VTK 绘制等值线

      VTK 绘制等值线

  使用vtk来绘制等值线，需要使用到vtkContourFilter，它所处理的数据需要是规则网格数据，如规则的正方形网格等。
  通过使用vtkContourFilter::SetValue(i,value)方法，i表示第几条等值线，从0开始计算，value表示等值线的值。多边形的每个点都有一个属性值，两个相邻的属性值之间要先通过插值，再把这些属性值作为该函数的输入。如：
            SetValue(0,5);
画一条等值为5的等值线。两个相邻的点的属性值为3和6，则先插值为（3,4,5,6），所以该先应该经过这条边的2/3处。
  也可以使用vtkContourFilter：：GenerateValues(count, start, end);来生成一些列的等值线，其中count参数指定等值线的条数，而start指定第一条等值线的值，end指定最后一条等值线的值，中间等值线的值则通过计算得到。

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

解释：
  SetValue和GenerateValues均为设置等值线值的函数；这两个函数的不同之处在于SetValue设置一条等值线值，而GenerateValues对应n条等值线的值，同时数量n包括两个边界值，其余n-2条等值线的值自动按照线性关系产生。
  SetValue和GenerateValues函数根据先后调用关系产生覆盖，即后调用的函数产生的等值线可能覆盖掉先调用函数产生的等值线，这与等值线的条数(GetNumberOfContours)相关。
  例如：首先调用GenerateValues(3, 100, 300)产生3条等值线，分别为100、200、300；然后调用SetValue(0, 125)，则最终效果为共有3条等值线，分别为125、200、300；如果改为调用SetValue(3, 400)，则最终效果为共有4条等值线，分别为100、200、300、400。
  SetValue一般用于覆盖某一条已经存在的等值线，或者增加一条等值线；GenerateValues一般重新设置等值线的条数。

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示例代码：

#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(caller);

        double value = static_cast(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 colors = vtkSmartPointer::New();

vtkSmartPointer imgData = vtkSmartPointer::New();
CreateData( imgData );

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

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

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

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

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

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

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

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

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

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

vtkSmartPointer interactStyle = vtkSmartPointer::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(pData->GetScalarPointer( x, y, 0 ));
            pixel[ 0 ] = sqrt( pow( x, 2.0 ) + pow( y, 2.0 ) );
        }
    }

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

}
结果如下图所示：