VTK 繪制等值線

??????VTK 繪制等值線

??使用vtk來繪制等值線，需要使用到vtkContourFilter，它所處理的數據需要是規則網格數據，如規則的正方形網格等。
??通過使用vtkContourFilter::SetValue(i,value)方法，i表示第幾條等值線，從0開始計算，value表示等值線的值。多邊形的每個點都有一個屬性值，兩個相鄰的屬性值之間要先通過插值，再把這些屬性值作為該函數的輸入。如：
????????????SetValue(0,5);
畫一條等值為5的等值線。兩個相鄰的點的屬性值為3和6，則先插值為（3,4,5,6），所以該先應該經過這條邊的2/3處。
??也可以使用vtkContourFilter：：GenerateValues(count, start, end);來生成一些列的等值線，其中count參數指定等值線的條數，而start指定第一條等值線的值，end指定最后一條等值線的值，中間等值線的值則通過計算得到。

????vtkContourFilter::SetValue(…)、GenerateValues(…)、GetNumberOfContours(…)

解釋：
??SetValue和GenerateValues均為設置等值線值的函數；這兩個函數的不同之處在于SetValue設置一條等值線值，而GenerateValues對應n條等值線的值，同時數量n包括兩個邊界值，其余n-2條等值線的值自動按照線性關系產生。
??SetValue和GenerateValues函數根據先后調用關系產生覆蓋，即后調用的函數產生的等值線可能覆蓋掉先調用函數產生的等值線，這與等值線的條數(GetNumberOfContours)相關。
??例如：首先調用GenerateValues(3, 100, 300)產生3條等值線，分別為100、200、300；然后調用SetValue(0, 125)，則最終效果為共有3條等值線，分別為125、200、300；如果改為調用SetValue(3, 400)，則最終效果為共有4條等值線，分別為100、200、300、400。
??SetValue一般用于覆蓋某一條已經存在的等值線，或者增加一條等值線；GenerateValues一般重新設置等值線的條數。

示例代碼：

#include "vtkAutoInit.h"
VTK_MODULE_INIT( vtkRenderingOpenGL2 );
VTK_MODULE_INIT( vtkInteractionStyle );

#include "vtkSmartPointer.h"
#include "vtkContourFilter.h"
#include "vtkInteractorStyleUser.h"
#include "vtkProperty.h"
#include "vtkOutlineFilter.h"
#include "vtkCommand.h"
#include "vtkSliderWidget.h"
#include "vtkSliderRepresentation.h"
#include "vtkSliderRepresentation3D.h"
#include "vtkImageData.h"
#include "vtkCellArray.h"
#include "vtkFloatArray.h"
#include "vtkPointData.h"
#include "vtkPolyDataMapper.h"
#include "vtkActor.h"
#include "vtkRenderer.h"
#include "vtkRenderWindow.h"
#include "vtkRenderWindowInteractor.h"
#include "vtkXMLImageDataWriter.h"
#include "vtkNamedColors.h"

namespace
{

void CreateData( vtkImageData* data );

class vtkSliderCallback : public vtkCommand
{
public:
    static vtkSliderCallback* New()
    {
        return new vtkSliderCallback;
    }

    virtual void Execute( vtkObject *caller, unsigned long eventId, void *callData )
    {
        vtkSliderWidget* sliderWidget = reinterpret_cast<vtkSliderWidget*>(caller);

        double value = static_cast<vtkSliderRepresentation*>(sliderWidget->GetRepresentation())->GetValue();
        this->ContourFilter->GenerateValues( 5, 10.0, value );
        //5: 5contour line 
        //10.0 * value: start value
        //value: end value
    }

    vtkSliderCallback() : ContourFilter( NULL ){}
    vtkContourFilter* ContourFilter;
};

}

int main()
{
vtkSmartPointer<vtkNamedColors> colors = vtkSmartPointer<vtkNamedColors>::New();

vtkSmartPointer<vtkImageData> imgData = vtkSmartPointer<vtkImageData>::New();
CreateData( imgData );

//Create an iso surface
vtkSmartPointer<vtkContourFilter> contourFilter = vtkSmartPointer<vtkContourFilter>::New();
contourFilter->SetInputData( imgData );
contourFilter->GenerateValues( 5, 10, 10 );

//Map the contours to graphical primitives
vtkSmartPointer<vtkPolyDataMapper> contourMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
contourMapper->SetInputConnection( contourFilter->GetOutputPort() );

//Create an actor for the contours
vtkSmartPointer<vtkActor> contourActor = vtkSmartPointer<vtkActor>::New();
contourActor->SetMapper( contourMapper );
contourActor->GetProperty()->SetLineWidth( 5 );

//Create the outline
vtkSmartPointer<vtkOutlineFilter> outlineFilter = vtkSmartPointer<vtkOutlineFilter>::New();
outlineFilter->SetInputData( imgData );

vtkSmartPointer<vtkPolyDataMapper> outlineMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
outlineMapper->SetInputConnection( outlineFilter->GetOutputPort() );
vtkSmartPointer<vtkActor> outlineActor = vtkSmartPointer<vtkActor>::New();
outlineActor->SetMapper( outlineMapper );
outlineActor->GetProperty()->SetColor( colors->GetColor3d( "Gray" ).GetData() );
outlineActor->GetProperty()->SetLineWidth( 3 );

//Visualize
vtkSmartPointer<vtkRenderer> renderer = vtkSmartPointer<vtkRenderer>::New();
vtkSmartPointer<vtkRenderWindow> renderWindow = vtkSmartPointer<vtkRenderWindow>::New();
renderWindow->AddRenderer( renderer );
vtkSmartPointer<vtkRenderWindowInteractor> interactor = vtkSmartPointer<vtkRenderWindowInteractor>::New();
interactor->SetRenderWindow( renderWindow );

renderer->AddActor( contourActor );
renderer->AddActor( outlineActor );
renderer->SetBackground( colors->GetColor3d( "Burlywood" ).GetData() );

vtkSmartPointer<vtkSliderRepresentation3D> sliderRep = vtkSmartPointer<vtkSliderRepresentation3D>::New();
sliderRep->SetMinimumValue( 0.0 );
sliderRep->SetMaximumValue( 30.0 );
sliderRep->SetValue( 10.0 );
sliderRep->SetTitleText( "Contour value" );
sliderRep->SetPoint1InWorldCoordinates( -20, -40, 0 );
sliderRep->SetPoint2InWorldCoordinates( 0, -40, 0 );
sliderRep->SetSliderWidth( 0.2 );
sliderRep->SetLabelHeight( 0.1 );

vtkSmartPointer<vtkSliderWidget> sliderWidget = vtkSmartPointer<vtkSliderWidget>::New();
sliderWidget->SetInteractor( interactor );
sliderWidget->SetRepresentation( sliderRep );
sliderWidget->SetAnimationModeToAnimate();
sliderWidget->EnabledOn();

vtkSmartPointer<vtkSliderCallback> callback = vtkSmartPointer<vtkSliderCallback>::New();
callback->ContourFilter = contourFilter;

sliderWidget->AddObserver( vtkCommand::InteractionEvent, callback );

vtkSmartPointer<vtkInteractorStyleUser> interactStyle = vtkSmartPointer<vtkInteractorStyleUser>::New();
interactor->SetInteractorStyle( interactStyle );

renderWindow->SetSize( 500, 500 );
renderWindow->Render();
interactor->Start();

return EXIT_SUCCESS;

}

namespace
{
    void CreateData( vtkImageData* pData )
    {
        pData->SetExtent( -25, 25, -25, 25, 0, 0 );
        pData->AllocateScalars( VTK_DOUBLE, 1 );
        int* extent = pData->GetExtent();

    for( int y = extent[ 2 ]; y <= extent[ 3 ]; y++ )
    {
        for( int x = extent[ 0 ]; x <= extent[ 1 ]; x++ )
        {
            double* pixel = static_cast<double*>(pData->GetScalarPointer( x, y, 0 ));
            pixel[ 0 ] = sqrt( pow( x, 2.0 ) + pow( y, 2.0 ) );
        }
    }

    vtkSmartPointer<vtkXMLImageDataWriter> writer =
        vtkSmartPointer<vtkXMLImageDataWriter>::New();
    writer->SetFileName( "data.vti" );
    writer->SetInputData( pData );
    writer->Write();
    writer->Update();
}

}
結果如下圖所示：