[vtkusers] Changing/setting the extent of an image for a custom reader

[kalin]

Hi there,

I have a questions about changing the extents of an image in VTK. I 
have followed the examples to create a reader for my data files. What I 
have so far seems to work OK, the reader subclasses vtkImageAlgorithm 
and produces an image. What I would like to know is if I can change the 
extents of the image to be rendered before my reader reads the entire 
file. Let's say that I want to look at only a small part of the image 
and I provide the extent as input to the reader. I have a test harness 
that I'm using for the reader, which is below (which will use the reader 
to read all of the data and then display it as an image).

	// Read the file
	vtkSmartPointer<MyReader> reader =
		vtkSmartPointer<MyReader>::New();
	reader->SetFileName ( argv[1] );
	reader->Update();

	//TODO: Figure out how to set the extent.

	vtkImageData* output = reader->GetOutput();

	// Convert the image to a polydata
	vtkSmartPointer<vtkImageDataGeometryFilter> imageDataGeometryFilter =
	vtkSmartPointer<vtkImageDataGeometryFilter>::New();
	imageDataGeometryFilter->SetInputConnection(reader->GetOutputPort());
	imageDataGeometryFilter->Update();

	vtkSmartPointer<vtkPolyDataMapper> mapper =
	vtkSmartPointer<vtkPolyDataMapper>::New();
	mapper->SetInputConnection(imageDataGeometryFilter->GetOutputPort());

	vtkSmartPointer<vtkActor> actor =
	vtkSmartPointer<vtkActor>::New();
	actor->SetMapper(mapper);
	actor->GetProperty()->SetPointSize(3);

	// Setup rendering
	vtkSmartPointer<vtkRenderer> renderer =
	vtkSmartPointer<vtkRenderer>::New();
	renderer->AddActor(actor);
	renderer->SetBackground(1,1,1);
	renderer->ResetCamera();

	vtkSmartPointer<vtkRenderWindow> renderWindow =
	vtkSmartPointer<vtkRenderWindow>::New();
	renderWindow->AddRenderer(renderer);

	vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
	vtkSmartPointer<vtkRenderWindowInteractor>::New();

	renderWindowInteractor->SetRenderWindow(renderWindow);
	renderWindowInteractor->Initialize();
	renderWindowInteractor->Start();


Any suggestions as to the best way to do this? I am not quite sure if I 
should be implementing the RequestUpdateExtent method or something else.

Thanks,
Kalin