A play
field
taller than the screen can be scrolled, or moved smoothly, up or down. A
play
field
wider than the screen can be scrolled horizontally, from left
"to
right or right
to
left. Scrolling is described
in
the section called "Moving (Scrolling) Play fields. "
In the Amiga graphics system, you can have up
to
thirty-two different colors
in
a single
playfield, using normal display methods. You can control the color of each individual
pixel in the playfield display by setting the bit or bits
that
control each pixel. A display
formed in this way
is called a bit-mapped display. For instance, in a two-color display,
the color of each pixel
is determined by whether a single
bit
is on
or
off.
If
the bit
is
0,
the pixel is one user-defined color; if the bit is
1,
the pixel is another color. For a four-
color display, you build two bit-planes
in
memory. When the play
field
is
displayed, the
two bit-planes are overlapped, which means
that
each pixel is now two bits deep. You
can combine up
to
five
bit-planes
in
this way. Displays made up of three, four,
or
five
bit-planes allow a choice of eight, sixteen, or thirty-two colors, respectively.
The color of a pixel
is always determined by the binary combination of the bits
that
define it. When the system combines bit-planes for display, the combination of bits
formed for each pixel corresponds
to
the number of a color register. This method of
coloring pixels
is called color indirection. The Amiga has thirty-two color registers, each
containing bits defining a user-selected color (from a total of 4,096 possible colors).
Figure
3-3
shows how the combination of up to
five
bit-planes forms a code
that
selects
which one of the thirty-two registers
to
use
to
display the color of a playfield pixel.
Playfield Hardware
31
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