Sunday, July 22, 2012

Level Setting

This project is about getting better-than-expected graphics output from the TRS-80 Color Computer (aka "CoCo").  With that in mind, it seems reasonable to review just what sort of graphics output one would normally expect from the CoCo.

Most graphics on the CoCo are produced using static graphics configurations -- the VDG registers are setup at the beginning of a program and changed only rarely while the program runs, if at all.  There are only a handful of available resolutions, and only a few color options.  Further, each color option brings its own set of restrictions and compromises.  I described the available statically configured graphics modes in an entry on the Fahrfall blog.  Those so interested might wish to review that information.

Original image used to generate examples below.

Color Commentary

The "color" graphics modes of the CoCo's VDG are only able to display 4 colors at a time.  Worse, the palette of colors must be chosen between two pre-defined groups: green, yellow, blue, and red; or buff (almost white), cyan, magenta, and orange.  Note that black is not an option in these modes.

Green, Yellow, Blue, and Red

Buff, Cyan, Magenta, and Orange

For the most part, these modes are a bit awful.  They are reasonably serviceable for a variety of game activities, but they don't make for good display of "real life" pictures.

Dithered Green, Yellow, Blue, and Red

Dithered Buff, Cyan, Magenta, and Orange

Dithering improves (or at least changes) the output a bit.  But for the most part, the resulting images are still horrible.

Block Party

The VDG's semi-graphics modes can use all eight of the aforementioned colors on-screen at once, even in a static configuration. Black is available as well.  On the downside, the semi-graphics resolution is only 64 pixels wide!  Also, for every pair of pixels either both must be the same color or one of the pair must be black.

Semi-Graphics (8 colors + black)

Dithered Semi-Graphics

Access to the full(!) range of colors improves the color fidelity of most images.  But the reduction in resolution isn't necessarily worth the trade-off.

Left Overs

I'm not going to cover the NTSC "artifact color" mode at this point.  It is more painful to use, since its color numbering changes somewhat randomly according to how the CoCo comes out of reset.  In any case, it is really just another 4 color mode.  Suffice it to say that it is comparable to the "color" graphics modes in terms of image fidelity.

Anyway, I hope that the above provides some good background on what CoCo graphics displays tend to be like.  There is plenty of room for improvement!  Luckily, dynamic VDG configuration changes greatly expand the possibilities for better CoCo graphics...

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