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Broken
Sword is a graphical adventure game re-implemented from scratch to
work on the Game Boy Advance. Selecting the right compression techniques
for various data is important for all games and never more so when you
want to squash as much as possible onto an 8MB cartridge without resorting
to content butchery. The largest chunk of our storage budget was given
to animation data, but the game also contains roughly 150 unique background
images, nearly 80 music cues complete with orchestral sample set, a suite
of SFX samples and five languages worth of dialogue.
Uncompressed,
the backgrounds in Broken Sword would occupy close to 7MB. This
article discusses the technique we used for storing lots of high detail
unique backgrounds for use with character (tiled) BG modes. There's no
glamor here I'm afraid; this article presents a useful, flexible solution
to the meat and potatoes problem of managing a large number of background
graphics.
Design
Deciding
on a graphics mode to best fit the game was first port of call after identifying
all visual requirements. We decided to use a screen mode which supports
256 colors because
it suited the in-game graphics best.
JPEG compression
suits detailed images particularly well, so it was a natural first choice
for storing our backgrounds. Not only is it very powerful, crunching images
down to impressive ratios, but it's very flexible too. JPEG compression
can accept a quality parameter, making it very simple to play the quality
of the resulting image against its compressed size. We exploited this
in our build process by applying an initial global quality level to all
backgrounds, set as low as we could get away with, then spending the remainder
of the budget individually improving any backgrounds that decompressed
with too many ugly artifacts (those with large areas of solid color in
particular). Figures 1 to 3 give an example of JPEGs flexible quality
control. It's also worth noting that both the artifacts and the decrease
in saturation subtleties that JPEG introduces are much less noticeable
on the Game Boy Advance LCD.
For ten
representative backgrounds, on average, JPEG compression, set at a quality
of 50/100 compresses images down to 30.2 percent of their original size.
With a quality setting of 35/100 this becomes 24.8 percent and at 20/100
its down to 18.1 percent.
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