Black and Transparency

[pfelberg]

Sorry if I sound dumb, but I am missing someting very basic here.
Is black allways the absence of light?
What if I want to use an alpha BG and still want my rendered shadows to show up?

I have to animated text with an alpha BG, but with rendered black/grey shadows being projected over a rocky wall. All black is gone.

tks

[thedawjones]

Shadows only occur in a lit environment. If you have an unlit surface a shadow won't/can't exist.
Therefore your background has to have some illumination for the presence of a light blocking shadow.
If I wanted to project a black square, I would actually be projecting an illuminated frame that causes the eye to perceive the inside square as black. Not project a black square...
If your b/g is black and your shadow is black-how can you see that?
- at this point you would only see "shadows" that lie between the gray (lit) areas.

So if you have a lit object it should be emitting light on the surrounding surface to be realistic, as well as an object blocking this light which would cause your "shadow" to appear.

Or your text has an external light source, with some light passing through the holes/spaces within the text to create the lit background, and the light blocked by the text creating shadow.
good luck.

[pfelberg]

You didn´t get the picture.
The shadows were rendered properly. The were part of a "shadow render pass".
On post production they were added to the whole animation, so I have a transparent background (alpha), the 3D text being annimated and the shadows behind.
Even with no BG at all you can see the shadows (in black) in the post production software window (vegas).
But when projected, they are gone. They only appear if I have another color layer as a BG on Arena.

So, just to conclude what it seems to be my basic and stupid question. Is black allways the absent of light in a projection? Can´t I have an alpha channel in a footage and still have black projected?

[drazkers]

...........................
Simple version of what Dawjones said:

Yea, projectors create black by not projecting any light.

Alpha Channel is nothing, so the absence of anything is black.

http://en.wikipedia.org/wiki/Black

[Joris]

Thedawjones did get the picture, Pfelberg.

Resolume can deal with alpha channels during rendering, but it can never project them. Think about it. Alpha indicates something is invisible. How can you project something which is not there?

The closest we can get to projecting nothing, is to project black. This is why Resolume puts black where ever there is alpha on the final render. This is called premultiplication.

Even when you have correctly rendered shadows, you cannot see black on alpha. Because it is black projected on a black background. In the editor you are probably seeing the shadows on a checkerboard, not black.

This actually how it is in the real world as well. You can only see shadows on a surface when there is some light source hitting the same surface as well. Otherwise the whole thing would just be black.

So to see the shadows, you have to premultiply with a different color by putting in a solid background in Resolume. Or carefully re-read Thedawjones suggestions again, because he is giving some very good advice.

[Joris]

And to make this very clear, projectors do *not* create black by projecting no light. Instead they create black by projecting very dark grey.

True black would be what Pfelberg calls the absence of light, but a *projector* can not, never ever project no light. Only very dark grey.

That doesn't change anything to the fact that when projecting, alpha and black are practically the same thing.

[pfelberg]

Thank you all.
Neurotic is when someone believes that 2+2=5.
Psychotic, me, knows that 2+2 isn't 5 but cannot comply to it ; )

I realize that I will never project black over an alpha channel. I got that. That´s the important thing to know.
But, I will never comply to it. ; )

btw... the projection happened today and I solved the issue by taking a picture of the wall itself and adding the image as a BG layer in Arena. I mapped the wall on itself, adjusted brightness and saturation (Arena Effects). That granted me all my shadows and the results were just fine. ; )

Again, thanks everyone.

Paulo

[thedawjones]

Congrats on your show!
Life is nothing without contrast.