algorithmic modeling for Rhino
Grasshopper is an application that deals with a lot of different types of data. These data types can come from 6 different sources and some of them will not be known when Grasshopper is written and compiled. The six potential sources are:
Although clearly some of these are known to the developer during the time of writing, not all of them can be. Yet Grasshopper still needs to be able to interpret and use types it may know nothing about. Some of the things Grasshopper needs to be able to do with data of any type are:
To overcome the problem of (A) needing to do so many things while (B) knowing nothing about the types in advance an interface is defined in the Grasshopper SDK and all data which is stored inside parameters must implement this interface. This allows GH to do the things from the second list to all the types from the first list.
The IGH_Goo interface is is usually nothing more than a wrapper around the actual data which provides a bunch of functionality for whatever it wraps. For example take the primitive Boolean (or bool in C#). It's a structure so it can never be null, and it can only exist in either a true or a false state.
GH uses booleans a lot so it provides an IGH_Goo implementation for Boolean. This wrapper class tells GH that a boolean value can be converted into an integer (false -> 0, true -> 1), into a colour (false -> black, true -> white), into a string (false -> "false", true -> "true) and so on. The wrapper class also knows how to write and read boolean values to and from *.gh files, and because we're now dealing with a wrapper class we can have null instances in a collection of boolean values. The wrapper doesn't tell GH how to preview or bake booleans, because booleans are not a geometric type of data.
Because so many data types are not in any way geometric, there is a second interface called IGH_GeometricGoo which extends IGH_Goo with stuff like transforming, bounding-boxing etc.
Almost always component developers and scripters can ignore any of the IGH_Goo types, because they are mostly used for internal bookkeeping. However sometimes a developer will either want to access the functionality that goo provides or they wish to inject a new, previously unknown type of data into a Grasshopper file. In these cases some knowledge of IGH_Goo and its derived interface and classes is required.
An additional note on conversion.
IGH_Goo requires that the implementor provides three different conversion methods:
As an elucidation, imagine the following conversation going on between Grasshopper (G), which is trying to push data of type A in one parameter into another parameter which only accepts data of type B.
G - "Hmm, two parameters that both store different types have been connected with a wire. That means the user wants to transfer all data from left to right, but I can't simply copy it because the parameter on the right only accepts instances of type B.
First let me check if A is a more derived type of B, in which case I can just 'trick' the parameter into accepting the original data."
G - "Nope, A is not a special flavour of B, which means some conversion needs to happen first... Hey! You there! Mr. A! I need to move you over to this other place but they don't take kindly to the likes of you. Is there any way I can persuade you to turn yourself into something more B-like?"
A - "Oh sorry, I have never heard of this B-thing you mentioned before, afraid I can't help you out here."
G - "Never mind, I'll ask over there. Oy, B-man! Any chance of you letting me in on the secret about how to take this A thing over there and turn it into a B thing?"
B - "Yeah I know how to do that, but you'll lose some information in the process."
G - "That's all right, something's better than nothing."
B - "All right, here you go, a B-ified version of the A data which is the best I can do under these imperfect circumstances."
G - "Thanks! I'll put it into this data tree over here right away."
More succinctly, GH calls the CastTo<B> method on the instance of A, asking it if it knows how to convert itself into a B type. If that fails, it'll construct a new empty B instance and ask it using the CastFrom<A> method whether it knows how to interpret the A data and mimic it. If both these methods fail then Gh will put up an error and leave that slot blank.
The ScriptVariable method allows data to protect itself from potentially ill-informed programmers. Using this method, Grasshopper tells some data that it's about to be handed over to a script (ie. a VB/C#/Python component) and whether it would like to maybe provide a different kind of data instead. For example a GH_Boolean might assume that the scripter does't really care for all that IGH_Goo malarkey and instead of pumping a GH_Boolean instance into a script just the value on the inside is provided.
Remark for the future:
Data in GH2 works nothing like this. If you're developing plug-ins/code for GH2 or beyond, you're reading the wrong document.