ACNews #61 21 января 2016

How to convert gfx for colour-per-pixel mode - how to convert gfx to "EGA" mode of ATM Turbo.

           How to convert gfx for colour-per-pixel mode
                        by Alone Coder

I was asked a question, how to convert gfx to "EGA" mode of
ATM Turbo. 

If you want just a photo with dithering:

1. Tune up brightness/contrast/saturation more vivid.

2. In Photoshop, convert to adaptive palette of ~30 colours.
Don't apply this palette though. Do edit it. 

3. Change every color by hand to match ATM Turbo's colours (i.e.
with components equal to 0/85/170/255 - howeverLVDmeasures 
there 0/93/162/255, but I'm not sure the authors of ATM Turbo 
wanted these exact values; the difference is subtle though). If 
there is uncertainty how to round - make a couple of versions of 
the colour (anyway the palette will have a lot of identical 
colours). 

4. See how many colours we get (apply, convert to RGB, then to
"exact" palette). If there are less than 16, add colour 
variants. If there are more, remove the rare shades (if they can 
be simulated with dithering). 

5. Apply the palette. Try dithering between 30% and 100%.

6. Convert to RGB.

7. Convert to "exact" palette - 16 colours.

8. Save as a 4-bit BMP.

While you select a palette, you can time to time save it, apply,
undo the result...

Hippiman suggests the following method:

───────────────────────────────────────────────────────────────┐ 
You must reduce the number of colour in the image to the
minimum. The best way is to do it gradually, in few steps.

First, experimentally convert the image to "adaptive" palette
with almost the original quality. Generally there are 60 to 70
colours.

Then "join" the similar colours, possibly to ATM colours. Try to
avoid small areas of rare colours.

Then convert to RGB and repeat many times, until you have around
30 colours.
───────────────────────────────────────────────────────────────┘ 

If you have to convert pixel art (as in Nedodemo 2 ), there is
more to do. You must try by hand several versions of colour
grades for each of the colour grades of the original palette.
Sometimes you must lose one colour or two (join it with
another). If you need 50% textures, first convert the picture to
~30 colours with components 0/42/85/128/170/212/255, then find
an ATM Turbo palette that covers all of your colours with 50%
textures, then convert every colour area to 50% texture (not all
at once - Photoshop can't make a clear 50% texture everywhere).
I found no better method than to copy the area to another
document (initially white) then setting a 3-colour palette
(white plus two colours, the given colour must be exactly in the
middle between them) then apply with diamond dithering. Then
copy the converted area back to the original picture.

Converting toDDp's palette (RЧGЧB4) is easier - just apply the
"adaptive" palette and save. When you remove the least
significant bits, there will be a little difference. If you want
it better, correct the "adaptive" colours by hand before you
apply them.

DiHalt Visitors' software for group conversion of BMPs to packed
memory dumps is there:
http://alonecoder.nedopc.com/zx/convega.rar

There is also a special utility byTmK that converts a picture
to two flickering screens, colour-per-pixel. However it can't
find the best palette