Harvey_Taylor@mindlink.UUCP (Harvey Taylor) (03/15/91)
Background: I want to display some large 256 level greyscale
video samples. The standard Amiga will output only 16 levels.
The questions deal with greyscale & how to get the max number
of greyscales out of the amiga hardware.
Q0: It seems the maximum greyscale range I can get is 16. Why? It seems
to me I should be able to get 32 using EHB mode. Yes? No?
Q1: In the A2024 docs [in the autodocs] this formula occurs:
*
* The function is:
* Gray Level = (77 * R + 150 * G + 29 * B + 128)/1024
*
What is the basis for this?
Q2: Dithering
Has anybody written an n-bit sample to dithered bitmap routine?
Q3: GreyScale to HAM [SHAM, SHAMM, Dynamic whatever]
Will HAM (and dynamic brethren) be any use?
Q4: A colour board
What greyscales do the popular 24 bit boards put out?
(ie. DCTV, ColorBurst, HAM-E, A2140, Firecracker 24)
Q5: Are there any other options?
---Summary---
Q0: It seems the maximum greyscale range I can get is 16. Why? It seems
to me I should be able to get 32 using EHB mode. Yes? No?
(peterk@cbmger.UUCP)
No, there are physically only 16 shades. In EHB mode, the color is
halved, so that you would get half steps, but then is rounded to
the next integer step.
(alex@bilver.uucp)
No. The Amiga video hardware only allows 16 intensity levels
for the red, green, and blue electron guns in the CRT. You will
notice in your palette setup that there are 4 bits per gun,
which means 16 levels.
Q1: In the A2024 docs [in the autodocs] this formula occurs:
*
* The function is:
* Gray Level = (77 * R + 150 * G + 29 * B + 128)/1024
*
What is the basis for this?
(alex@bilver.uucp)
This formula approximates the NTSC formula for converting color to
black & white.
Q2: Dithering
Has anybody written an n-bit sample to dithered bitmap routine?
(kholland@hydra.unm.edu)
It would probably be better to get TAD from ASDG becuase
the dithering techniques give a closer simulation of
256 shades of grey (and more natural look, seems to improve resolution).
(jbickers@templar.actrix.gen.nz)
I'd go with dithering. If you can accept HAM (LORES), then you could
also use EXTRA_HALFBRITE mode, which (if you picked some off-gray
colors) could give you 64 levels to dither into.
> Has anybody written an n-bit sample to dithered bitmap routine?
I have, to convert .GIF files to HIRES grayscales. Just a simple
ordered dither, though, that only uses a 2x2 thingos, so only 6 bits
of the original is considered relevant. I convert color .GIF files
into greyscale, then dither away.
(chris@kessner.denver.co.us)
This should help some. I don't know if you'll get 256 grey-levels, but
it's worth a try. Will look very grainy.
(James_Hastings-Trew@tptbbs.UUCP)
Other than than, you best bet is dithering - HAM and Dynamic
modes don't alter the palette available, so still only have 16
greys to work with in those modes.
The Art Department does a very nice job of converting 256
greyscale images to 16 greys with Floyd-Steinberg dithering. I
daresay at hi-res interlace, the results would be
indistinguishable from real 256 grey once you get it into video.
(peterk@cbmger.UUCP)
Should work. Remember, you can take a HiRes screen, so your
resolution is still at least LoRes. (Don't know from heart
how many grey scales you can get from this, sure not 256 :-)
Dithering references:
---------------------
Digital Halftoning - Robert Ulichney
MIT Press 1988
ISBN 0-262-21009-6
Computer Graphics, Principles & Practice - Foley, vanDam, Feiner & Hughes
Addison Wesley 1990
ISBN 0-201-12110-7
Q3: GreyScale to HAM [SHAM, SHAMM, Dynamic whatever]
Will HAM (and dynamic brethren) be any use?
(kholland@hydra.unm.edu)
As you said the Amiga has only 16 grey scales, therfore
HAM is useless since it follows the same principle.
I mean, HAM can use only 16 shades but many hues.
(chris@kessner.denver.co.us)
Won't help. HAM and SHAM (And the dynamic-doodads) all have the
same number of gun intensity levels as the normal display modes.
(peterk@cbmger.UUCP)
If you don't have extra hardware connected: NO solution.
Q4: A colour board
What greyscales do the popular 24 bit boards put out?
(ie. DCTV, ColorBurst, HAM-E, A2140, Firecracker 24)
(es1@cunixb.cc.columbia.edu)
You can get 256 grey scales with the HAM-E board using 384x480
resolution. The higher model will do anti-aliasing to get you
the equivalent of 768x480 resolution.
(kholland@hydra.unm.edu)
The other way to go is use a 24-bit graphics card. DCTV and the
other under 6000 dollar graphics cards are somewhat jokes except
for HAM-E becuase you can't really view 24-bit color since NTSC
is limited to ~ 21-bit color resolution (2 million colors). It
would probably be better to use either HAM-E (300 dollars) or
Firecracker 24 (1000-1500 dollars) to do this stuff.
(jbickers@templar.actrix.gen.nz)
The properly 24-bit ones will put out "enough", by definition. :)
At 8 bits per primary, they must be able to do 256 levels of gray.
(chris@kessner.denver.co.us)
Now this WILL get you somewhere.
Well, my experience tells me that DCTV, ColorBusrt, A2410, and
FireCracker, all being 24-bit display boards, can give you 256
gun-intensities, and therefore 256 greyscales. HAM-E is a
palette-based display. It allows you to display any 256 colors
out of 16 million. It can also just as easily display 256
greyscales. It has the added advantage of color-cycling, if you
should so need it.
(James_Hastings-Trew@tptbbs.UUCP)
A 24 bit colour card should be able to give you a true 256 grey
scale picture.
(peterk@cbmger.UUCP)
If it's 24 bits, then yes.
Those which really provide 8 bits per color component RGB,
give you exactly 256 grey shades.
Q5: Are there any other options?
(oliphant@telepro.UUCP)
I've achieved some rather nice results by usings shades of
purple to emulate 32 shades of grey.
Here are the colors I use:
UWORD colortable[32]={0x000,0x102,0x111,0x213,0x222,0x324,0x333,0x435,0x444,
0x546,0x555,0x657,0x666,0x768,0x777,0x879,
0x888,0x98a,0x999,0xa9b,0xaaa,0xbac,0xbbb,0xcbd,0xccc,
0xdce,0xddd,0xedf,0xeee,0xfef,0xfff,0xfff};
---
-het
PS.
Got another idea? Send it along...
"But now some other stranger seems to want you to ignore his dreams,
as though they were the burden of some other." -L.Cohen
Harvey Taylor Meta Media Productions
uunet!van-bc!rsoft!mindlink!Harvey_Taylor
a186@mindlink.UUCPford@amix.commodore.com (Mike "Ford" Ditto) (03/17/91)
Harvey_Taylor@mindlink.UUCP (Harvey Taylor) writes: > Background: I want to display some large 256 level greyscale > video samples. The standard Amiga will output only 16 levels. And someone refered him to this book: > Computer Graphics, Principles & Practice > - Foley, vanDam, Feiner & Hughes Please note that this book is an enhanced edition of an ealier work by (only) Foley and VanDam. Make sure that you get the new edition, not the first one, since Feiner Hughes are precisely what you are interested in. Hee hee, -=] Ford [=- "Like a pizza in the rain, (In Real Life: Mike Ditto) no one wants to take you home." ford@amix.commodore.com - David Byrne, "Loco de Amor" uunet!cbmvax!ditto ford@kenobi.commodore.com