Query about Extra Half-Brite (EHB)

[Foebane]

So OCS/ECS Amigas had this mode which on principle takes the 32 colours of the first five bitplanes and with the extra bits of the sixth bitplane, halves the brightness if the bit is set.

What I've always wondered about is if the halved brightnesses were tied into the 4096 colours of the first 32, or unique accurate shades in their own right. What I mean is, say if a colour register with R01 G02 B03 is used in an image, would the EHB equivalent be R0.5, G01 and B1.5 or would it be the closest colours to be compatible with the 16 levels of RGB that this chipset could produce?

[robinsonb5]

Originally Posted by Foebane

What I've always wondered about is if the halved brightnesses were tied into the 4096 colours of the first 32, or unique accurate shades in their own right. What I mean is, say if a colour register with R01 G02 B03 is used in an image, would the EHB equivalent be R0.5, G01 and B1.5 or would it be the closest colours to be compatible with the 16 levels of RGB that this chipset could produce?

The video DAC inputs only have four bits per gun, so there are no extra colours in the cracks between the 16 levels that are normally available. Instead the levels are simply shifted one bit to the right, truncating the lowest bit, so R01 G02 B03 would become R00 G01 B01.

[Foebane]

Thanks, robinsonb5, I figured it would be that. Just wanted confirmation. I assume the same is true for AGA as well?

[saimon69]

@foebane
Wish it was so,and have a 512 colors EHB mode for aga, but afaik does not.

[commodorejohn]

I think he means, does the EHB mode work the same on the AGA chipset, since the DACs there are 6 bits per element?

[Foebane]

I thought when it came to EHB in AGA, it was 8 bits per element?

Come to think of it, I'm even more confused now. I'm thinking of five bitplanes in OCS for EHB, so how many are there in AGA? Can someone describe in detail how EHB in AGA works, please?

[eXeler0]

Can palette switching be used in AGA? Dividing screen into blocks / slices and modifying palette registers on the fly during vertical scans?
Using this trick it was apparently possible to produce 512 colors on ECS, but with limitations of course (one 32 color palette / block)

[Mrs Beanbag]

of course, AGA machines also have copper, you can change any hardware register approximately every 8 pixels. Writing a 24-bit colour value requires 3/4 register writes though.

[Toni Wilen]

EHB works exact same in AGA, 6 bit plane mode without BPLCON2 KILLEHB bit set. (KILLEHB was originally introduced in ECS Denise, meant for genlock mode where 6th plane can be genlock transparency mask, it was not AGA-only bit).

BPLCON4 upper 8 bits can be used to make quick color "bank" switches on AGA machines.

[Mrs Beanbag]

so what happens if you use EHB on an AGA machine, but use 24-bit colour values in the palette?

[Toni Wilen]

Originally Posted by Mrs Beanbag

so what happens if you use EHB on an AGA machine, but use 24-bit colour values in the palette?

AGA EHB always uses full AGA palette range.

[Mrs Beanbag]

Originally Posted by Toni Wilen

AGA EHB always uses full AGA palette range.

but if OCS EHB truncates the Halfbright colours to 12 bit... and AGA EHB doesn't... they don't work the same way, right?

[Foebane]

Originally Posted by Mrs Beanbag

of course, AGA machines also have copper, you can change any hardware register approximately every 8 pixels. Writing a 24-bit colour value requires 3/4 register writes though.

Could the OCS copper change a hardware register every pixel? There is a demo (Arte by Sanity) which has the most amazing plasma effect I've ever seen, and it's so fluid it could only be the copper changing a colour every single pixel!

And HAM8 is another query, as well.

HAM6 is simple to understand, the first two bits being:

0 - Colour register
1 - Red
2 - Green
3 - Blue

(OK, maybe not in that order)

and the remaining four bits being used for a value of 0-15 for each one, which makes full use of the OCS 4096 colours, or 12-bit palette.

BUT...

HAM8 is the first two bits used as described above, but leaving only 6-bit precision (64 levels) of Red, Green, and Blue. So what does AGA do then? Take the colour register with the 24-bit RGB, convert that to the closest 18-bit RGB and then modify the R, G or B in 6 bits?

[Toni Wilen]

Originally Posted by Mrs Beanbag

but if OCS EHB truncates the Halfbright colours to 12 bit... and AGA EHB doesn't... they don't work the same way, right?

Results are still the same. Same colors.

[Mrs Beanbag]

Originally Posted by Toni Wilen

Results are still the same. Same colors.

how can they be?

On OCS $FFF -> $777 = $777777
On AGA $FFFFFF -> $7F7F7F

[Toni Wilen]

Originally Posted by Foebane

Could the OCS copper change a hardware register every pixel?

No

HAM8 is the first two bits used as described above, but leaving only 6-bit precision (64 levels) of Red, Green, and Blue. So what does AGA do then?

AFAIK two lowest bits of color component are not modified.

Originally Posted by Mrs Beanbag

how can they be?

On OCS $FFF -> $777 = $777777
On AGA $FFFFFF -> $7F7F7F

You are right, for some reason I thought them as 2 separate 4 bit shifters (which probably isn't right, unless EHB really was designed to be fully OCS/ECS compatible)