the command-line utility as follows:



rcs Cobra.scr



This will generate a RCS encoded file called "Cobra.scr.rcs", that you must now

compress using your favorite compressor (such as ZX7).



Afterwards, you have the following choices to restore the original screen from

the compressed data:



* First decompress it to a temporary buffer, then use "buffered RCS decoder" to

decode it to the screen. However this option requires a 6912 bytes buffer to

decompress a full screen, therefore this is a good choice only if your program

is already using a large buffer area (such as shadow screen) anyway.



* First decompress it directly to the screen, then use "on-screen RCS decoder"

to decode it. However this option will display some "garbage" on screen for

a fraction of a second (unless you compress bitmaps and attributes separately,

so you can hide the screen using the same INK/PAPER, decompress bitmaps only,

decode it, then finally decompress attributes).



* Decompress and decode at the same time, directly to the screen, using the

"Smart" integrated RCS+ZX7 decompressor. When decompressing anything to the

screen area, the "Smart" version assumes the compressed data was also RCS

encoded, so it automatically decodes it. When decompressing to anywhere else,

it assumes the compressed data is not RCS encoded, thus it works exactly like

regular ZX7 decompressors. However this option only works for ZX7 compression

(obviously) and the "Smart" version is about 3 times slower than other ZX7

decompressors.



* Decompress and decode at the same time, directly to the screen, using the

"Agile" integrated RCS+ZX7 decompressor. It works exactly the same as "Smart"

version, except it runs much faster (about the same speed as regular "Turbo"

version) when decompressing data outside the screen (without RCS). However the

"Agile" decompressor version is larger than "Smart".





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PARTIAL SCREENS

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Since RCS format reorders bitmap sectors separately, it can also be used before

compressing only 1/3 or 2/3 of a ZX-Spectrum screen.



Notice however that bitmaps and attributes are stored in separate memory areas

for each part of the screen:



* Upper 1/3 screen: bitmaps from 16384 to 18431, attributes from 22528 to 22783

* Middle 1/3 screen: bitmaps from 18432 to 20479, attributes from 22784 to 23039

* Lower 1/3 screen: bitmaps from 20480 to 22527, attributes from 23040 to 23295



Because of this, storing a partial screen requires either compressing bitmaps

and attributes as two distinct blocks, or using a contiguous temporary area to

copy both.



RCS provides exactly the same choices:



* Distinct blocks: First save the bitmap contents from 1/3 or 2/3 screen (2048

or 4096 bytes) as binary file, encode it with RCS and compress it. Then save

another binary file with the corresponding attributes (256 or 512 bytes) and

compress it separately. Later, decompress each block separately to the screen

(either using a regular decompressor and running the "on-screen RCS decoder"

afterwards, or using an integrated RCS+ZX7 decompressor).



* Temporary area: Copy the bitmap and attribute contents from 1/3 or 2/3 screen

to a temporary area (2048+256=2304 or 4096+512=4608 bytes), save it as binary

file, encode it with RCS and compress it. Later, decompress it to a temporary

area, then use a "buffered RCS decoder" to copy bitmaps and attributes to the

screen. In this case, the existing "buffered RCS decoder" will need trivial

changes since it currently supports full-screen images (contact me if you need

assistance on this).





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TECH STUFF

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The following program helps visualize the regular ZX-Spectrum screen ordering:



10 CLS

20 FOR F=0 TO 6143

30 POKE 16384+F,255

40 NEXT F



The RCS format reorganizes this data as follows:



10 CLS

20 FOR S=0 TO 2

30 FOR C=0 TO 31

40 FOR R=0 TO 7

50 FOR L=0 TO 7

60 POKE 16384+S*2048+L*256+R*32+C,255

70 NEXT L

80 NEXT R

90 NEXT C

100 NEXT S





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LICENSE

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This utility can be used freely within your own ZX-Spectrum programs, even for

commercial releases. The only condition is that you indicate somehow in your

documentation that you have used RCS.





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CREDITS

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This utility was created by Einar Saukas.



Many thanks to joefish for suggesting to implement the "on screen" decoder, and

to Antonio Villena for additional suggestions to improve it.



Further information about ZX7 compression is available at:

http://www.worldofspectrum.org/infoseekid.cgi?id=0027996