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Old 14 February 2020, 16:03   #1
roondar
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Question Converting a given palette to Extra Half-Brite

I've been wondering lately how you'd properly convert colours to an EHB specific palette. For non-EHB I can think of several approaches, but for EHB I'm kind of stumped on how to deal with the half-bright colours.

Are there any good algorithms available for this (including source)?
I did try to find some on EAB, but my search-fu was not particularly good so I found lots of threads on EHB in general.
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Old 15 February 2020, 13:10   #2
Solo Kazuki
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To reduction colours in specific picture, i was using PPaint, which have good palette reduction parameters.

But i suppose it's for overall work, not one picture?
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Old 15 February 2020, 13:50   #3
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I used (for another color reduction problem, not EHB) a modified k-means algorithm, where I substituted the euclidean distance by a custom distance function. I guess in the case of EHB, the centers would be your 32 direct palette colors, but when computing the distance to a pixel color, you'd use the min of the distance to the direct color and the associated half brite color.

It's a bit of a sloppy approach, because all the convergence theorems etc. for k-means assume euclidean distance, but for me it worked reasonably well. K-means is quite slow, however, so it's rather not well suited if you want to write something running on the Amiga.
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Old 15 February 2020, 14:06   #4
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Quote:
Originally Posted by Solo Kazuki View Post
To reduction colours in specific picture, i was using PPaint, which have good palette reduction parameters.

But i suppose it's for overall work, not one picture?
Well, I want to convert a whole bunch of pictures (on the order of 200-300). I suppose I could use PPaint for it, but I thought it might be simple to just make a small program that does it manually.

I also had a look at http://tool.anides.de/ but that doesn't actually convert to EHB. It claims to do so, but the results are identical to the 32 colour version it generates
Quote:
Originally Posted by chb View Post
I used (for another color reduction problem, not EHB) a modified k-means algorithm, where I substituted the euclidean distance by a custom distance function. I guess in the case of EHB, the centers would be your 32 direct palette colors, but when computing the distance to a pixel color, you'd use the min of the distance to the direct color and the associated half brite color.

It's a bit of a sloppy approach, because all the convergence theorems etc. for k-means assume euclidean distance, but for me it worked reasonably well. K-means is quite slow, however, so it's rather not well suited if you want to write something running on the Amiga.
This makes me wonder how the old Amiga programs did this. I can't imagine them using something this slow.

Anyway, I must confess my own ideas where considerably simpler. I was looking at something more along the lines of modifying existing code like https://rosettacode.org/wiki/Color_quantization/C

Perhaps it's more efficient to select an existing program and scripting around it so it can do the slew of images I want to try. Which leads to the obvious question: which program is the best bet here. Probably an Amiga one, as I don't suppose modern programs support EHB output in any meaningful way.
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Old 15 February 2020, 17:47   #5
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If you want to use an existing program, then there's also hamconvert
which, despite the name, supports EHB and has a batch mode.

PS: Rosetta code has an example of color quantization using k-means - unfortunately it is written in J, which at least for my eyes looks quite similar to banging your head on the keyboard. But it's very short!

Last edited by chb; 15 February 2020 at 17:53.
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Old 15 February 2020, 21:02   #6
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Wow. J looks almost as bad as Brainf**k. Completely unreadable and unusable language.
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Old 15 February 2020, 22:51   #7
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Yeah, J is something else. Give me 68000 assembly any day over that!

I'll be checking out HAMconvert. See if that actually does create 64 colour images when converting to EHB
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Old 15 February 2020, 23:21   #8
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You can see here where I gave it a go to create an EHB converter:

https://github.com/alpine9000/amiga_...r/011.ehb_mode

I didn't really expend much effort at the time as EHB wasn't something I really saw myself using much.
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Old 15 February 2020, 23:32   #9
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Thanks, I'll check it out!
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Old 17 February 2020, 03:46   #10
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alpine9000, do you have any ideas how the code could be improved to make better use of EHB?

Not necessarily asking you to actually make the changes, I'm happy to have a play around with it.
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Old 17 February 2020, 08:53   #11
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The pic to raw converter I use (KingCon by Hannibal/Lemon.) has a EHB conversion option. Not sure what it does but I can run a picture through it and spit out the raw + dc.w palette data if you have a test picture handy?
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Old 17 February 2020, 09:22   #12
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Quote:
Originally Posted by Hewitson View Post
alpine9000, do you have any ideas how the code could be improved to make better use of EHB?

Not necessarily asking you to actually make the changes, I'm happy to have a play around with it.
I am pretty sure my code just picks the “best” 32 colour palette, then does a quantisation of the original image with the 32 colour + 32 half bright colours.

Off the top of my head, I guess two ways to improve it would be picking the base palette with the intelligence that half bright colours exist and also taking advantage of the half bright colours in a more sophisticated way (like being used in a dithering algorithm for example).
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Old 17 February 2020, 10:41   #13
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I'm going to be using hamconvert for now (which is a very nice tool, by the way). Writing a proper colour quantisation algorithm for EHB is probably going to take more time than I want to spend on it at the moment.

As for the why I'd want to.. Well, I've always been fascinated by EHB and don't always like the "HAM look". So I want to experiment a bit with images converted to EHB instead and see how that ends up.

Last edited by roondar; 17 February 2020 at 11:08.
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Old 17 February 2020, 13:05   #14
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Hi Roonar, I remember converting Rygar graphics to HalfBright mode. One of my approaches was to go to the export gif option in photoshop and here you can lock colours. Another setting first needs to happen, that is display most commonly used colours first, from this I’d then try to Eliminate colours that are very close in rbg value. The aim of course was to get 64 colour palette exported.

Other option was to import the arcade graphics as a 256 colour image into Brilliance and let it convert. Happy to share what I came up with.

By the way if you want any of us to experiment with converting graphics with a particular palette in mind please share the file

Good luck and look forward to seeing what you come up with
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Old 19 February 2020, 16:48   #15
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Once I have something, I'll be sure to share it. But it may be a while yet
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Old 20 February 2020, 16:23   #16
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I'd like to see a dynamic/sliced EHB converter/viewer for OCS...
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Old 21 February 2020, 00:04   #17
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Thought I'd post the experimentation of HalfBright for Rygar, If nothing else was an interesting process of reducing arcade colours (256?) down to 64.

The example at top left of the lava enemy shows 3 colours replaced using the darker versions., considering its only two red colours in the 32 colour palette it works pretty well.
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Old 24 February 2020, 19:40   #18
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Perhaps you can use my image converter? (http://bgafc.t-hosting.hu/prgv.php?p=2)
If you have a palette which you want to convert into EHB, then you can just save an image with it and convert it with the converter.
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