HALFCOLORBUG fix

[Valken]

I was using DamienD's set and noticed some games looked off.

The games basically look HALF as bright as it should be on the Amiga vs an Atari ST (real or emulated).

I really noticed it in Vixen.

So I did a search it and it turns out there might be an Amiga Half Colour Bug which FS-UAE appears to have a fix for:

http://eab.abime.net/showpost.php?p=...&postcount=229

The games noted are here:

http://openretro.org/browse/halfcolorbug

It would be great to offer such a fix or shader in WinUAE as that is my primary platform for testing.

[dlfrsilver]

About Iron Lord, it was a pork job. I have checked the files, they did some conversion on some graphic files, but they kept some files in atari format. The result is that the palettes have indeed to be corrected.

[Retro-Nerd]

WinUAE already has it. Look under Display settings where you can change brightness etc. There is a "Dark palette fix" option.

[mark_k]

WinUAE already has a feature to correct the colours, and more options.

On Display settings, in the Brightness drop-down box select "Dark palette fix". There are four options selectable using the slider to the right: 0.0, 1.0, 2.0, 3.0

You can see how those settings work in the WinUAE source, video_calc_gammatable() function in gfxutil.cpp.

0 means no correction.
1 means ST_value -> ST_value × ($FC / $EE)
2 means ST_value -> ST_value × 2
3 means ST_value -> ST_value × ($FC / $77)

To explain that: The FS-UAE shader just multiplies by 2.0 (like the "2" setting in WinUAE). But a better option is to multiply by ($FC/$77), since then "ST white" is closer to actual 24-bit white. That's the WinUAE "3" setting.

Why would you use the "1" and "2" settings if "3" is best? Well...

"1" can be used for Amiga ST-ports which have doubled ST colours. With those on a real Amiga, "ST white" $777 corresponds to Amiga $EEE which isn't quite white. Since PC palette is 24-bit, you can have that not-quite-white map to $FCFCFC instead. So almost full white.
Example Amiga game whose palette is like that: Gods

"2" is what a simple doubled palette would look like. That's the best you can do on a real Amiga with 12-bit palette.

"3" is used for unadjusted (half bright) ST ports. $777 maps to $FCFCFC, almost full white.

[Valken]

Thanks RN! Will look into it and tweak the settings against Hatari to see how it turns out!

Originally Posted by mark_k

...
0 means no correction.
1 means ST_value -> ST_value × ($FC / $EE)
2 means ST_value -> ST_value × 2
3 means ST_value -> ST_value × ($FC / $77)

How do we check for this as a plain bog standard user without God powers like Toni or the other hackers here? I guess we have to eyeball it unless there is a list somewhere so we can setup and share presets or fixes...

[mark_k]

Basically: if a game is obviously half-brightness, use setting "3". That should give the same result as an ST emulator like Hatari.

If a game is an ST port but the colours look OK on a real Amiga (or emulator), there might still be some benefit from setting "1" instead of no correction. To check, you could take a screenshot of the game and load into a paint package. See what colour the white pixels are. If white pixels are colour 238:238:238, setting "1" should give a better result (but the difference should be fairly subtle, only ~6% brighter).