Are transformations active or passive?

I foreseen this for a while, but I think this ambiguity is finally biting somewhere. See https://github.com/JuliaImages/ImageTransformations.jl/issues/25 for context.

The problem is that when you have a transformation, it isn't clear whether it is active or passive.

• Active transformations take a point and moves them within the same frame. It is a physical change of location.
• Passive transformations change the coordinate system itself - there is no physical interpretation of the point moving, just our description of it.

While the difference might seem pretty inane/abstract, I think the distinction can have important consequences. In ImageTransformations.jl the difference matters when your warp an image - are you moving the pixels to a new location, or is the "canvas" being warped (i.e. mapping a new canvas to pixels on the old canvas)? Note that the two transformations are inverses of one another (and further note that not all transformations are invertible, or otherwise that creating the inverse may be a slow/inaccurate operation).

I'm not yet certain what the solution is, but I feel that if it were clear whether a given transformation were to be interpretted as active or passive then these kinds of ambiguities would not occur downstream.

Start by reading this issue and the linked ImageTransformations.jl issue 25, then inspect the transformation API and how warp consumes transformations. Done means the project has an explicit, agreed convention for active versus passive transformations and the resulting behavior is clear for downstream image warping.