How difficult would it be to write a conversion tool to convert older animations to a new format that included Skew? Or build auto-conversion into Spine after Skew is implemented into the new version.
Scaling using skew is completely different behavior from the current scaling. There's no choice but for runtimes to support both if old animations still need to work (which they probably do). Newer versions of Spine will always open older projects, there is no problem evolving how things work as far the editor, project files, etc go. Spine has already gone through many extensive evolutions and can still open any project file ever saved, back to the very first version.
Note that skew of a single attachment can be achieved using a mesh, since the mesh vertices can be translated independently.
We've put some more thought into this. If we were to change how scaling works in Spine so we get rid of the old non-skewing scale and only have skewing scale, what old skeletons and animations would be affected? It would only affect animations that use non-uniform scale that is applied to multiple bones. I have a feeling that no one or very close to no one is relying on this.
If you scale uniformly (ie, the same on X and Y), then it will work the same. If you scale non-uniformly (ie, a different on X and Y) and only affect a single bone, then it will work the same. If you scale non-uniformly and allow a child bone to inherit the scale, only then will it work differently. With the old non-skewing scale, allowing a child bone to inherit the scale means the child bone gets scaled along it's local axes. This is so odd that I really doubt anyone has found a situation where this is the scaling they want. Is that true? Do you use non-uniform, non-skewing scale to affect multiple bones?
As of Spine 2.1.18 you can try out the skewing scale by first downloading and reinstalling Spine to get the latest launcher, then run Spine with the `
command line argument. On Windows:Spine.exe
Please try it with your existing skeletons and report back if anything went wrong. Obviously don't rely on skewing scale since it isn't in the runtimes yet.
Moving to the new scaling is important for a few reasons. First, currently bone flip is always done using the world axes. This can be functional for some types of flipping, but for many cases it is unintuitive. The new scaling allows bone flip to use the local axes. The new scaling also allows for fancy effects using skewed rotation and easy squash/stretch of an entire skeleton:
This is a bit extreme to make it easy to see, but all I did was scale the root bone in order to squash the whole skeleton.