[Coco] Artifact purple/green wasRe: CoCo Monitor

John Kowalski sock at axess.com
Mon Jun 28 11:50:51 EDT 2004

At 10:24 PM 27/06/2004 -0500, CoCoList for Color Computer Enthusiasts wrote:
>Some of the emulators have an artifact mode, but now I wonder:  How far 
>do they go?  Do they just simply do the "odd is red, even is blue" or 
>vise versa colors, or did they actually try to emulate all the 
>artifacting in NTSC?  That would let our PAL friends see them.
>		Allen

Most emulators just do red,blue,white and black.
MESS and Mocha, on the other hand simulate complex artifacting.

What I did was hook up a CoCo to my PC's ATI All-In-Wonder video input, and
adjust all adjustments (color saturation,hue,etc..) until the display most
closely duplicated how I remember seeing colors on an old TV.   In other
words, adjusted until all the extra colors like purple,green,yellow and cyan
started appearing in PMODE4.  (New displays do a good job of eliminating all
these 'unwanted' colors.)

Then, on another CoCo3, I loaded a sample PMODE4 bitmap into memory, and
wrote a program to re-interpret the B&W image into 16 color 256x192
resolution.   I chose a set of 16 colors that would be used to duplicate all
PMODE4 artifacting effects, then coded up a lookup table for the PMODE4 bit
patterns.  The table was made to extend a few pixels before and after the
'target' pixel.  That is, the pattern of pixels extending to the left and
right of any given pixel may have an effect on the appearance of the pixel
in the center.
Run the program, see what happens... adjust the tables... Run the program,
see what happens... until eventually the simulated PMODE4 artifacting looked
just like the real thing.

MESS and Mocha both use the resulting data and algorythm to simulate
artifacting.  Mocha even simulates color artifacting in the green and black
PMODE4 screen! (You can get many hues of green, plus blue and yellowish-green).

And possibly, these CoCo emulators may be/have been the very first emulators
ever to simulate composite video color artifacting realistically.   As far
as I know, not even Apple II (which relied heavily on artifact colors)
emulators do this yet.

                                         John Kowalski (Sock Master)

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