Cache Proxy Tag

[JohnThomas]

Hello,

I have a question when it comes to the Selection tags such as "Tcacheproxytagpointselection". In the SDK they are listed as private for generators and not referenced again.

Is there a method available to utilize them or an equivalent method. Specifically I'm trying to achieve the behavior shown in this clip, where the Text object is able to be changed by the Plain effector without being made editable due to its selection tag, while the Rectangle spline has to be made editable to be changed.

Any help would be greatly appreciated.

John Thomas

[ferdinand]

Hey @JohnThomas,

Thank you for reaching out to us. We just talked about your question in our morning meeting, and none of us does truly understand it. So, let me state a few facts about the literal question you asked:

Specifically I'm trying to achieve the behavior shown in this clip, where the Text object is able to be changed by the Plain effector without being made editable due to its selection tag, while the Rectangle spline has to be made editable to be changed.

Plugin authors should treat caches as read-only although they technically are not. Internally, we pull a lot of caching tricks, as this is certainly possible and desirable in some cases. But you can also quite easily crash Cinema 4D when you do not know what you are doing. Especially allocations or deallocations are absolutely off limits, as something could have already established a pointer to some internal data. E.g., you cannot add or remove points to/from a PointObject when poking around irregularly in caches.

1. Editing things without making them editable just means modifying their cache. So, for your Rectangle spline, you could call myRect->GetCache() and then find there a LineObject which you could edit. See geometry_caches_s26.py for an explanation of the cache model of Cinema 4D, as navigating caches can become quite complex.
2. The natural model to modify caches are deformers, i.e., ObjectData::ModifyObject, as for example done by the Bend deformer. That is also what MoGraph is doing with its effectors once you enable one of the deformer modes. Tags in general or specifically Tcacheproxytagpointselection are only tangentially relevant.
3. Another (although not recommended) way to modify the caches of things can be TagData::Execute or ObjectData::Execute, i.e., a tag or an object acting like a tag. You simply reach there into the cache and do stuff, but other than for ModifyObject you are not as well shielded for access violations/update anomalies there. The reason why one would do this, is to put a possible deformation step somewhere else in the excution pipeline (as you can specify when in the scene evaluation the Execute methods are called).
4. Modifying the cache of an object is volatile, i.e., will just be discarded on the next scene update. So, unless you do it in regularly called places such as ModifyObject or Execute, this will not work. And when you do this outside of the context of these two methods, you are pretty much guranteed to crash Cinema 4D sooner or later.

When I read here a bit between the lines, the implicit question does not seem to be actually about deforming the output of other generators but how selections work in the context of generators.

As a rule of thumb we can say that selections do not work with generators. It is only for generators which yield a simple (i.e., one object deep cache) and some hard coded extra cases where you can put a point/edge/polygon selection tag on that generator and Cinema 4D then being able to interpret that selection tag correctly. Because a selection tag is effectively just a list of selection states such as [True, False, False, True], where this list would mean that the elements with the indices 0 and 3 are selected while the elements with the indices 1 and 2 are not selected. When you now have a scene structure like this:

myGenerator (BaseObject)
├── Tags
│   └── myPointSelection (SelectionTag): {True, False, False, False}
└── Cache
    └── myNull (BaseObject)
        ├── poly.0 (PolygonObject)
        └── poly.1 (PolygonObject)

The selection tag myPointSelection is ambiguous as it is not clear which of the PolygonObject instances in the cache of myGenerator it is referring to. So, Cinema 4D will just ignore it. Only when it is clear to which Point/Line/PolygonObject a selection tag is referring to, it will be used. There are some hardcoded exceptions in the case of MoGraph, but you must implement them manually in the 'user' of such selection, i.e., the thing that takes such selection as an input. So, long story short, this is why this works for the text spline but not the text object, one has a simple cache structure and the other has not.

Cheers,
Ferdinand

[JohnThomas]

Thanks for the response.

For clarification what I am trying to do is have my modifer inside of ModifyObject change the points of the modified object. For simplicity sake it takes in the four points and moves them inwards so the Rectangle spline is smaller. The idea with the selection tag is to mimic the behavior of the things like the Selections in the Text object, so I could create a selection tag that would have some amount of the points from the Rectange spline selected, these points would be set in the Selection tag inside of ModifyObject. The selection tag would ideally be able to now be dragged into something like the Plain effector where it can effect the selected points, without the parent Spline object being made editable.

When it comes to Selection tags is it possible, via code, to create the Tcacheproxytagpointselection style selection tags or is that purely limited to things like the Selections in a Text object?

John Thomas

[ferdinand]

Hello @JohnThomas,

Thank you for your clarifications, but unfortunately they do not bring much clarity for me. It is still unclear to me what you want to achieve.

1. Tcacheproxytagpointselection and its edge, polygon, vertex color, and vertex map counter parts are a MoGraph mechanism to link from a generator, i.e., the tags visible in the Object Manager to tags in the cache of something, i.e., the invisible part of a scene graph. So that the user can interact with tags buried in the cache of something. They are one of the MoGraph workarounds for the very problem I described above. The generator using them has of course to support them/create them, because you need a selection/vertex tag on some point object in the cache in the first place, to which the proxy can link. I.e., you cannot just create proxy tags on anything.
2. You can create a proxy tag yourself. Since it is just a variable tag, you can just the generic VariableTag interface for that. But the concrete interface is private, so you would have to reverse engineer how they work, but they are effectively a hollow shell which just carry a base link to their linked entity in their data container (and some other meta data). The real challenge would be to keep that meta up to date, i.e., mimic how MoGraph operates these tags.
3. You also talk about fields implicitly all the time, specifically a variable tag field, i.e., a field which can sample things like selection tags and their proxies. Variable tag fields of course only support variable tags and nothing you would write yourself.

Cheers,
Ferdinand