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RCS (Reverse Computer Screen)

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.

Usage

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 ZX0, ZX1, or 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 a "buffered RCS decoder" to decode it to the screen. In this case, there are 2 variants of this routine that you can choose: "compact" (that's very small) or "rapid" (about 2.5 times faster). 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 a "Smart" integrated decompressor (either RCS+ZX0, RCS+ZX1, or RCS+ZX7). 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 ZX0, ZX1 or ZX7 decompressors. However this option only works for currently supported compressors (ZX0, ZX1, and ZX7), and the "Smart" version is about 3 times slower than standard ZX0, ZX1, or ZX7 decompressors.
Decompress and decode at the same time, directly to the screen, using the "Agile" integrated decompressor (either RCS+ZX0, RCS+ZX1, or RCS+ZX7). It works exactly the same as "Smart" version, except it runs much faster (about the same speed as regular "Turbo" decompressor) when decompressing data outside the screen (without RCS). However the "Agile" decompressor version is larger than "Smart".

Partial Screens

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+ZX0, RCS+ZX1, or 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).

Tech Stuff

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

License

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.

Credits

RCS was created by Einar Saukas.

Many thanks to joefish for suggesting to implement the "on screen" decoder, Antonio Villena for additional suggestions to improve it, and James 'Arkannoyed' Slasor for providing the compact version of the "buffered" RCS decoder.