o In low-resolution mode, the normal playfield has a width of 320 pixels.
o High-resolution mode gives finer horizontal resolution 640 pixels in the same physical
display area.
o In non-interlaced mode, the normal NTSC playfield has a height of 200 video lines. The
normal mal PAL screen has a height of 256 video lines.
o Interlaced mode gives finer vertical resolution 400 lines in the same physical display
area in NTSC and 512 for PAL.
These modes can be combined, so you can have, for instance, an interlaced, high-
resolution display.
Note that the dimensions referred to as "normal" in the previous paragraph are nominal
dimensions and represent the normal values you should expect to use. Actually, you can
display larger playfields; the maximum dimensions are given in the section called "Bit-
Planes and Playfields of All Sizes." Also, the dimensions of the playfield in memory are
often larger than the playfield displayed on the screen. You choose which part of this
larger memory picture to display by specifying a different size for the display window.
A playfield taller than the screen can be scrolled, or moved smoothly, up or down. A
playfield 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) Playfields."
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 playfield 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 colouring 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.
- 36 Playfield Hardware -
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