EnigmA Amiga Run 1998 July/EnigmA AMIGA RUN 29 (1998)(G.R. Edizioni)(IT)[!][issue 1998-07 & 08].iso/recent/iib122.lha/IIB/Threads/Transparent_background

Internet Message Format | 1998-04-13 | 9KB

Date:         Fri, 3 Apr 1998 20:16:48 +0200
From: Enrico Piria <[e-mail]>
Subject: [IML] QUEST:Transparent color and Imagine

Hi all!
I'm sorry if I repost this message, but the first time I forgot to
precede the subject with "QUEST:"
I'm a Web designer.
I'm trying to make an animation in Imagine 4.0 for DOS and to use it on
a web page.
To do it, first I make the animation, then I convert each frame in a GIF
file and finally I place all the frames in a single animated GIF image.
So far so good.
For my purposes I've to make this image have a transparent background
color (a very useful feature of GIF format). So I can change the
background of the Web page without changing the background color of the
images showed in the page.
To do this in Imagine, I've tried two ways, but without success.

1. I change the +Zenith Horizon -Zenith Global settings to the same
color.
Doing this, also the image rendered acquires some bit of the background
color.
It's not what I want.

2. I change the background picture Global setting to a previously
prepared image that shows only the uniform background color.
Even doing this the image rendered reflects the background color.

What else can I try?

Thank you

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Date:         Sat, 4 Apr 1998 00:37:46 +0400
From: Charles Blaquiere <[e-mail]>

Enrico, I'm not getting the same results. I just placed a reflective
sphere above a ground object in Stage, with the camera horizontal, so
the ground would only occupy the bottom half of the image. Rendered with
default globals (all black), and got the following reflections on the
ball: black in the upper half, ground in the lower half, and the sky
behind it was black.

I loaded an all-red backdrop image and re-rendered. The sky was now red,
but the ball did not change. The backdop doesn't exist in the Imagine 3D
universe; it's added in the background after the renderer has done its
job.

If you use the Zenith/Horizon settings to specify an environment, the
imaginary sphere created does indeed play a role in the renderer. For
example, setting all three controls to full green will create green
reflections in the ball, and the sky will be green.

If you use both a backdrop and Zenith/Horizon colors, the sky will be
the color of the backdrop, yet objects will reflect the Zenith/Horizon.

I believe your problem occurred because you forgot to nullify
Zenith/Horizon settings when you loaded a backdrop.

So, the solution is definitely to use the desired background color as a
backdrop image, and set Zenith/Horizon to the colors you wish reflected
by your object (or leave them black). I guarantee this will render the
way you want.

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Date:         Sat, 4 Apr 1998 11:51:39 +0100
From: George Cole <[e-mail]>

> 1. I change the +Zenith Horizon -Zenith Global settings to the same
> color.
> Doing this, also the image rendered acquires some bit of the
> background
> color.
> It's not what I want.

That's because at the edge of a curve the pixels take up a bit of the
background colour so the object blends in better. Just have an idea of
what your background colour is going to be e.g. if it will be navy or
black when render with a navy background and it will look seemless on
any dark background, when you make the background colour of the image
transparent. If however the background may be bright yellow or bright
red then you may have to render two versions: 1 with a bright yellow
background and one with a bright red background. Alternatively you could
render it with an orange background (a mix of the red and yellow) and
see if that is seemless enough. I think you are going to find that an
animation with a transparent background is going to look a bit strange
around the edge pixels of the image (non-transparent part). Also make
sure that your background colour is found nowhere else on the picture as
those parts will become transparent also. Another problem is that when
you load the frames into a gif animator it changes the palette to a
global palette (one they all share) which means that the background may
actually change or become dotted, so if you can make your background
colour one that is already in the gif palette.

> 2. I change the background picture Global setting to a previously
> prepared image that shows only the uniform background color.
> Even doing this the image rendered reflects the background color.

I don't see why this should happen as it didn't happen to me the other
day whilst rendering a chrome logo (highly reflective).

hope this helps,

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Date:         Sat, 4 Apr 1998 06:50:05 -0500
From: Gregory Denby <[e-mail]>

Enrico Piria wrote:
>To do it, first I make the animation, then I convert each frame in a GIF
>file and finally I place all the frames in a single animated GIF image.
<snip>
>Doing this, also the image rendered acquires some bit of the background
>color.
&
>Even doing this the image rendered reflects the background color.

I'm not quite sure what is happening from your description, but my
guess is this: when you first make the animation, the compression
routine is creating "artifacts" which average together the background
and foreground pixels in some places. Try rendering each frame seperately
as a .bmp, and then find a utility to batch process the images into
.gifs that share the same palette (I think an old freeware program
called "display" did this, but its been years since I used it, so
maybe not). Then compile the gif anim.

Other than this, maybe the hard way would work. Load each frame into
an appropriate paint/image processor program and remove the offending
pixels.

Hope this helps

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Date:         Sat, 4 Apr 1998 11:00:40 -0700
From: Bill Graham <[e-mail]>

Enrico, you are travelling down a path that many web designers have, and it
can be frustrating. However, besides editing each GIF frame separately, this
is the only way I know of to get consistent results.
(a) If your GIF is going to be over a solid background, render the anim with
that color as the background, or at least a color close to it. Then make
that color transparent.
(b) If your GIF will be over a busy background, then render it over a SOLID
color that most closely resembles the average of the colors of the target
background image. Then make that color transparent.
(c) It is a PITA, but you can use an image editing proggy to remove an
outline of the image in each frame of the GIF. But I just recook the anim to
match instead of dealing with that, usually.

Good luck!

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Date:         Mon, 6 Apr 1998 09:12:15 PDT
From: Ian Stearns <[e-mail]>

>Hi all!
>I'm a Web designer.
>I'm trying to make an animation in Imagine 4.0 for DOS and to use
>it on a web page.

>For my purposes I've to make this image have a transparent background
>color (a very useful feature of GIF format). So I can change the
>background of the Web page without changing the background color of

Hi. I've done this too. You were doing everything correctly.
Turn of anti-aliasing. If it's on, the background(Blue-Screen) will get
blended with the foreground.  When you choose Blue as the transparent
GIF color...you get transparent lace around the edges of the object.
Have fun!

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Date:         Wed, 8 Apr 1998 01:19:50 -0500
From: "Stephen G." <[e-mail]>

The best way to do this is not to turn off antialiasing but to use Mark
Willis' textures for creating Alpha Channels  "AAalpha.itx"  Once you
have created your normal render with the full image, apply the Alpha
texture to your objects and create an alph channel image that you can
load into Photoshop as an alpha channel.  Once you have done this create
a separate layer underneath your image and add the background you are
going to use on your web page into this layer.  Once you have done this
select again the layer of your rendered objects using your alpha
channel.  Then inverse the selection and delete the rendered
background.  At this point you will have your objects overlaying your
background image. Flatten your image and convert the image to 256
colors.  You should still have your selection active.  At this point
delete the background of your selection which will leave only your
objects that you want to use in the transparent gif.  At this point
change the background color to whatever color you like that is not used
in your image and export as a transparent gif.  The advantage of doing
it this way is that the edges of your images will have been blended with
the background image of your webpage and there will be no discoloration
involved at the edges of your pictures when you load them into your
webpage.

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