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View Full Version : What's light mapping or wrapping ?


everlite
11-05-2006, 05:36 PM
I know this is more composition end, but if someone could explain what it is and more so how this is applied that would be great.

Cheers - Dave

jussing
11-06-2006, 01:58 PM
Hmm, what's the context? I think those terms could have more than one meaning, but here's my go:

Light mapping: "baking" a texture map with all the light information. Useful for rendering geometry with ambient occlusion or other render-heavy light. -The basic principle is rendering the light once, to a texture map, instead of calculaing it for each frame. Once the light map is generated, the lighting calculations are "free" when you render the animation.

Wrapping: you mean "wrapping" or "light wrapping"? In camera mapped matte painting context, I guess this is when you projected picture "wraps" onto the backside of your model. For instance, if you project an image onto a cylinder, it will look good from one side, but on the sides it will stretch, and on the backside it will shine right through the model. This could be called wrapping.

Light wrapping: When backlight appears to wrap a little bit onto the other sides of objects. I real-life I guess this effect is a combination of translucency, sub-surface scattering, and tiny objects sticking up from the surface (like hair on an arm).

I'm not sure these are the answers you're looking for, but if not I hope I'm not misleading you. :)

Cheers,
- Jonas

everlite
11-07-2006, 10:20 PM
Hmmmm, not quite sure i don't think it's any of the above. It's just i've seen a few demo reels recently that show the composition breakdown and it sometimes says, i'm taking this from memory; light wrap pass or something to that discription. I think it has something to do with blending say a 3d object with its surroundings by effecting the edges of the object or something, not sure.

Cheers again - Dave.

jussing
11-08-2006, 07:51 AM
OK, I think that's what I tried to say with this. :)

Light wrapping: When backlight appears to wrap a little bit onto the other sides of objects. In real-life I guess this effect is a combination of translucency, sub-surface scattering, and tiny objects sticking up from the surface (like hair on an arm).

I've tried to do it on the blimp in this clip (http://www.duck.dk/gallery/agenthugo_cinematics.html). (second clip). I just called it "bloom" instead. I guess the difference between bloom and light wrap is:
Bloom is a lens effect as a consequence of bright light, and light wrapping physically happens on the surface of the object.
But both have the effect of a bright environment "bleeding" onto objects.

When applying light wrap, I think it's a good idea to combine it with a falloff/fresnel pass, because surfaces that are parallel to viewing direction bounce more environment light than surfaces ortogonal to viewing direction.

Cheers,
- Jonas

everlite
11-11-2006, 12:48 PM
Yep that's the one, where the environment bleeds into the object. So how do you go about doing this? Say if i have a 3d object on a 2d live plate, a blimp passing through the clouds maybe? I think this can be done in after effects somehow by comping layers, maskinf and feathering, in some kind of order, note sure.

- Dave.

jussing
11-12-2006, 09:08 AM
Well, a simple way of doing it is to copy the the blimp's alpha channel, blur it, and put it through the original alpha channel once more. Then you have a b/w matte that "bleeds" intro the original shape.

However, I think this tends to look a bit fake, so a good trick is to combine it with a falloff pass, so the effect depends on the angle of the surface.

And finally, when you have a proper matte for the bleeding, take the background pass, blur it (a lot), and then add it on top of the original blimp using the new matte.

....I hope this makes sense. :)

Cheers,
- jonas

jussing
11-12-2006, 09:18 AM
Check out the bottom clip here:

http://features.cgsociety.org/story_custom.php?story_id=3802&page=2

They don't speak a lot of the effect, but you can see it's there.

There's also a bit here:

http://forums.creativecow.net/cgi-bin/new_page_wrapper.cgi?forumid=1&page=http://www.creativecow.net/articles/dewar_john/light_spill/index.html

And here: This is actually the best example, because it shows the light wrap pass:

http://www.digitalfilmtools.com/cs/cslightwrap.htm

Cheers,
- Jonas

DoubleSupercool
11-27-2006, 03:36 AM
basically taking the CG elements alpha, eroding it, blurring it, minusing the original alpha and then using this as a mattemask on the BG. Then, comping this "halo" over the CG pass and fading it.

Essentially you have a stroked mask for the CG element that is filled with the BG plate and laid over the CG

dellis
11-27-2006, 06:46 PM
Read this...should shed some light on things....pun intended!

http://www.neilblevins.com/cg_education/specular_bloom/specular_bloom.htm

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