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PCHROME.DOC
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1992-11-01
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21KB
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449 lines
Atari ST/E PhotoChrome v3.00
-----------------------------
Concept, Design and Programming
by Douglas Little.
(C) 1992 Pixel Twins Shareware Utilities.
You are now the proud owner of the PhotoChrome graphics card
emulator. It is the only one of its type currently available for the
Atari ST and is capable of out-performing all other screen modes
including the infamous HAM and HalfBrite displays on the Commodore
Amiga.
It's basic specification is the ability to load most IFF formats,
24-Bit True colour screens (16 million colours), GIF files and
VidiChrome digitised 'RGB' colour separations (4096 colours) before
converting to one of six powerful screen modes.
*---------------------------------------------------------------------*
THE 6 CONVERSION MODES
----------------------
The six screen modes are...
1. Spectrum 512 > 42/45 cols per scanline from 512 on an ST
or STE.
2. Spectrum 4096 > 42/45 cols per scanline from 4096 on an STE.
3. PCS-ST > 48 colours per scanline from 512 on an ST
or STE.
4. PCS-STE > 48 colours per scanline from 4096 on an STE.
5. Super HAM > A full 4096 colours out of 4096 on an ST
or STE.
6. STE PhotoChrome > 19200 colours out of 32768 on an STE.
Modes 1 and 2 are basically real Spectrum-512 files. They are not
compressed, but Spectrum can do this if you need it. STE mode seems to
handle pictures better than I had expected, since with such a large
palette the colour assignments tend to be used up twice as quickly.
Modes 3 and 4 are my own custom versions of the Spectrum display.
They both have a few extra colours to play with, filling up the streaks
and dots Spectrum can often leave behind. This is my own 'PCS' format
(PhotoChrome Screen) and uses the best of 2 compression methods.
Modes 5 and 6 are actually impossible, so just accept they work
and don't ask too many questions!
SUPER HAM is an emulation of the Hold And Modify mode normally
exclusive to the Commodore Amiga. After some experimentation with my
own Amiga, I found that HAM is not actually as capable of displaying
Çsome of the more detailed colour images (especially with high levels of
contrast or lots of sharp lines) and so renamed my own format 'SUPER
HAM'. Many HAM pictures are of course better as they have many more
colours per line, but have to smudge and blend to obtain them.
PHOTOCHROME is only available when the source picture contains in
excess of 4096 colours (i.e. QRT 16-million colour pictures), and then
only to STE owners. However, if you do have an STE and some RAW files
knocking about (STF cover disk 35!) then you are in for a treat indeed!
Eat your heart out, VGA! This is the most powerful mode available short
of a real Graphics card, with 19200 colours on-screen at once from a
largish palette of 32768! (15-bit colour). The picture requires two
conversion passes, but is well worth the wait!
N.B. Pictures saved out as Super HAM or PhotoChrome tend to be
anywhere from large to enormous, even with fancy compression. Bear this
in mind when testing out the hatched and fractal stippling options!
(Check out PCSVIEW.DOC for info on better compression...)
*---------------------------------------------------------------------*
FEARURES
--------
1> STIPPLING. (COLOUR REDUCTION ONLY)
Generally, if the source picture holds more colours than the
destination format (i.e. RAW > SPU) then you are allowed stippling.
The three options are CLEAN - just leave the picture as it is, HATCHED
- use levels of hatching to make up in-between colours and FRACTAL -
the weird one. This uses fractal numbers to highlight/leave each pixel
on the screen, the result being that graduated colours look more obvious
but you get a weird 'oil-painting' effect. It really only works well
in PhotoChrome and super-HAM modes.
2> ERROR/FLICKER FREE. (SUPER HAM/PHOTOCHROME ONLY)
Error-free mode makes a better job of mega-colourful pictures
but is not normally needed (a sort of last-resort). 'Flicker-free'
should be used where available, the only problem being that it chews
through the colour tables at an alarming rate (keep an eye on QRT
screens). N.B. STE PhotoChrome mode is fixed at ERROR-FREE in 60Hz,
since flicker is minimal with such a large palette.
FLICKER-FREE mode CAN sometimes reduce the quality of the screen...
3> 50/60 Hz MODES. (SUPER HAM and QRT PICTURES ONLY)
Basically, everything runs at 50Hz, with a couple of exceptions.
QRT pictures tend to have a 60Hz aspect ratio (not fixed but the option
is there) causing the screen to look squashed at 50Hz. Also, PHOTOCHROME
mode looks best at 60Hz anyway, so it is left as the default frequency.
60Hz also helps when 'flicker-free' option has been avoided for
accuracy. To change the default, click on the 50Hz button. All screens
will now run at 50Hz regardless of how they might look. To switch modes
while viewing the picture, hit the '*' key.
4> SPECTRUM OPTIONS.
Spectrum 512 does not use all of it's available colours. Colour 0
Ç(background colour) is always black and colour 15 is reserved for drawing
and for the mouse pointer etc. leaving 14 variable colours for use in the
picture. There are 3 palette changes per line meaning (16-2)*3 or 42
cols available for alteration on each individual scanline of the
screen. The 'LEGAL COLOURS' option uses just these colours. The 'ALL
COLOURS' option however releases colour 15 for use, meaning (16-1)*3 or
45 cols per line. The only disadvantage is that Spectrum will crack up
if you try to load them in. Any pixels using colour 15 will turn bright
orange and look a right mess. You have been warned... (N.B. slideshows
should display these pictures no probs.)
*---------------------------------------------------------------------*
LIMITATIONS
-----------
Hmmm, I'm not sure there are any, really..
No, seriously, there are a few bits you should know about before
creating slideshows full of pretty pictures!
1> RAW's
QRT screens can be any width or height (<=320*200) and will be
centred on the screen with a black border. If you use the 'flicker-
free' option (SUPER HAM only) some loss of detail or palette-errors
might occur on very complex pictures. To avoid this you may have to use
the 'error-free' mode instead (at 60Hz there is very little flicker
anyway). To cut down any flicker you may still see, reduce the contrast
setting on your monitor.
2> GIF's
The program can load GIF's of any height or width and will offer
the option to either squash the picture, or select a 320*200 zone.
320*200 GIF's will bypass this stage. To select a zone, use the arrow-
keys and 'SPACE' when satisfied.
2> IFF's
The program can load ILBM IFF's of any height or width but will
crop them to 320*200 (a later version will contain squash/interlace
options). The real bonus is that it eats up colours like nothing else.
All of the Amiga pictures I fed it looked identical to the originals,
with no loss of colour or detail! (HalfBrite 64 colour IFF's are
supported properly)
3> RGB's
VidiChrome RGB screens suffer from very few limitations. Generally
you should choose the 'flicker-free' option all the time and if you
spot dots/errors on the screen (rare) you can resort to 60Hz and
'error-free' mode.
4> SYNC PROBL