How get an image with a transparent background color

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2 total messages Started by Randy Young Thu, 10 Jan 1991 16:26

#7984

Author: Randy Young
Date: Thu, 10 Jan 1991 16:26

12 lines
1042 bytes

Hello:

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.
I want to be able to draw images to the windows without damaging the background
picture.  I have done some work with the Image stuctures to draw to a window,
but it seems that when I cut a square brush and then use it in my program
it erases part of the background.  I am not sure of the technique to use, 
should I use bobs or something like that.  Any help would be welcomed.

Tanks in advance
Randy Young

Re: How get an image with a transparent background color

#8094

Author: Jean-Michel Forgeas
Date: Sat, 12 Jan 1991 10:57

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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 ?

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