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marc
07-28-2003, 03:41 PM
Hello, everyone,

here's my request, maybe even in time for LW8:
I want to render objects against a true TRANSPARENT BACKDROP.

The problem I'm having with the current Lightwave is that I always have to select a backdrop colour. When I save the image as 32bit PNG (my preferred format, which also includes the alpha channel), the anti-aliased edges of my object show traces of the backdrop colour. That is of course logical since the rendered image (with that backdrop colour) is multiplied with the alpha image.

The following examples will illustrate what I mean. Everytime the image was rendered with a backdrop colour different from the one in the composite.

Example 1 - Rendered with green backdrop:

http://www.themarcside.com/forumimages/transparent_bg_1.jpg

Example 2 - Rendered with black backdrop:

http://www.themarcside.com/forumimages/transparent_bg_2.jpg

Example 3 - Rendered with black backdrop and DOF:

http://www.themarcside.com/forumimages/transparent_bg_4.jpg

There is a workaround: SpriteEdger. Check out the examples:

Example 4 - Rendered with black backdrop and SpriteEdger:

http://www.themarcside.com/forumimages/transparent_bg_3.jpg

Example 5 - Rendered with black backdrop and DOF and SpriteEdger:

http://www.themarcside.com/forumimages/transparent_bg_5.jpg

But I am not sure whether this will always do the trick. Or maybe I am doing something wrong, but I always wondered, how people would do composites without a transparent backdrop...

Cheers,
Marc

Panikos
07-28-2003, 03:47 PM
Render your scene over a backdrop colour that is similar to the background footage that you want to compose.
Compose with Alpha Channel.

Or if your compositing application allows premultiplied color selection (like AfterFX) use the same colour as your LW backdrop.

marc
07-28-2003, 04:19 PM
Well, if I want to reuse elements, I do not know, what a similar background colour would be.

But the "premultiplied" tip was EXCELLENT. At last I understand what this means in Commotion. That's exactly what I was looking for. DFX+ will probably be able to do the same.

See, I am only just starting to learn the art of compositing (Commotion came with my editing software and now I have preordered LW8 only to get DFX+).

Still, having LW8 render with transparent backdrops would be great for importing into other painting apps that do not offer this premultiplied thing.

Thanks again for the hint.

Marc

Panikos
07-28-2003, 04:26 PM
Its impossible to have transparent backdrop.
There is no such thing.

The closest you can get is black.
I realize that the problem is DOF or motionblur.

Your compositing app, when Alpha channel is not solid white or black, it subtracts your premultiplied colour from the RGB value of your foreground image.

This is the method all compositing apps are using.

Good luck
:cool:

wacom
07-28-2003, 04:34 PM
Originally posted by marc

But the "premultiplied" tip was EXCELLENT. At last I understand what this means in Commotion. That's exactly what I was looking for. DFX+ will probably be able to do the same.

Marc

DFX+ will let you do it to any footage you put into it- and more. Here is a screen shot.

Just so the people who got the bundle know...

marc
07-29-2003, 12:29 AM
Originally posted by Panikos
Its impossible to have transparent backdrop.
There is no such thing.

But how do painting apps do this, then? Photoshop, Fireworks or Photopaint all let me start out with a transparent canvas if I want to, don't they?

Marc

Panikos
07-29-2003, 12:36 AM
You cannot directly compare a 3d-app with a painting.

Is like oranges with apples.

Both are fruits, but are not the same in taste, colour, shape etc

Let say that Backdrop is transparent and you have a reflective ball, or a transparent. What will be the reflection or refraction ????
If you answer this, then you realize that you need something to begin with.

Prost !

:cool:

marc
07-29-2003, 02:42 AM
Originally posted by Panikos
Let say that Backdrop is transparent and you have a reflective ball, or a transparent. What will be the reflection or refraction ????

Well, it'll reflect nothing, of course... ;-)

OK, you got me. But thanks to your tip, this feature request wasn't in vain. I learned something.

Cheers,
Marc

Mylenium
07-29-2003, 03:11 AM
Hi Marc,

Even PS or other painting apps have no transparent backdrop - it's mathematically impossible. Instead they constantly manipulate alpha info (which is more like saying "Fill anything that is zero with the values from the other layers/ background!") for allowing other elements to show through. The funny thing is, that especially PS simply does not show alpha unless you explicitely create a channel for it.

Mylenium

marc
07-30-2003, 05:11 AM
Wacom,
I tried it with DFX+ (with the DF4 demo actually). It didn't work the way you said. So I asked in the Digital Fusion forum at CG-Talk. Jayk2k, the incredible helpful DF expert over there, solved the problem for me. You might want to check it out:

http://www.cgtalk.com/showthread.php?s=&threadid=78682

Marc

trison
07-30-2003, 10:45 AM
Cinema 4D has a great way of handling the situation, it can take the color of the outside pixel and fill in the pixels to be antialiased with that color. It looks strange and jagged when you render but once you apply the alpha it smooths out.

trison
07-30-2003, 10:48 AM
I meant to attach a picture with that. This is a ball with a slight glow. It fills in the whole glow and lets the Alpha do it's thing

Anttij77
07-30-2003, 05:56 PM
LightWave does that too. Check Render Options->Output Files->Fader Alpha Mode button... That should work!

Maimo
08-02-2003, 03:00 AM
in simple cases (not blur or dof) you could use a matte choke filter, i do this in aftereffects, and im sure most other compositing programs have that as well.

also when you want to add volumetric fx to stuff and reuse the same renderd footage over and over you can comp it using the photoshop transfer modes, AfterFX and DFX+ do this.

I did a spot for lilo and stitch last year and made a holographic projector type thing (volumetric spotlight) and reused the same renderd clip for a Finding Nemo AD this time it was the sun shining through the water, I dont think ill ever have lw render that type of effect again its so much faster just to comp it in.

just make sure you render it over black and use screen or add mode.


-Mychael

pixelranger
08-02-2003, 11:03 AM
In Photoshop try:

Layers -> Matting -> Defringe (contracts the edges in the layer by 1 or 0,5 pixel or so)
-> Remove black matte (the same as premultiplied with black in AFX)
-> Remove white matte (the same as premultiplied with white in AFX)