VTK (pyqt): from world coordinate to display coordinate

Support
1

I am using VTK for my project. Now, I need to convert the point from world coordinate to display coordinate, and the result is wrong. The following pyqt code is used to reproduce the problem I meet.

from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
import sys, os
import numpy as np
import vtk, qimage2ndarray

class vtkLabel(QLabel):
    def __init__(self):
        super(vtkLabel, self).__init__()
        cone = vtk.vtkConeSource()
        mapper = vtk.vtkPolyDataMapper()
        mapper.SetInputConnection(cone.GetOutputPort())
        actor = vtk.vtkActor()
        actor.SetMapper(mapper)

        ren = vtk.vtkRenderer()
        ren.AddActor(actor)
        renWin = vtk.vtkRenderWindow()
        renWin.AddRenderer(ren)
        renWin.SetOffScreenRendering(1)
        imageFilter = vtk.vtkWindowToImageFilter()
        imageFilter.SetInput(renWin)

        self.ren = ren
        self.renWin = renWin
        self.imageFilter = imageFilter

    def mousePressEvent(self, QMouseEvent):
        super().mousePressEvent(QMouseEvent)
        pos = QMouseEvent.pos()
        x = pos.x()
        y = pos.y()
        self.lastPos = [x, y]


    def mouseReleaseEvent(self, QMouseEvent):
        super().mouseReleaseEvent(QMouseEvent)
        self.lastPos = None

    def mouseMoveEvent(self, QMouseEvent):
        super().mouseMoveEvent(QMouseEvent)
        pos = QMouseEvent.pos()
        x = pos.x()
        y = pos.y()
        if self.lastPos != None:
            if QMouseEvent.buttons() == Qt.RightButton:
                dy = self.lastPos[1] - y
                center = self.ren.GetCenter()
                dyf = dy/center[1]
                import math
                val = math.pow(1.1, dyf)
                self.ren.GetActiveCamera().Dolly(val)

            if QMouseEvent.buttons() == Qt.LeftButton:
                dx = x - self.lastPos[0]
                dy = self.lastPos[1] - y
                size = self.renWin.GetSize()

                delta_elevation = -20.0 / size[1]
                delta_azimuth = -20.0 / size[0]
                rxf = dx * delta_azimuth
                ryf = dy * delta_elevation

                camera = self.ren.GetActiveCamera()
                camera.Azimuth(rxf)
                camera.Elevation(ryf)
                camera.OrthogonalizeViewUp()

            self.ren.ResetCameraClippingRange()
            self.ren.UpdateLightsGeometryToFollowCamera()
            self.ren.Render()
            self.calculationForDisplay()

    def resizeEvent(self, QMouseEvent):
        super().resizeEvent(QMouseEvent)
        self.renWin.SetSize(self.width(), self.height())
        self.calculationForDisplay()

    def calculationForDisplay(self):
        self.renWin.Render()
        self.imageFilter.Modified()
        self.imageFilter.Update()
        displayImg = self.imageFilter.GetOutput()

        dims = displayImg.GetDimensions()
        from vtk.util.numpy_support import vtk_to_numpy
        numImg = vtk_to_numpy(displayImg.GetPointData().GetScalars())
        numImg = numImg.reshape(dims[1], dims[0], 3)
        numImg = numImg.transpose(0, 1, 2)
        numImg = np.flipud(numImg)

        displayQImg = qimage2ndarray.array2qimage(numImg)
        pixmap = QPixmap.fromImage(displayQImg)
        self.pixmap = pixmap
        self.update()

    def paintEvent(self, QPaintEvent):
        super(vtkLabel, self).paintEvent(QPaintEvent)
        painter = QPainter(self)
        width = self.width()
        height = self.height()
        painter.drawPixmap(QPoint(0, 0), self.pixmap, QRect(0, 0, width, height))

        coordinate = vtk.vtkCoordinate()
        coordinate.SetCoordinateSystemToWorld()
        point = [-0.411, 0.203, 0.894]
        coordinate.SetValue(point[0], point[1], point[2])
        displayCoor = coordinate.GetComputedDisplayValue(self.ren)
        x = displayCoor[0]
        y = displayCoor[1]
        y = self.height() - y
        painter.setPen(QPen(QColor(255, 0, 0), 5))
        painter.drawPoint(x, y)

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.resize(500,500)
        self.imageLabel = vtkLabel()
        self.setCentralWidget(self.imageLabel)

app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

In the above code, the world point is hard coded as [-0.411, 0.203, 0.894] in the paintEvent function, and show it as a red point. Then, I zoom in/out the object by mouse press and move (right button), and rotate the object by mouse press and move (left button). It can be seen that the shown red point doesn’t change along with the object.

2

The paint event will not necessarily be called when VTK updates its scene, since it takes place in an OpenGL viewport (that is, in the graphics card) without Qt noticing it. Try adding a print("") to the paintEvent() function to tell whether it is being called repeatedly.

3

Firstly, I show the cone and point by QLabel rather than VTK. Thus, I need to update the results in paint event.

More important, the thing makes me confuzed is that the displayed point would not rotate or zoom in/out along with the object, which is calculated from a fixed world point. I am doubt that the point convertion between world system and display system is wrong. Could you please help me to check the point convertion code?

    coordinate = vtk.vtkCoordinate()
    coordinate.SetCoordinateSystemToWorld()
    point = [-0.411, 0.203, 0.894]
    coordinate.SetValue(point[0], point[1], point[2])
    displayCoor = coordinate.GetComputedDisplayValue(self.ren)
4

Please, put a print("Repainted!") in the paintEvent() function and try again. Post the results here when you interact with the cone.

5

I have added print(‘repainted!’) in the paintEvent(), and it actually print the expected information when I iteract with the cone. However, the red point still not change along with the cone.

from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
import sys, os
import numpy as np
import vtk, qimage2ndarray

class vtkLabel(QLabel):
def init(self):
super(vtkLabel, self).init()
self.k = 0
cone = vtk.vtkConeSource()
mapper = vtk.vtkPolyDataMapper()
mapper.SetInputConnection(cone.GetOutputPort())
actor = vtk.vtkActor()
actor.SetMapper(mapper)

    ren = vtk.vtkRenderer()
    ren.AddActor(actor)
    renWin = vtk.vtkRenderWindow()
    renWin.AddRenderer(ren)
    renWin.SetOffScreenRendering(1)
    imageFilter = vtk.vtkWindowToImageFilter()
    imageFilter.SetInput(renWin)

    self.ren = ren
    self.renWin = renWin
    self.imageFilter = imageFilter

def mousePressEvent(self, QMouseEvent):
    super().mousePressEvent(QMouseEvent)
    pos = QMouseEvent.pos()
    x = pos.x()
    y = pos.y()
    self.lastPos = [x, y]


def mouseReleaseEvent(self, QMouseEvent):
    super().mouseReleaseEvent(QMouseEvent)
    self.lastPos = None

def mouseMoveEvent(self, QMouseEvent):
    super().mouseMoveEvent(QMouseEvent)
    pos = QMouseEvent.pos()
    x = pos.x()
    y = pos.y()
    if self.lastPos != None:
        if QMouseEvent.buttons() == Qt.RightButton:
            dy = self.lastPos[1] - y
            center = self.ren.GetCenter()
            dyf = dy/center[1]
            import math
            val = math.pow(1.1, dyf)
            self.ren.GetActiveCamera().Dolly(val)

        if QMouseEvent.buttons() == Qt.LeftButton:
            dx = x - self.lastPos[0]
            dy = self.lastPos[1] - y
            size = self.renWin.GetSize()

            delta_elevation = -20.0 / size[1]
            delta_azimuth = -20.0 / size[0]
            rxf = dx * delta_azimuth
            ryf = dy * delta_elevation

            camera = self.ren.GetActiveCamera()
            camera.Azimuth(rxf)
            camera.Elevation(ryf)
            camera.OrthogonalizeViewUp()

        self.ren.ResetCameraClippingRange()
        self.ren.UpdateLightsGeometryToFollowCamera()
        self.ren.Render()
        self.calculationForDisplay()

def resizeEvent(self, QMouseEvent):
    super().resizeEvent(QMouseEvent)
    self.renWin.SetSize(self.width(), self.height())
    self.calculationForDisplay()

def calculationForDisplay(self):
    self.renWin.Render()
    self.imageFilter.Modified()
    self.imageFilter.Update()
    displayImg = self.imageFilter.GetOutput()

    dims = displayImg.GetDimensions()
    from vtk.util.numpy_support import vtk_to_numpy
    numImg = vtk_to_numpy(displayImg.GetPointData().GetScalars())
    numImg = numImg.reshape(dims[1], dims[0], 3)
    numImg = numImg.transpose(0, 1, 2)
    numImg = np.flipud(numImg)

    displayQImg = qimage2ndarray.array2qimage(numImg)
    pixmap = QPixmap.fromImage(displayQImg)
    self.pixmap = pixmap
    self.update()

def paintEvent(self, QPaintEvent):
    super(vtkLabel, self).paintEvent(QPaintEvent)
    print('repainter: ', self.k)
    self.k += 1
    painter = QPainter(self)
    width = self.width()
    height = self.height()
    painter.drawPixmap(QPoint(0, 0), self.pixmap, QRect(0, 0, width, height))

    coordinate = vtk.vtkCoordinate()
    coordinate.SetCoordinateSystemToWorld()
    point = [-0.411, 0.203, 0.894]
    coordinate.SetValue(point[0], point[1], point[2])
    displayCoor = coordinate.GetComputedDisplayValue(self.ren)
    x = displayCoor[0]
    y = displayCoor[1]
    y = self.height() - y
    painter.setPen(QPen(QColor(255, 0, 0), 5))
    painter.drawPoint(x, y)

class MainWindow(QMainWindow):
def init(self, parent=None):
super(MainWindow, self).init(parent)
self.resize(500,500)
self.imageLabel = vtkLabel()
self.setCentralWidget(self.imageLabel)

app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

Actually, the QLabel.update is called in the calculationForDisplay, which will be called in the mouseMoveEvent. Thus, the paintEvent would be called when I interact with the cone.

6

Please, replace this:

    x = displayCoor[0]
    y = displayCoor[1]

with this:

    x = random.randint ( 0 , 500 )
    y = random.randint ( 0 , 500 )

in your paintEvent() method. Report the results when you drag the mouse to interact with the cone.

7

Yes, I have modified my code as the suggestion. The red point would randomly change when I rotate the cone.

8

Would or IS? It is important to make sure the paintEvent() method is being called as you drag the mouse. I’d try updating the coordinates in another event, say mouseMoveEvent().

9

The paintEvent() is called when I rotate the cone. The coordinate is also updated in the mouseMoveEvent(). In the mouseMoveEvent(), I call the calculationForDisplay() function, and in this function, the update() is called thus the paintEvent() is called.

10

Try translating the cone with the middle mouse button or SHFT + left mouse button.

11

Sorry, but I do not implement the translation function.

Are you doubt that the display coordinate is not updated when I interact with the cone?

However, when I roatate the cone, the displayed red point also move. The problem is that the movement of red point do not along with the cone.

Could you please run the code? It may show the problem more clearly.

12

Hello, Zhang,

You hard coded the world coordinate of the test point. So, its screen coordinate won’t necessarily coincide with the cone’s. To achieve the coherent effect you desire, you have to make the point’s world coordinate equal one of the cone’s points (e.g. its center). But other parts of the cone will appear to move with respect to the point due to parallax. That’s inevitable because the cone is a 3D object.

cheers,

Paulo

13

Thank you for you suggestion. You mean that the point I provide is not a point of cone, thus, it will not move along the cone. Yes, it may be the reason.
In order to avoid this possible problem, I obtain the world point by converting a displayed point:

picker = vtk.vtkPropPicker()
picker.Pick(250, 250, 0, self.ren)
if picker.GetActor():
    coordinate = vtk.vtkCoordinate()
    coordinate.SetCoordinateSystemToDisplay()
    coordinate.SetValue(250, 250, 0)
    position = coordinate.GetComputedWorldValue(self.ren)
    self.position = position
    print('the point is one of the cone point')
else:
    raise RuntimeError('the point is not one of the cone point')

I convert the displayed point (250, 250) to world point, and it print that “the point is one of the cone point”. Thus, the self.position is definitely one point of cone. Am I right?

However, when I rotate the cone, the point still not fall on the cone:
1

The whole code is:

from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
import sys, os
import numpy as np
import vtk, qimage2ndarray

class vtkLabel(QLabel):
    def __init__(self):
        super(vtkLabel, self).__init__()
        cone = vtk.vtkConeSource()
        mapper = vtk.vtkPolyDataMapper()
        mapper.SetInputConnection(cone.GetOutputPort())
        actor = vtk.vtkActor()
        actor.SetMapper(mapper)

        ren = vtk.vtkRenderer()
        ren.AddActor(actor)
        renWin = vtk.vtkRenderWindow()
        renWin.AddRenderer(ren)
        renWin.SetOffScreenRendering(1)
        imageFilter = vtk.vtkWindowToImageFilter()
        imageFilter.SetInput(renWin)

        self.ren = ren
        self.renWin = renWin
        self.imageFilter = imageFilter

    def mousePressEvent(self, QMouseEvent):
        super().mousePressEvent(QMouseEvent)
        pos = QMouseEvent.pos()
        x = pos.x()
        y = pos.y()
        self.lastPos = [x, y]


    def mouseReleaseEvent(self, QMouseEvent):
        super().mouseReleaseEvent(QMouseEvent)
        self.lastPos = None

    def mouseMoveEvent(self, QMouseEvent):
        super().mouseMoveEvent(QMouseEvent)
        pos = QMouseEvent.pos()
        x = pos.x()
        y = pos.y()
        if self.lastPos != None:
            if QMouseEvent.buttons() == Qt.RightButton:
                dy = self.lastPos[1] - y
                center = self.ren.GetCenter()
                dyf = dy/center[1]
                import math
                val = math.pow(1.1, dyf)
                self.ren.GetActiveCamera().Dolly(val)

            if QMouseEvent.buttons() == Qt.LeftButton:
                dx = x - self.lastPos[0]
                dy = self.lastPos[1] - y
                size = self.renWin.GetSize()

                delta_elevation = -20.0 / size[1]
                delta_azimuth = -20.0 / size[0]
                rxf = dx * delta_azimuth
                ryf = dy * delta_elevation

                camera = self.ren.GetActiveCamera()
                camera.Azimuth(rxf)
                camera.Elevation(ryf)
                camera.OrthogonalizeViewUp()

            self.ren.ResetCameraClippingRange()
            self.ren.UpdateLightsGeometryToFollowCamera()
            self.ren.Render()
            self.calculationForDisplay()

    def resizeEvent(self, QMouseEvent):
        super().resizeEvent(QMouseEvent)
        self.renWin.SetSize(self.width(), self.height())
        self.calculationForDisplay()
        picker = vtk.vtkPropPicker()
        picker.Pick(250, 250, 0, self.ren)
        if picker.GetActor():
            coordinate = vtk.vtkCoordinate()
            coordinate.SetCoordinateSystemToDisplay()
            coordinate.SetValue(250, 250, 0)
            position = coordinate.GetComputedWorldValue(self.ren)
            self.position = position
            print('the point is one of the cone point')
        else:
            raise RuntimeError('the point is not one of the cone point')

    def calculationForDisplay(self):
        self.renWin.Render()
        self.imageFilter.Modified()
        self.imageFilter.Update()
        displayImg = self.imageFilter.GetOutput()

        dims = displayImg.GetDimensions()
        from vtk.util.numpy_support import vtk_to_numpy
        numImg = vtk_to_numpy(displayImg.GetPointData().GetScalars())
        numImg = numImg.reshape(dims[1], dims[0], 3)
        numImg = numImg.transpose(0, 1, 2)
        numImg = np.flipud(numImg)

        displayQImg = qimage2ndarray.array2qimage(numImg)
        pixmap = QPixmap.fromImage(displayQImg)
        self.pixmap = pixmap
        self.update()

    def paintEvent(self, QPaintEvent):
        super(vtkLabel, self).paintEvent(QPaintEvent)
        painter = QPainter(self)
        width = self.width()
        height = self.height()
        painter.drawPixmap(QPoint(0, 0), self.pixmap, QRect(0, 0, width, height))

        coordinate = vtk.vtkCoordinate()
        coordinate.SetCoordinateSystemToWorld()
        point = self.position
        coordinate.SetValue(point[0], point[1], point[2])
        displayCoor = coordinate.GetComputedDisplayValue(self.ren)
        x = displayCoor[0]
        y = displayCoor[1]
        y = self.height() - y
        painter.setPen(QPen(QColor(255, 0, 0), 5))
        painter.drawPoint(x, y)

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.resize(500,500)
        self.imageLabel = vtkLabel()
        self.setCentralWidget(self.imageLabel)

app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

I am so confuzed.

14

And when you drag (translate) the cone with the mouse?

15

Yes, if I drag the cone, we can see that the point is seperated from cone.

When I drag the cone, the displayed cone and displayed point will simultaneously be updated.

Could you please run the code?