DP Tool Club 8/CDASC08.ISO/VRAC/MS_ARCAD.ZIP/ARC2_1.CHP

Text File | 1993-06-15 | 7KB | 190 lines

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                               ~W~IOverview~Y~I


Modern microcomputers come with |video subsystems| (or else the subsystem
can be installed). There are a few basic parts to the video subsystem on
your computer.

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The first is the most obvious, ~G~Ithe screen~Y~I. A computer |monitor| is a
cathode ray tube (CRT) with one or more phosphorus coatings on the front.
Phosphorus is a chemical that glows when energized. When we want to send
an image to the screen, we spray electrons at the phosphorus to energize
it. The energized phosphorus then starts glowing for a small portion of
a second.

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The coating is divided into discrete dots called |pixels|. The pixels are
arranged into rows, called scan lines. On monochrome screens each pixel
may be on or off. That is, it can be glowing or dark. Color systems have
three (and possibly more) electron guns inside the CRT. By directing the
beams at different portions of a pixel, each of which are colored
differently, colors in various gradations can be produced.

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Put more simply, each pixel on a color monitor is composed of at
least three little dots. The dots are usually colored red, green,
and blue. That's why such monitors are called RGB monitors - the
RGB stands for red, green, and blue.

The dots are small enough so that our eyes can't separate them.
By controlling the intensity of the red, blue, and green portions
of each pixel, a wide variety of colors and shades can be produced
on the screen. How many shades can be produced is determined by the
number of different intensities a monitor will allow and how much
memory your video adaptor has. On PCs, memory is the governing
factor.


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~KFor each pixel on the screen, there has to be at least one location in
video memory. On monochrome systems, one bit in memory corresponds to one
pixel on the screen. This is because a monochrome pixel may only have one
of two possible values, ~C~Ion~Y~I or ~R~Ioff~Y~I. As it happens, a bit may also have one
of two values, ~C~Ion~Y~I or ~R~Ioff~Y~I.~k

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One the ~W~ICGA ~Y~Iadaptor, the very first adaptor for the IBM PC that was able
to do color graphics, there are ~M~Itwo modes~Y~I. The first is called ~C~Itwo-color
mode~Y~I, and it has a resolution of |640 x 200 pixels|. The second is
~G~Ifour-color mode~Y~I, and it has a resolution of 320 x 200 pixels.

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The ~C~Itwo-color mode~Y~I has each pixel mapped into a single bit. That is all
that is needed to represent the two different color combinations, which
are typically |black and white|. In ~G~Ifour-color mode~Y~I, the possible color
combinations are mapped into two bits. With two bits, each of which can
be set to one of two values, there are four possible values that each
pixel can have, hence four colors can be displayed.

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Because video RAM is grouped together into bytes of eight bits, one
byte contains the information for four pixels. The CGA also has the
added headache of using interleaved memory. Interleaved memory means
that the pixel data for all the even numbered scan lines (rows of
pixels) are kept in one place in memory, while the data for the odd
numbered lines are kept in another.

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The ~R~IEGA~Y~I and ~M~IVGA~Y~I (an adaptor called |SuperVGA| exists as well)
adaptors use a completely different scheme to map their pixels into memory.
These adaptors use what are called bit planes. Standard EGA and VGA adaptors
have four bit planes.

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Since SVGA is a very non-standard (but popular) adaptor, it is
possible that there are more than four bit planes in SVGA modes.

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The four bits that store the pixel information are each kept in a separate
bit plane, one bit to a bit plane. All of the bits that are associated with
a particular pixel can be accessed in |parallel|. This organization not only
decreases access time, it saves memory.

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The standard modes available for the ~C~IEGA~Y~I can display as many as ~C~I16 colors~Y~I at
a time, with resolutions up to ~C~I640 x 350 pixels~Y~I. A standard ~R~IVGA~Y~I can display
up to ~R~I256 colors ~Y~Iwith a resolution of ~R~I320 x 200 pixels~Y~I. ~SThe VGA also has a
16 color mode with a resolution of 640 x 480.~s

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SuperVGA systems vary both in the number of colors they can display and the
screen resolution. However, it's not uncommon for them to be able to display
~W~I~F262,144 ~Y~I~Ncolors at a resolution of ~W~I1024 x 768 pixels~Y~I.

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There are some SVGA adaptors that will display in excess on 4
million colors at a resolution of 1280 x 1024.

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There are also higher resolution adaptors available for PC-compatible
computers. Currently, these adaptors are so much more expensive than VGA or
SuperVGA that they are mostly purchased by professionals that use high
resolution graphics as part of their job. Look for this to change, however.
Since Windows 3.x supports 24-bit graphics, it's much easier now to produce
programs for high end adaptors. I don't think it will be too long before
extremely high resolution video systems push EGA and VGA completely off the
market.

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Any code written for EGA |won't run on a CGA|, and code written for the
VGA may not run on an EGA adaptor. It is for this reason that I chose to
write the software for this book using the Borland Graphics Interface
(BGI). The BGI handles the particulars of dealing with the hardware so we
don't have to. This enables us to write games that are portable across a
wide variety of adaptors.

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The demonstration game included with this book is  written to run on EGA,
VGA, and SVGA compatible adaptors. The use of CGA adaptors is presented in
the next section. However, it is not demonstrated in this book. That's
because CGA is dying out very quickly.

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The another reason for not bothering with the CGA is that the EGA,
the VGA, and the SVGA support a feature called paging. We will make
limited use of this ability in Space Attackers.  A page is the
amount of memory that it takes to display one screenful of graphics.
That amount is different for each adaptor. A high resolution adaptor
like the VGA requires much more memory per page than the CGA.

The number of pages available on EGA, VGA and SuperVGA adaptors
varies from machine to machine. However, it's reasonable to expect
that there are at least two pages that you can use, especially in
medium and low resolution modes. For Space Attackers, that's all
we'll need.

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