Half-brite mode

[leonard]

Hi!

I'm thinking about half-brite mode to improve image conversion but I have few questions.
First of all, I think it's not supported on all A500 right? Doesn't work on OCS?

Second, I get that a grey color like $fff will produce a new color between $777 and $888.
But what if input color is $eee? will half-brite version be exactly the same as an original $777 color?

[Jobbo]

All A500's support EHB. Only some early US A1000's don't.

I always thought the half-brite colors were still 4bit per channel, it just takes the original values and halves them, rounding down.

It's probably just shifting them down in the hardware.

[a/b]

Half-bright works on all A500, only the very early produced A1000s are without it.
Both $f and $e are mapped to $7 (basically right-shift once, or integer div by 2).

[Estrayk]

That's right, as the above guys have said, very few units of the first A1000 sold in the USA in 1985 (the ones that came with ks1.0) do not support HB. So you could say that 99.99% of the world's Amigas support it.

[leonard]

ok so basically it doesn't "create" new colors. It just avoid to use all of them in the palette? Like there is no color in between $000 and $111 when using halfbrite on $111 value.
It's just that is you need a complete greyscale palette, you don't need all colors from $000 to $fff, you just need 8 values, and there 8 "halfbrite" values
$000,$222,$666,$888,.. $eee
So it's totally useless if you want to improve the 4bits per color component. Damn

[Jobbo]

Yeah, EHB is not as awesome and you were hoping!

I haven't considered how to determine the best possible palette when converting an image to EHB but it seems like it'd be challenge.

Also, I'm sure you're aware of the flickering techniques common in some newer ST games and in Batman Rises and Transhuman?

It'd be interested to see how far EHB can be pushed with similar flickering tricks.

[a/b]

Yeah, if you were to use a HB mode to display a greyscale image you would end up with something like:
00: 000
01: fff
02: eee
03: ddd
04: ccc
05: bbb
06: aaa
07: 999
08: 888
09: 333
0a: 222
0b-1f: free
"Auto-generated" HB colors would then be:
20: 000
21: 777
22: 777
23: 666
24: 666
25: 555
26: 555
27: 444
28: 444
29: 111
2a: 111
You would need all the 8-f color components (since HB color components can only be 0-7), plus whatever is missing. So it's not really a fifty-fifty scenario.

[Toni Wilen]

If anyone cares, reason why EHB can't create "half colors": Denise has 3x4-bit digital RGB output pins and EHB logic is inside Denise. It can't create more than 16*16*16 = 4096 colors.

[Karlos]

I've often wondered what the main reason for EHB was. Was it just that having 64 colour registers would take too much silicon?

[chb]

Originally Posted by Karlos

I've often wondered what the main reason for EHB was. Was it just that having 64 colour registers would take too much silicon?

Yes, pretty much. An additional set of 32 12-bit registers needs about 32*12*6=2304 transistors, which was substantial at that time.
I guess they had 6 bitplane dma channels for dual plafield and ham anyway, so someone had a clever idea to get 64 colors on the cheap.

[leonard]

Originally Posted by Jobbo

I haven't considered how to determine the best possible palette when converting an image to EHB but it seems like it'd be challenge.

I'm playing around with a brute force search, take a while even on my 16 threads AMD CPU. At the end I discovered the palette is important in HAM but *not* so much. Like, you could even have a completly black palette (so just using HAM delta colors), and you can easily recognize the picture (obviously with plenty of typical horizontal ham glitchs). Then, adding colors in the palette one by one improve the image. ( I brute force the color adding one by one in the palette by lowering an average "color match score" over the complete picture)

Originally Posted by Jobbo

Also, I'm sure you're aware of the flickering techniques common in some newer ST games and in Batman Rises and Transhuman?

yeah Atari has some really nice tech to display high color recently, also using color flickering ( like flickering $333 and $444 is kinda creating a in between color). I thought about using that in my HAM converter but don't know how to properly add this feature in my brute force search right now

Originally Posted by Jobbo

It'd be interested to see how far EHB can be pushed with similar flickering tricks.

You mean without HAM mode? Like, standard OCS 5 bitplans, but with EHB active, so having "kinda" 64 colors palette? I don't know, could be interesting to try.

[leonard]

Originally Posted by Toni Wilen

If anyone cares, reason why EHB can't create "half colors": Denise has 3x4-bit digital RGB output pins and EHB logic is inside Denise. It can't create more than 16*16*16 = 4096 colors.

thanks for technical detail! this definitely kills all my hopes of having more than 4096 colors without using flickering technics

[pandy71]

Originally Posted by leonard

thanks for technical detail! this definitely kills all my hopes of having more than 4096 colors without using flickering technics

"flickering technics" are temporal dithering and it is commonly used by modern display technology... so there should be plenty of papers and patents on this in web.

btw as Amiga video DAC a.k.a. Vidiot produce B/W in analog domain so on CVBS Amiga output there is more than 16 levels of gray.

Forget to add about neat software using "dynamic" to number of colors, sadly it is only for Atari ST(E) and Windows http://www.leonik.net/dml/sec_pcs.py - but using same methodology should be straightforward in Amiga.

[duga]

Originally Posted by leonard

I'm playing around with a brute force search, take a while even on my 16 threads AMD CPU. At the end I discovered the palette is important in HAM but *not* so much. Like, you could even have a completly black palette (so just using HAM delta colors), and you can easily recognize the picture (obviously with plenty of typical horizontal ham glitchs). Then, adding colors in the palette one by one improve the image. ( I brute force the color adding one by one in the palette by lowering an average "color match score" over the complete picture)



yeah Atari has some really nice tech to display high color recently, also using color flickering ( like flickering $333 and $444 is kinda creating a in between color). I thought about using that in my HAM converter but don't know how to properly add this feature in my brute force search right now



You mean without HAM mode? Like, standard OCS 5 bitplans, but with EHB active, so having "kinda" 64 colors palette? I don't know, could be interesting to try.

Isn't that how it's done in at least one Amiga Doom port? https://aminet.net/package/game/shoot/ADoom-1.3

[Karlos]

I had an idea years ago that I never tried beyond a simple mockup because I had AGA. The idea was to take a 320*256 image using 15bpp palette and to display it interlaced with every pixel split vertically and spreading the LSB difference between them, one halve with all channels rounded up, the other rounded down. To improve the effect the idea was to alternate the low and high halves in a typical checker board fashion and finally to invert the whole thing every frame. It looked like it might have legs for static images at least.

[sandruzzo]

Originally Posted by chb

Yes, pretty much. An additional set of 32 12-bit registers needs about 32*12*6=2304 transistors, which was substantial at that time.
I guess they had 6 bitplane dma channels for dual plafield and ham anyway, so someone had a clever idea to get 64 colors on the cheap.

Maybe a trick to switch palette like Aga, could have been possible..

[phx]

Originally Posted by Estrayk

That's right, as the above guys have said, very few units of the first A1000 sold in the USA in 1985 (the ones that came with ks1.0) do not support HB.

I bought a PAL Piggyback-board A1000 with Kickstart 1.2 and US keyboard in the first half of 1987 here in Germany. It had a ceramic Denise without EHB mode.
So there are problably more A1000s affected.

[Estrayk]

Originally Posted by phx

I bought a PAL Piggyback-board A1000 with Kickstart 1.2 and US keyboard in the first half of 1987 here in Germany. It had a ceramic Denise without EHB mode.
So there are problably more A1000s affected.

Is weird because in Spain the A1000's that were sold in 1986 with kickstart 1.1 had EHB. That is why I assumed what I said.

[Jobbo]

What does a machine without EHB do when code tried to display an image in EHB?

Does it just look like it's ignoring the 6th bitplane?

[ross]

Originally Posted by Jobbo

Does it just look like it's ignoring the 6th bitplane?

This.