PEG vobscene_interp--tjl:

Author:	Tuomas J. Lukka
Last-Modified:	2002-12-19
Revision:	1.7
Status:	Implemented
Affects-PEGs:	vob_considerations--benja, vobcoorder_parents--tjl, vobscene_hierarchies_clicks--tjl
Type:	Architecture
Scope:	Limited
Stakeholders:	mudyc, benja

As witnessed by mudyc, vobscene interpolation needs a bit of adjustment: it is very easy to accidentally get non-interpolating behaviour, especially when reparenting coordinate systems.

This PEG clarifies the relationship of the different hierarchies and sets simple principles for interpolation.

Issues

Are there compatibility issues with this change?

RESOLVED: Not really. This change only affects the frames rendered during interpolation, and mostly by allowing interpolation where there previously was none.

If some coordinate systems are defined without being careful, this MIGHT lead to floating-point overflows that did not occur previously, but this is unlikely.

For some situations, it may remove a jumping behaviour, which should be a desirable change in just about all situations...
What about the situation where we have
```
vs1.cs1 = translate(0, null, 500, 500)
vs1.cs2 = rotate(cs1, "A", 50)

vs2.cs1 = translate(0, null, 500, 500)
vs2.cs2 = rotate(cs1, "A", 150)
```
The cs1 systems do not have keys but cs2 are to be interpolated to each other.

If the rotations are interpolated point-wise, the coordinate system will shrink during the animation; if parameter-wise, the animation will be just a rotation.

Should the system try to discover that cs1s are the same and interpolate parameter-wise?

RESOLVED: No. This would be too much work at a time-critical stage, and also error-prone. It is possible to add this later, but for now, point-wise interpolation is ok.

The result should be easy to detect and fix in most situations.

Principles

In the following, vs1.cs is a coordinate system of the first vobscene.

If vs1.cs and vs2.cs match in the VobMatcher tree, then their transformations will be interpolated to each other. The types of the coordinate systems, their parents in the vobcoorder hierarchy and their determining coordinate systems shall not affect this.
If vs1.cs and vs2.cs are to be interpolated and are of the same type and share interpolating parents, they will be interpolated parameter-wise.

Changes

If two completely unrelated coordinate systems are to be interpolated, linear (or other) interpolation of the transformed points shall be used as a fallback. This means that a point x will be transformed as:

interp.cs(x) = (1-fract)*vs1.cs(x) + fract*vs2.cs(x)

where interp.cs is the interpolated coordinate system at fraction fract and cs(x) is x transformed according to cs.

We shall not define the exact circumstances when this shall happen; we shall define one circumstance where this shall not happen: if the types of the coordinate systems, their parents and their keys are identical in the two vobscenes, then parameterwise interpolation shall be used.