[vtkusers] Coding Errors in VTK Java

[Dale Greer]

Here's a simple class for testing vtkPoints.GetPoint(). It's only using one
point, but it behaves the same as my larger app that uses an array of
points.

import vtk.*;

import java.io.*;

public class TestGetPoint {
 static {
  System.loadLibrary("vtkCommonJava");
  System.loadLibrary("vtkFilteringJava");
  System.loadLibrary("vtkIOJava");
  System.loadLibrary("vtkImagingJava");
  System.loadLibrary("vtkGraphicsJava");
  System.loadLibrary("vtkRenderingJava");
 }

 public TestGetPoint() {
 }
 public void testing() {
  vtkPoints points = new vtkPoints();
  points.SetDataTypeToDouble();
  int slotID = 0;
  double[] p = new double[3];
  p[0] = 1.1;
  p[1] = 2.2;
  p[2] = 3.3;
  slotID = points.InsertNextPoint(p);
  p[0] = 0;
  p[1] = 0;
  p[2] = 0;
  points.GetPoint(slotID, p);
  System.out.println("p1 " + slotID + " " + p[0] + " " + p[1] + " " + p[2]);
  p = points.GetPoint(slotID);
  System.out.println("p2 " + slotID + " " + p[0] + " " + p[1] + " " + p[2]);
 }
 public static void main(String[] args) {
  TestGetPoint tester = new TestGetPoint ();
  tester.testing();
 }
}

This is the output.

p1 0 0.0 0.0 0.0
p2 0 1.100000023841858 1.056176589181958E-38 1.2611686178923354E-44

In p1, p is unchanged because Java can't modify an object using a pointer
reference. Java can only pass back objects on the stack.
In p2, the first element is returned correctly. I think this is because Java
doubles only add bytes to the mantissa and don't change the exponent, so you
can overlay a float on top of a double structure, and it will be read the
same. But the other two elements of p are incorrect because the bytes are no
longer aligned on the stack. Java is popping three doubles off a stack that
only has three floats. Of course, Java won't let you cast it to a float
pointer, so you're stuck.

Dale

----- Original Message -----
From: "Jeff Lee" <jeff at cdnorthamerica.com>
To: "Dale Greer" <dmgreer at airmail.net>
Cc: <vtkusers at public.kitware.com>
Sent: Tuesday, May 21, 2002 5:14 AM
Subject: Re: [vtkusers] Coding Errors in VTK Java


> Dale,
> Do you have some sample code that exhibits the problem?  I assure you
> that what you have below is compeletly valid with jni.
> -Jeff
>
> Dale Greer wrote:
>
> >There are some fatal coding errors in the Java interface to the vtk
dll's.
> >For example, in vtkPoints, there's a declaration for GetPoint which
returns
> >a pointer to a float on the stack. But in Java, GetPoint returns a
reference
> >to a double array. If GetPoint is used in Java, the first element in the
> >double
> >array is valid, but the other two are not.
> >
> >C++
> >
> >float* vtkPoints::GetPoint( vtkdType id)
> >
> >Java
> >
> >  private native double[] GetPoint_26(int id0);
> >  public double[] GetPoint(int id0)
> >    { return GetPoint_26(id0); }
> >
> >
> >Even worse, the C++ way to get back an array of doubles is to pass by
> >reference in the argument, which isn't possible in Java, but which is
> >nevertheless attempted by the autogenerated Java interface to the dll.
> >
> >C++
> >
> >void vtkPoints::GetPoint( vtkIdType id, double x[3])
> >
> >Java
> >
> >  private native void GetPoint_27(int id0,double id1[]);
> >  public void GetPoint(int id0,double id1[])
> >    { GetPoint_27(id0,id1); }
> >
> >
> >Having pinpointed the problem, I'm not sure if I know how to fix this. Is
> >there someone else who could put a fix for this into the nightly build?
> >
> >Dale
> >
> >
> >_______________________________________________
> >This is the private VTK discussion list.
> >Please keep messages on-topic. Check the FAQ at:
<http://public.kitware.com/cgi-bin/vtkfaq>
> >Follow this link to subscribe/unsubscribe:
> >http://public.kitware.com/mailman/listinfo/vtkusers
> >
> >
>
>
> _______________________________________________
> This is the private VTK discussion list.
> Please keep messages on-topic. Check the FAQ at:
<http://public.kitware.com/cgi-bin/vtkfaq>
> Follow this link to subscribe/unsubscribe:
> http://public.kitware.com/mailman/listinfo/vtkusers