[vtkusers] VTK Java/C Wrapping/Coding Problem

[Ambar C]

I think Luke's solution worked - I didn't realize casting the long
type vtkID would do the trick. Thanks much!

On Thu, Aug 19, 2010 at 7:28 AM, Luke Dodd <i.like.bread at gmail.com> wrote:
> I was going for a simple but dirty approach.
>
> Get the vtkObjectBase.vtkId IS a pointer to the vtk object. You could
> pass it into C++ via JNI as a long and then simply cast/memcpy it into
> a pointer with the correct type. We can avoid thinking about how to
> use a java object from C, we can just cut out the java object "middle
> man".  If you actually need to call methods on a java object from C
> (say a java object you have written yourself) then it is a different
> matter entirely.
>
> Sketched out in code:
>
> JAVA:
> vtkObjectBlah blah = new vtkObjectBlah(); // java vtk object that we
> want to now pass into C land
> long id = blah.vtkId // won't work - it's a protected field so have to
> do something horrible, or make changes to vtkObjectBase.java to get it
> giveToC(id) // giveToC is native method backed by JNI code - it takes a long
>
> C:
> void giveToC(long id){
>   vtkObjectBlah * blah = (*vtkObjectBlah)id // somehow cast long into
> pointer - don't know enough C to know if this is an allowable cast
>   blah->Method(); // should work, we now have a normal vtk object pointer
> }
>
> .........
>
> I know very little about JNI - that's just a sketch of what I meant.
> You don't really need to consider vtkGlobalJava hash, it takes a C
> pointer and turns it into a _Java _reference to the object that's
> wrapping it if there is already one - this is the other way round from
> what you seem to want.
>
> If you do this you'll have to make extra considerations for memory
> management and all that stuff.
>
> Hopefully this is enough to get you started. It's tricky for me to go
> into more detail because I don't use JNI much myself, so I can't throw
> together a quick example without figuring all that stuff out first.
>
> On 19 August 2010 08:13, Ambar C <ambarc at gmail.com> wrote:
>> So I'll add some more on this.
>>
>> What I need to do is be able to somehow get a hold of the
>> vtkGlobalJavaHash.PointerToReference (which seems to be a global hash)
>> in Java's VTK wrapping system, into my C++ code. I'm not quite sure
>> what kind of JNI drudgery I'd need to use - so any input on that or
>> whether I'm looking at the wrong part of the C++/Java wrapper would be
>> very helpful. My plan after being able to access this would be to feed
>> it the vtkID of the object I need to work with (both of which I've
>> managed to isolate) and then use the 'get' function from the
>> PointerToReference hash to get the actual C++ reference and attempt to
>> use some ugly casts on them.
>>
>> Once again, any recommendation, link, resource, suggestion,
>> encouragement/discouragement (:) would be helpful.
>>
>> Best.
>>
>> On Wed, Aug 18, 2010 at 3:22 PM, Ambar C <ambarc at gmail.com> wrote:
>>> Hi Luke,
>>>
>>> From what I see - the field vtkID in the vtkObjectBase class is of
>>> type 'long' - how would I use this to cast into C++ recognizable
>>> objects? Is there some hash that the ID is a key to; or something of
>>> the sort?
>>>
>>> Thanks,
>>> Ambar
>>>
>>> On Thu, Aug 12, 2010 at 1:52 PM, Luke Dodd <i.like.bread at gmail.com> wrote:
>>>> Hi Ambar,
>>>>
>>>> If you look in vtkObjectBase.java (will only exist after comping vtk,
>>>> in the build directory - not the source) you will see that it has a
>>>> (protected) field called vtkId. This is essentially a C pointer to the
>>>> vtk object the java object is wrapping. If you retrieve value and get
>>>> it into C++ with a few horrible casts you could have a pointer to the
>>>> object. You'll have to work around trying to get the protected field -
>>>> this is possible in java (google it) or you could tweak
>>>> vtkObjectBase.java (this is annoying because vtk object base is
>>>> generated when building).
>>>>
>>>> Hopefully that should be enough to get you started. If you need more
>>>> detailed help just ask.
>>>>
>>>> Best Wishes,
>>>> Luke
>>>>
>>>> On 10 August 2010 23:12, Ambar C <ambarc at gmail.com> wrote:
>>>>> Hello VTK users,
>>>>>
>>>>> I currently have a project that's in both Java and C++, and part of
>>>>> the project involves a wrapper using the Java Native Interface (JNI)
>>>>> where I use functionality from my C++ libraries in the Java code.
>>>>>
>>>>> Now the problem that I face is, both my Java and C++ codes use VTK
>>>>> objects - and when I pass down, say a vtkPolyDataAlgorithm object
>>>>> created in Java through to the C++ code, I'm not quite sure how to
>>>>> cast/use it.
>>>>>
>>>>> Please feel free to ask any questions that would help you to answer my question.
>>>>>
>>>>> Best Regards,
>>>>> Ambar
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>>>
>>
>