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From: aozer@next.com (Ali Ozer)
Newsgroups: comp.sys.next
Subject: Re: Caching Images for Views ....
Message-ID: <742@rosie.NeXT.COM>
Date: 16 May 91 19:00:49 GMT
References: <1991May15.161837.27550@noose.ecn.purdue.edu> <1991May15.183632.22391@agate.berkeley.edu>
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In article <1991May15.183632.22391@agate.berkeley.edu> Izumi Ohzawa writes:
>Bitmap object is obsolete.  Its functionality has been
>expanded by NXImage object.
>
>	myImage = [[NXImage alloc] initSize:&myRect.size];
>	[myImage useCacheWithDepth: NX_TwoBitGrayDepth];
>	[myImage lockFocus];	/* focus on NXImage to work on */

Nice description! However, one point: Caling "useCacheWithDepth:" with
the NX_TwoBitGrayDepth argument forces the cache to be 2 bits; you
really want a cache which can hold the cached image in its full glory.
Thus you should use NX_DefaultDepth as the argument, which causes the
cache to be depth limited to the depth limit of the system. And,
because of the lazy depth promotion, the cache will remain at 2 bits
(even on a deep machine) as long as you happen to draw 2-bits worth of
gray into it (which is what most views do). But if the view happens to
draw color, and you are on a color machine, you'd really like your
cache to hold color as well. (You might want to refer to the recent
discussions on window depths and depth limits.)

It turns out that if your image has no representations, simply calling
lockFocus creates a cache with the default depth. Thus calling
useCacheWithDepth: becomes unnecessary. (This is a feature which allows
Bitmap based apps to be converted more easily.)

A final point is that the lockFocus method for NXImage returns a BOOL to 
indicate errors.  Thus the correct way to use the lockFocus/unlockFocus pair is
to

	if ([myImage lockFocus]) {
	    // ... draw ...
	    [myImage unlockFocus];
	}

You probably won't get any errors trying to focus on a simple cache;
however, you might get errors if the image was built from an EPS or
TIFF file, which are interpreted lazily, at time of composite: or lockFocus.

Ali, Ali_Ozer@NeXT.com