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From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga
Subject: Re: Layers Lib./Clipping
Message-ID: <8712031916.AA04108@cory.Berkeley.EDU>
Date: Thu, 3-Dec-87 14:16:29 EST
Article-I.D.: cory.8712031916.AA04108
Posted: Thu Dec  3 14:16:29 1987
Date-Received: Mon, 7-Dec-87 06:03:32 EST
Sender: daemon@ucbvax.BERKELEY.EDU
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>        Also, can anyone give me any idea of the processing costs of 
>      implementing Layers Lib. clipping.  I am working with 3-D images
>      and must clip in Z space anyway.  Will the Layers functions be
>      faster/slower than algorithmically croping the images?
>
>                                        Thanks again,
>
>                                                        Wade.

	There is a noticeable overhead.   The layers lib is setup to handle
more general clipping... multiple overlapping windows etc...   Also the fact
that the library calls use only the lower 16 bits for X,Y positions means that
you cannot pass arbitrary 32 bit quantities and assume they will be properly
clipped.

	I suggest you do your own clipping.  There are numerous algorithms
for clipping more complex objects like polygons.  Line clipping is relatively
trivial.  I think the layer functions would both be slower and inadequate.

USING LAYERS TO CLIP:
        You need to create a layer.  Creation of a layer requires a 'base'...
	A Layer_Info.  If using Intuition screens simply use the Screen
	structure's Layer_Info.  Otherwise use the NewLayerInfo() 
	layers.library call.

	Then use CreateUpfrontLayer() with the screen's Layer_Info and
	screen's bitmap.  Use the layer->rp (the layer's rastport) to
	render into.

					-Matt