Path: utzoo!attcan!uunet!cbmvax!cbmehq!cbmfra!swinjm!forgeas
From: forgeas@swinjm.UUCP (Jean-Michel Forgeas)
Newsgroups: comp.sys.amiga.tech
Subject: Re: How get an image with a transparent background color
Message-ID: <1882acab.ARN0b89@swinjm.UUCP>
Date: 12 Jan 91 10:57:47 GMT
References: <1991Jan10.162634.690@vort.uucp> <729@cbmger.UUCP>
Reply-To: forgeas@swinjm.UUCP (Jean-Michel Forgeas)
Followup-To: comp.sys.amiga.tech
Organization: The Software Winery
Lines: 34

In article <729@cbmger.UUCP>, Peter Kittel GERMANY writes:

> In article <1991Jan10.162634.690@vort.uucp> randy@vort.uucp (Randy Young) writes:
> >I am programming a new game for the amiga.  I would like to use the effect that
> >dpaint uses when they pick up a brush and the background color is transparent.
> >[...]
>
> You could use bobs or do the same they do yourself. The technique is
> called "Cookie cutting". This involves the following steps:
> 1. Make up a "mask" of the object to be placed. This mask is 1 where
>    any of the object bitplanes is non-zero, 0 else. It can be computed
>    by ORing together all object bitplanes.
> [...]

You have another other solution:
After making a mask like Peter Kittel says, you can use the graphic.library
function BltMaskBitMapRastPort() where you take a bitmap (a small one which
contains your brush) its mask, and draw it into the big rastport.

I use this function a lot instead of DrawImage() because like you said
DrawImage() is good but doesn't have transparency.
I never look at the speed of BltMaskBitMapRastPort(). But since it does
clipping when it has to, be sure it cannot by having no layer attached
to your rastport (rastport->Layer == 0).
Perhaps it is fast enough for a game ?

Best regards,
    Jean-Michel
--
                                     \___/
Jean-Michel Forgeas                   \-/
cbmvax!cbmehq!cbmfra!swinjm!forgeas    |    The Software Winery
                                      -^-
                           And, where is the universe ?