Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!well!shf
From: shf@well.UUCP (Stuart H. Ferguson)
Newsgroups: comp.sys.amiga.tech
Subject: Re: CAMG
Message-ID: <12340@well.UUCP>
Date: 23 Jun 89 06:52:48 GMT
References: <1373@psueea.UUCP>
Reply-To: shf@well.UUCP (Stuart H. Ferguson)
Organization: The Blue Planet
Lines: 41

+-- bartonr@psu-cs.cs.pdx.edu (=> robart <=) writes:
| In article <12085@well.UUCP> shf@well.UUCP (Stuart H. Ferguson) writes:
| >  The only bits you should really look at (and save!) in the CAMG word are
| >the HAM and HALFBRIGHT bits.  The way to decide if the image should be
| >displayed as interlace or not is to look at the aspect ratio and image size
| >in the BMHD property.  The BMHD is always provided -- if not, the ILBM
| >is broken.
|   I think a better way to decide if the image is LACE or HIRES is to look at
| the information in the CAMG chunk.  After all, if you're going to write it out
| anyway what is the point in leaving out important information?  The less there
| is for the reader program to have to figure out, the better.  Besides, using
| the aspect ratio can be unreliable.  Some programs don't set it correctly, and
| even if they do the ratios are different for NTSC and PAL machines, and you
| have no information in the file to tell you which type it was created on.

Ok, I agree.  You should look at the CAMG and if you can make sense of
what's there, you use it.  Several things to note, however:

1.) Many programs write inconsistent ILBM's.  Some even create 640
x 200 bitmaps with no CAMG chunk and ask for a pixel aspect ratio of 10/11.
(You want square pixels?  I'll give you square pixels... ;-)

2.) Many ILBM's are not created on Amigas.  For them, the CAMG chunk is
much less meaningful and may be missing.  Here the pixel aspect ratio is
much more important to observe.

3.) Many ILBM's are not displayed on Amigas.  Same caveats as above.  (We're
talking device-independent here, folks...) 

4.) Many bits in the viewmodes word as used in the graphics library are
very dangerous to just jam into a new viewport.  Bits such as VP_HIDE and
the GENLOCK bits.  Always be sure to mask these out when writing and
especially when reading (kids, you don't know where these files have been...)

5.) The ECS viewmodes, support for multisync monitors, and the way the
system people plan to address this in the future will make this CAMG issue --
let's just say -- VERY interesting.  (You think it's hard to understand
NOW...)
-- 
		Stuart Ferguson		(shf@well.UUCP)
		Action by HAVOC