Mail Archive - Plus/4 World



	Home Search Games Tapes Covers Cheats Maps Software New Stuff Hall Of Fame HVTC Game Endings Solutions Remakes Publications Magazines Effects Top List Members Groups Features Plus/4 Encyclopedia Hardware Tools Options Forum Home Search Games Tapes Covers Cheats Maps Software New Stuff Hall Of Fame HVTC Game Endings Solutions Remakes Publications Magazines Effects Top List Members Groups Features Plus/4 Encyclopedia Hardware Tools Options Forum	Login

Previous | Next

From: Gaia (all posts)
Date: 2001-03-23
Subject: RE: Plus/4 palette

Hi All,

I made some experiments with the Color Hue values Zsolt posted to the list a couple of days ago. I got some help from an excellent document on the VIC-II palette.
(http://www.pepto.de/projects/colorvic)

The result is a calculated plus/4 palette which is quite close to the digitised one, and will allow to add runtime brightness/contrast/gamma setting to the emulator.

However, I had to use some wild guessing at a few places,
because I'm far from being a color expert, and also because of the lack of information.

First I had the Hue values (hue_angle), from which I could calculate the U and V color components (YUV conversion):

U = saturation * COS( hue_angle )
V = saturation * SIN( hue_angle )

I'll come to saturation a bit later. Now on to the luminance levels: the plus/4 have 8 luminence levels, which is effectively 9, if we take into account the black... Here are the luminance voltage levels from a doc:

Luminance Voltage
00 2.00 V
01 2.4 V
02 2.55 V
03 2.7 V
04 2.9 V
05 3.3 V
06 3.6 V
07 4.1 V
08 4.8 V

I'm sure I was wrong, but I used the following equation for normalising the luminances to a 0-255 scale (i is the luminance level index):

Y[i] = (Voltage[i] - 2.0)* 255.0 / (5.0 - 2.0);

I used 5.0 as I guess this is a default max. voltage...
(?!??)
Now comes the saturation... This was rather annoying as I needed some time until I found a workaround. In short, I used "brute force" methods to find suitable saturation values for all luminance levels, so that the resulting RGB components would still be within the valid range [0..255].
I ended up with the following figures (indexed on luminance):

saturation[9] = {
17, 23, 30, 38, 55, 60, 38, 8
};

And from here I could calculate the RGB components based on the following formulae:

R = Y + 1.367 * V G = Y - 0.336 * U - 0.698 * V B = Y + 1.732 * U

The end result, as I mentioned before, is a palette that quite closely resembles a real plus4 palette ( maybe a bit darker). I posted this to the list in the hope that someone could help me make it better. I might send the lastest beta of my emulator that has this new palette, in case someone is interested.

Regards,
Attila

PS. Maybe I could post the palette screengrab to the list?
It's only 3 KB...