fhpack - compression for Apple II hi-res images

fhpack is a compression tool with a singular purpose: to compress Apple II hi-res graphics images. It uses a modified version of the LZ4 compression format, called LZ4FH ("fadden's hi-res").

Origins

I've had an idea for a project involving hi-res graphics compression for several years, but didn't do much about it. After learning about LZ4, and seeing uncompressors written for the 6502 and 65816, I decided to see if I could apply LZ4 to hi-res images.

A few hi-res compressors were written back in The Day, usually employing run-length encoding, which is easy to write and fast to encode and decode. In the spirit of LZ4, I decided to put together an asymmetric codec, meaning compression is very very slow, but uncompression is very very fast.

The result is a modified form of LZ4 that consistently beats LZ4-HC, and generally comes close to (and occasionally beats) ShrinkIt's LZW/II. The decoder is tiny and extremely fast, especially on the 65816 where the bulk data copy instructions can be used.

About the fhpack Tool

The compressor has two modes, similar to LZ4's "fast" and "high". The fast mode uses greedy parsing and is not particularly fast, while the high-compression mode uses optimal parsing and takes 12 times as long. Both employ simple brute-force algorithms, which we can get away with because we're only compressing 8KB of data. The high-compression mode does about 4% better on average -- not huge, but not negligible.

Other compression programs, such as gzip, produce significantly smaller output, but uncompression is much slower and requires more memory.

The comments in fhpack.cpp describe the data format. It's essentially LZ4 modified to work better on a system with 8-bit registers.

There is no implementation of the compression side for the 6502. An implementation that uses greedy parsing is feasible, as the bulk of the time is spent comparing 8-bit strings that are less than 256 bytes long, and the 6502 series is pretty good at that. The optimal parser could theoretically be done on a machine with 128KB of RAM, but would take a very long time to run.

Screen Holes

The hi-res screen has a curious interleaved structure that leaves "holes" in memory -- parts of the frame buffer that don't affect what appears on screen. The screen layout is divided into 128-byte sections, with 120 bytes of visible data followed by an eight byte "hole". The holes tend to be filled with zeroes, though sometimes they may contain garbage or program state.

fhpack can do one of three things with the screen holes:

1. Preserve them. This mode is enabled with the "-h" flag. If you want the uncompressed data to exactly match the original, you must specify this flag.
2. Fill them with zeroes.
3. Fill them with a pattern that matches the data immediately before or after the hole.

In some cases #2 provides the best results, in others #3 wins. The difference is usual minimal, with outliers in the 70-90 byte range. On modern hardware fhpack runs very quickly, so when not in hole-preserve mode the tool compresses everything twice, and keeps whichever approach yielded the smallest output.

Apple II Code and Demos

The 6502/65816 versions of the uncompressor (source and binaries), as well as two slideshow applications written in Applesoft and a number of sample files, are provided on the attached disk images (click "view raw" to download them from github).

There are six disk images. The first three hold the slide show demo:

• LZ4FHDemo.do (/LZ4FH, 140KB) - Source and object code for the uncompression routines, plus a few test images and the Applesoft "SLIDESHOW" program.
• UncompressedSlides.do (/SLIDESHOW, 140KB) - A set of 16 uncompressed hi-res images.
• CompressedSlides.do (/SLIDESHOW, 140KB) - A set of 42 compressed hi-res images.

To view the demo, put the LZ4FHDemo image in slot 6 drive 1, and one of the "slide" disks in slot 6 drive 2. Boot the disk and "-SLIDESHOW". Just hit return at the prompt to accept the default prefix.

The slideshow program will scan the specified directory and identify files that appear to be compressed or uncompressed hi-res images. It will then start a slide show, moving through them as quickly as possible. By swapping the compressed and uncompressed disks and restarting the program, you can compare the performance with and without compression. (For a 5.25" disk, it's generally faster to load a compressed image and uncompress it than it is to load an uncompressed image.)

There is a second demo, called "HYPERSLIDE", which shows off the raw performance by eliminating the disk accesses. A set of 15 images is loaded into just 24KB of memory -- overwriting BASIC.System -- and presented as a slide show as quickly as possible. The demo and selected images are on this disk:

• HyperSlide.po (/HYPERSLIDE, 140KB)

To run the demo, put the disk image in slot 6 drive 1, boot the disk, and "-HYPERSLIDE". If you are running on a IIgs, you may want to try it with the 65816 uncompressor, which is much faster than the 6502 version. If you want to compute frame timings, you can set an iteration count, and the slide show will beep at the start and end.

A larger set of images is available on a pair of 800KB disks. One disk has the compressed form, the other the uncompressed form:

• UncompressedImages.po (/IMAGES, 800KB)
• CompressedImages.po (/IMAGES.LZ4H, 800KB)

It's worth noting that the images on CompressedSlides.do take up about 135KB of disk space, but are about 104KB combined. The rest of the space is used up by filesystem overhead. Storing them in a ShrinkIt archive would be more efficient, but would also make them far more difficult to unpack.

Decoder Performance

Running under AppleWin with "authentic" disk access speed enabled, a slide show of uncompressed images runs at about 1.7 seconds per image (about 0.6fps). With compressed images the time varies, because the size of the compressed image affects the amount of disk activity, but it averages about 1.4 seconds per image (about 0.7 fps).

Removing disk activity from the equation, HyperSlide improves that to about 4.3 fps, with very little variation between files. The decode time is dominated by byte copies, and we're always copying 8KB, so the consistency is expected.

HyperSlide incurs a fair bit of overhead from Applesoft BASIC. The "blitz test", included on the LZ4FH demo disk, generates machine language calls that uncompress the same image 100x, eliminating all overhead (and simulating what HyperSlide could do if it weren't written in BASIC). The speed improves to 5.6 fps. To put that into perspective, you could unpack a green image twice in the time it takes "CALL 62454" to clear the screen to green.

The most significant boost in speed comes from using the 65816 data move instructions. With a 65816 implementation, still running at 1MHz, HyperSlide hits 7.7 fps, and BLITZTEST tops 12 fps.

Code Notes

The uncompressor takes as arguments the addresses of the compressed data and the buffer to uncompress to. These are poked into memory locations $02FC and $02FE. In the current implementation, the output buffer must be $2000 or $4000 (the two hi-res pages).

Packed images use the FOT ($08) file type, with an auxtype of $8066 (0x66 is ASCII 'f'). These files can be viewed with CiderPress v4.0.1 and later.

Experimental Results

I grabbed a set of about 70 images, most from games, a few from early "contributed program" disks. The latter include what look like digitized scans that don't compress especially well.

All images were compressed with LZ4 r131 in high-compression mode (lz4 -9), NuLib2 with LZW/II, and LZ4FH (fhpack -9). fhpack output has a one-byte magic number, while LZ4-HC has 15 bytes of headers and footers, so for a fair "raw data" comparison the numbers should be adjusted appropriately.

Most source images are 8192 bytes long, some are a few bytes shorter.

Image File	LZ4-HC	fhpack	LZW/II
contrib/BABY.JANE	3664	3617	2851
contrib/CHARACTERS	1874	1836	1614
contrib/CHURCHILL	4759	4723	3749
contrib/DIP.CHIPS	3840	3785	3048
contrib/DOLLAR	3838	3790	3483
contrib/DOUBLE.BESSEL	3066	3010	2566
contrib/GIRLS.BEST.FRND	5967	5933	4659
contrib/HOPALONG	3394	3331	2713
contrib/JOE.SENT.ME	7569	7546	7702
contrib/LADY.BE.GOOD	4119	4060	3292
contrib/MACROMETER	4405	4354	3613
contrib/MUSIC	1077	1030	929
contrib/RANDOM.LADY	5754	5720	5426
contrib/ROCKY.RACCOON	5864	5754	5598
contrib/SHAKESPEARE	4933	4884	4286
contrib/SPIRALLELLO	5722	5704	5345
contrib/SQUEEZE	3209	3163	2533
contrib/TEQUILA	5881	5828	5484
contrib/TEX	4463	4413	3677
contrib/TIME.MACHINE	3628	3578	3023
contrib/UNCLE.SAM	3057	3012	2784
contrib/WORLD.MAP	2792	2723	2429
games/ABM.TITLE	1387	1353	1581
games/ARCHON.TITLE	5607	5589	4991
games/AZTEC.TITLE	6378	6362	6414
games/BAM.TITLE	5380	5377	4902
games/BANDITS.TITLE	1442	1388	1376
games/BARDS.TALE.1	3853	3815	3523
games/BILESTOAD	2140	1559	2552
games/BORG.TITLE	2327	2559	2009
games/CAPT.GOODNIGHT	2839	2820	2726
games/CHOPLIFTER	1088	1007	1070
games/CRISIS.MT.GAME	4144	4085	4037
games/CRISIS.MT.TITLE	2290	2246	2384
games/DAVID.MIDNIGHT	3362	3322	3225
games/DEFENDER	728	696	678
games/EAMON.TITLE	3744	3665	3252
games/GALACTIC.EMPIRE	2161	2087	1956
games/GERMANY.1985	2566	2460	2390
games/HARD.HAT.MAC	1524	1457	1678
games/KADASH.DEMO	1796	1736	2120
games/KADASH.TITLE	5317	5294	5393
games/KARATE.TITLE	4040	3845	3952
games/KARATEKA.FORT	4050	3955	3452
games/KARATEKA.GAME	948	904	1108
games/LODE.RUNNER	1133	1102	1428
games/MARIO.BROS	1472	1406	1372
games/MAZE.CRAZE	2703	2659	2485
games/MICROWAVE.TITLE	2812	2737	2434
games/NIGHT.FLIGHT	1109	1024	1183
games/ODYSSEY.TITLE	3994	3953	3752
games/OUTWORLD	2222	2157	2296
games/PCS	1897	1837	1861
games/PCS.TITLE	1881	1841	1882
games/QUESTRON.DEMO	2569	2518	2253
games/QUESTRON.TITLE	1536	1499	1837
games/RASTER.BLASTER	2687	2636	2553
games/RESCUE.RAIDERS	5377	4961	4883
games/ROADWAR2K.TITLE	2063	1983	2068
games/SPARE.CHANGE	2058	2009	2268
games/STAR.MAZE	1253	1208	1600
games/STARSHIP.CMDR	1453	1427	1845
games/STELLAR.7	1629	1412	1845
games/SUNDOG.TITLE	3250	3188	3270
games/SWASHBUCK.GAME	4690	4608	4286
games/SWASHBUCK.TITLE	5077	5035	5085
games/TRANQUILITY	1409	1363	1273
games/ULT2.LORD.BRIT	1529	1514	1592
games/ULTIMA2.TITLE	2220	2176	2201
games/WASTELAND.TITLE	3540	3078	3510
games/WAYOUT	1691	1669	1864
games/WOLFEN.TITLE	2638	2588	2610
games/ZAXXON	1884	1862	1769
misc/CRAPS.TABLE	2286	2266	2548
misc/GHOSTBUST.LOGO	1829	1724	1594
misc/LINE.CHART	1753	1655	1578
misc/MICKEY	3369	3316	2945
misc/WHO.LOGO	1138	1084	1218
test/allgreen	63	137	215
test/allzero	62	136	38
test/nomatch	8211	7928	7414
TOTAL	248473	242771	232201
%	37.4%	36.5%	34.9%

Note: test/nomatch is not compressible by LZ4 encoding. fhpack was able to compress it because it zeroed out the "screen holes". When processed in hole-preservation mode, test/nomatch expands to 8292 bytes.

LZSS, which was used by HardPressed to get reasonable compression with fast decode speeds, reduces the corpus to 243991 bytes (36.7%), making it a viable alternative. It's generally inferior to LZ4 as the maximum match length and offset are much shorter, but that's not too significant for hi-res images. Literals are identified with individual flag bits, rather than as runs of bytes, which reduces performance for long strings of literals.