How do 3D objects coincide with traditional 2D animation?


I was watching an episode of _Shirobako_. I love how the anime gives you a
slightly in-depth view of the production process. However, something I can't
get my mind around is those few segments that involve CGI (in anime in
general) with the interaction of traditional 2D animation such as stationary
or moving objects.

In the episode, a scene in _Exodus_ where the girls ride a jeep while avoiding
an explosion was shown in key animations/"genga":
<https://sakugabooru.com/post/show/10903>.

As you can see, the 3D bike and characters basically move together on a plane.
Could it be that the bike is animated frame by frame in traditional animation
somehow? Or the other way around? Or could it be more technical than those?




The bike can be animated in a 3D model editor application and overlayed upon
the ready 2D frames. It doesn't really matter which application both come
from, since they can all export frames to static images. 3D editors have
motion direction, of course, or else we would not see much 3D movies. And you
can not add anything else aside from the main 3D mesh with applied texture, so
that the background will be transparent upon frame export. You know how they
have those "layers" in Photoshop? Well, that is it. One of them comes from 2D
app, the other from 3D. Nothing special.

Images taken [from this Autodesk 3ds Max™
tutorial](http://forums.cgarchitect.com/34787-help-rendering-vray-reflection-
transparency.html) showing how not much of a rocket science it is to create 3D
scenes with transparency (click images for full resolution):

[![](https://i.stack.imgur.com/D6FG5l.jpg)](https://i.stack.imgur.com/D6FG5.jpg)

The image with transparent background and white shadow with gradual
transparency changing, after exporting from a 3D editor, can be imported into
a 2D editor (such as Adobe Photoshop) and be laid over some colored rectangles
like this:

[![](https://i.stack.imgur.com/6hsccm.jpg)](https://i.stack.imgur.com/6hscc.jpg)
[![](https://i.stack.imgur.com/gPazqm.jpg)](https://i.stack.imgur.com/gPazq.jpg)

We also have sites like
[GraphicDesign](https://graphicdesign.stackexchange.com/) on [StackExchange
Questions and Answers sites network](http://stackexchange.com/sites) that [ask
questions on similar
topics](https://graphicdesign.stackexchange.com/questions/27507/how-can-i-
create-and-batch-this-3d-planes-effect) with such illustrations:

![](https://i.stack.imgur.com/nLvit.png)

You say "traditional animation", but that really means collaging paper
elements on a table, taking a shot of the scene and later on gluing together
the resulting frames into a cinema roll. You must want to know about the
modern day way, so that is what I described above.

Older animation techniques require the use of celluloid sheets, which are
transparent sheets of plastic, to make the things simple. Are you familiar
with the term _cel-shading_? Well, this is where the modern day 3D animation
technique comes from − the older days method of drawing the outline of objects
on a cel sheet. It goes something like this: You draw the black outline and
the fill inside areas with colored paint. Then you place it above the scene
image and move around or swap for other cels with different character poses.

[![](https://i.stack.imgur.com/rXMAYm.jpg)](https://i.stack.imgur.com/rXMAY.jpg)
[![](https://i.stack.imgur.com/GhRpdm.jpg)](https://i.stack.imgur.com/GhRpd.jpg)

There are a number of techniques used to overlay 3D objects onto ready frames
of anything else, used by all sorts of "magicians" in their shows. You could
ask google how they did it before computers and find out something new.




The bike can be animated in a 3D model editor application and overlayed upon
the ready 2D frames. It doesn't really matter which application both come
from, since they can all export frames to static images. 3D editors have
motion direction, of course, or else we would not see much 3D movies. And you
can not add anything else aside from the main 3D mesh with applied texture, so
that the background will be transparent upon frame export. You know how they
have those "layers" in Photoshop? Well, that is it. One of them comes from 2D
app, the other from 3D. Nothing special.

Images taken [from this Autodesk 3ds Max™
tutorial](http://forums.cgarchitect.com/34787-help-rendering-vray-reflection-
transparency.html) showing how not much of a rocket science it is to create 3D
scenes with transparency (click images for full resolution):

[![](https://i.stack.imgur.com/D6FG5l.jpg)](https://i.stack.imgur.com/D6FG5.jpg)

The image with transparent background and white shadow with gradual
transparency changing, after exporting from a 3D editor, can be imported into
a 2D editor (such as Adobe Photoshop) and be laid over some colored rectangles
like this:

[![](https://i.stack.imgur.com/6hsccm.jpg)](https://i.stack.imgur.com/6hscc.jpg)
[![](https://i.stack.imgur.com/gPazqm.jpg)](https://i.stack.imgur.com/gPazq.jpg)

We also have sites like
[GraphicDesign](https://graphicdesign.stackexchange.com/) on [StackExchange
Questions and Answers sites network](http://stackexchange.com/sites) that [ask
questions on similar
topics](https://graphicdesign.stackexchange.com/questions/27507/how-can-i-
create-and-batch-this-3d-planes-effect) with such illustrations:

![](https://i.stack.imgur.com/nLvit.png)

You say "traditional animation", but that really means collaging paper
elements on a table, taking a shot of the scene and later on gluing together
the resulting frames into a cinema roll. You must want to know about the
modern day way, so that is what I described above.

Older animation techniques require the use of celluloid sheets, which are
transparent sheets of plastic, to make the things simple. Are you familiar
with the term _cel-shading_? Well, this is where the modern day 3D animation
technique comes from − the older days method of drawing the outline of objects
on a cel sheet. It goes something like this: You draw the black outline and
the fill inside areas with colored paint. Then you place it above the scene
image and move around or swap for other cels with different character poses.

[![](https://i.stack.imgur.com/rXMAYm.jpg)](https://i.stack.imgur.com/rXMAY.jpg)
[![](https://i.stack.imgur.com/GhRpdm.jpg)](https://i.stack.imgur.com/GhRpd.jpg)

There are a number of techniques used to overlay 3D objects onto ready frames
of anything else, used by all sorts of "magicians" in their shows. You could
ask google how they did it before computers and find out something new.

