Center/Right Align a Widget in QTreeWidget

Qt comes with a QTreeWidget class which can be used to show data in a tree view. It can be used to show hierarchical data using either Model/View framework or by manually creating the hierarchy. The QTreeWidget supports multi-columns for each row and also allows editing of the individual cells. However, sometimes we need to present a QWidget in a cell to allow user to interact with data. For example, we might want a user to choose a Boolean value for a cell and instead of asking the user to type Yes/No or True/False, we can present the user with a checkbox. This eliminates human errors in type the values. Often, there is also a need to embed a QPushButton within a cell to allow user to run some action.

Let’s dig in a single example on how to embed a QWidget in a cell of a QTreeWidget.

// Create a tree widget with three columns.
QTreeWidget* treeWidget = new QTreeWidget();
QStringList columnNames;
columnNames << "Column 1" << "Column 2" << "Column 3";

// Add a top level tree widget item.
QTreeWidgetItem* item = new QTreeWidgetItem();

// Add a check box to the second column.
QCheckBox* checkBox = new QCheckBox("Click Me!");
treeWidget->setItemWidget(item, 1, checkBox1);

// Add tree widget to the parent widgets layout.
this->setLayout(new QVBoxLayout());
this->setContentsMargins(0, 0, 0, 0);

The above code produces the output as shown in the below image. The check box is left aligned and QTreeWidget does not offer a way to center or right align it out of the box. A standard solution offered online is to inherit from QTreeWidgetItem and take control of painting the item directly.

QTreeWidget by default aligns inserted widget to the left.

Inheriting from QTreeWidgetItem is unnecessarily complicated and there is no need to do it. A simpler way is to put the checkbox in another widget and use a horizontal layout with stretch before and after the checkbox! This trick can be used to right align as well by omitting the stretch before the checkbox. Here is the code:

// Create a check box.
QCheckBox* checkBox = new QCheckBox("Click Me!");

// Put the check box in a wrapping widget with appropriate layout.
QWidget* checkBoxWrapper = new QWidget;
QHBoxLayout* layout = new QHBoxLayout();
layout->setContentsMargins(0, 0, 0, 0);

// Add it to the tree widget.
mTreeWidget->setItemWidget(item, 1, checkBoxWrapper);

This code is going to produce the following output:

Center aligned widget by using a wrapping widget with a horizontal layout.

The complete Visual Studio 2019 solution for this demo can be downloaded here.

Display FPS for VTK on Python

In the last post, I discussed how to get started with VTK on Python. In this post, I will show how to add support to show frames per second (FPS). The idea to calculate FPS is straight forward: keep track of the number of frames (N) that were rendered in last T seconds. Then fps defined a N/T fps.

To calculate FPS we will add an observer to the EndEvent command of the vtkRenderer. In the callback function, we will count the number of frames rendered in the last T seconds and calculate FPS. Here is the complete code of the FpsObserver:

import vtk
from timeit import default_timer as timer

class FpsObserver:
	def __init__(self, renderer, x=0, y=0):
		self.mRenderer = renderer
		self.mRenderer.AddObserver(vtk.vtkCommand.EndEvent, self)
		self.ActorPosX = x
		self.ActorPosY = y
		self.mFrameCount    = 0         # Number of frames collected since last FPS was calculated.
		self.mStartTime     = timer()   # The last time FPS was calculated.
		self.mFpsUpdateRate = 1         # How often to update FPS in seconds.
	def setPosition(self, x, y):
		self.ActorPosX = x
		self.ActorPosY = y
		self.mFpsActor.SetPosition(self.ActorPosX, self.ActorPosY)
	def __call__(self, caller, event):
		if event == "EndEvent":
			self.mFrameCount = self.mFrameCount + 1
			if timer() - self.mStartTime > self.mFpsUpdateRate:
				_currentTime     = timer()
				_duration        = _currentTime - self.mStartTime
				_fps = self.mFrameCount/_duration
				self.mFpsActor.SetInput("FPS: {:.2f}".format(_fps))
				self.mStartTime  = _currentTime
				self.mFrameCount = 0
	def _createFpsTextActor(self):
		self.mFpsActor = vtk.vtkTextActor()
		self.mFpsActor.GetTextProperty().SetColor([1, 1, 1])
		self.mFpsActor.SetPosition(self.ActorPosX, self.ActorPosY)

To use FpsObserver, we just need to initialize it as self.mFpsObserver = FpsObserver.FpsObserver(self.mRenderer). That’s it, this will display the FPS for last one seconds!

Getting Started with VTK for Python

The visualization toolkit (VTK) is a open source library displaying scientific data. VTK is maintained by Kitware, the same company which gave us CMake. VTK is written in C/C++ but it comes with Python bindings and can be installed from In this post, I am going to show how to start using VTK from Python using PyQt5.

Qt has two package for using with Python: PySide2 and PyQt5. PySide2 is the official module for Python but for a long time there was no official module and only PyQt5 was available. You can refer to to understand the differences (they are mostly same) between two modules. I am going to use PyQt5 but the VTK module itself supports both Qt modules.

VTK provides a QVTKRenderWindowInteractor class which inherits from QWidget, QGLWidget, or any other custom class inherited from QWidget. We will add QVTKRenderWindowInteractor to a QMainWindow and use vtkRenderer to render a Hello, World sphere. To decouple user interface (Qt) and rendering (VTK) I will create a VtkWindow class and use it from a MainWindow which is purely for VTK.

Lets first create the MainWindow:

from PyQt5 import QtCore, QtWidgets
import sys
import VtkWindow

class MainWindow(QtWidgets.QMainWindow):
	def __init__(self, parent=None):
		super(MainWindow, self).__init__(parent)
		self.mVtkWindow = VtkWindow.VtkWindow()
if __name__ == '__main__':
	app = QtWidgets.QApplication(sys.argv)
	window = MainWindow()

If we comment lines 10 and 11 and run the, it will display a blank Qt Window. Now lets see how to add VTK support to it by adding a VtkWindow class:

from PyQt5 import QtWidgets
import vtk
import vtkmodules.qt
vtkmodules.qt.QVTKRWIBase = "QGLWidget"
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor

# VtkWindow must be derived from QFrame:
class VtkWindow(QtWidgets.QFrame):
	def __init__(self, parent=None):
		super(QtWidgets.QWidget, self).__init__(parent)
		# Create a VTK widget and add it to the QFrame.
		self.mVtkWidget = QVTKRenderWindowInteractor(self)
		self.layout().setContentsMargins(0, 0, 0, 0)
		# Get the render window and set an interactor.
		self.mRenderWindow = self.mVtkWidget.GetRenderWindow()
		self.mInteractor   = self.mRenderWindow.GetInteractor()
		# Create a new renderer and set the background color.
		self.mRenderer = vtk.vtkRenderer()
		self.setBackgroundColor([0.5, 0.5, 0.5])
		# Set the Vtk Window title.
		self.mTitleActor = None
		self.setTitle("pyVtkLib Demo")
	# Called when QFrame is resized.
	def resizeEvent(self, newSize):
		textSize = [0, 0]
		self.mTitleActor.GetSize(self.mRenderer, textSize)
		width  = int( (self.width() - textSize[0]) / 2.0)
		height = self.height() - textSize[1]
		self.mTitleActor.SetPosition(width, height - 10)
	def setBackgroundColor(self, color):
	def setTitle(self, title):
		if not self.mTitleActor:
			self.mTitleActor = vtk.vtkTextActor()
			self.mTitleActor.GetTextProperty().SetColor([1, 0, 0])
			self.mTitleActor.SetPosition(0, 0)

VTK module for Python comes with a QVTKRenderWindowInteractor class which by default inherits from QWidget for PyQt5. In lines 4-5, we first change it to to use QGLWidget so that rendering will be done using OpenGL instead of software renderer. Next, we create a class called VtkWindow which inherits from QWidget so that it can be use from Qt UI. Note, that it is recommended to inherit from QFrame and not QWidget as QVTKRenderWindowInteractor cannot be reparented. More discussion on this topic can be found at EmbedPyQt example on VTK website. Next, we create an instance of QVTKRenderWindowInteractor and add it to VtkWindow class through a QVBoxLayout.

After that it is usual VTK stuff of creating a vtkRenderingWindow, vtkRenderWindowInteractor, and vtkRenderer. I prefer to use vtkInteractorStyleTrackballCamera which I find far more intuitive than the default vtkInteractorStyleJoystickCamera.

I render scene title at the top-middle of the screen and in order to place it here I listen to QFrame::resizeEvent to determine te current width and height of the QFrame.

Run the from a terminal and it will display a windows with text pyVtkLib Demo printed in the middle-center of the window. In the next tutorial I will show how to measure and show frames per second to the VtkWindow.

The code from this tutorial and any other future enhancements I will do will be available from saurabhg17/pyVtkLib repository at GitHub.

QWidgetLogger: A Qt logging device for SLogLib

SLogLib is a flexible logging library written in C++. It comes with three logging devices: console, file, and memory but often there is a need to show log directly on the user interface. I heavily use Qt for all projects requiring user interface, so I wrote a logging device for QWidget. It is available in SLogLib repository in the Contrib/QWidgetLogger folder.

Here is how to use the QWidgetLogger:

QWidgetLogger* logger = new QWidgetLogger(new HtmlFormatter(), "WidgetLogger");

Once above code is added, any of the SLOGLIB_LOG_MSG_* logging macros will write to the QWidgetLogger as well as any other loggers added to SLogLib. QWidgetLogger internally uses SLogTextEdit class derived from QTextEdit. Instance of SLogTextEdit used by QWidgetLogger can be retrieved by QWidgetLogger::widget() function. This instance should be added to the UI to show the logging messages. QDockWidget is a good choice to show logging widget with QMainWindow.

SLogTextEdit sets a monospace 10 point font and it can be changed using style sheet. The color and style of the messages logged can also be changed using the Formatter. The HtmlFormatter used in the example above define different colors for different types of logging messages using HTML codes.

In Qt, UI elements can only be updated from the main thread but the logging messages might come from any thread. So SLogTextEdit checks if the message was posted from the main thread or some other worker thread. If the message was posted from a worker thread, SLogTextEdit emits a signal to only of its own private slot and updates itself in the slot. In Qt slots always run in the context of the main thread. This method works well but signal and slot mechanism is slow and update to widget lags while logging too many messages in a short period of time.

Added Multi-threading Support in SLogLib

Recently, I worked on a project which made heavy use of C++ threads. To use SLogLib in this project I added multi-threaded support in SLogLib with the help of c++ 11 std::mutex and std::lock_guard. Over the last few months multi-threaded support in SLogLib has been extensively tested and there are no known bugs.

All threading support is located in LoggingManager.cpp. The functions which modify internal state in LoggingManager are protected by std::mutex. There is a support for building call stack through the use of SLOGLIB_ADD_TO_CALLSTACK macro. In the latest build, there is a separate callstack for each thread.

Checkout the latest commit from

Error loading numpy arrays

If you received “Object arrays cannot be loaded when allow_pickle=False” while loading a numpy array using numpy.load() it is because numpy has changed the default loading behaviour since version 1.16.3. If you are using numpy version newer than this, at many places on internet it is advised to simply downgrade the numpy version. This is not the correct solution at all. From the numpy documentation:

allow_pickle : bool, optional
Allow loading pickled object arrays stored in npy files. Reasons for disallowing pickles include security, as loading pickled data can execute arbitrary code. If pickles are disallowed, loading object arrays will fail. Default: False
Changed in version 1.16.3: Made default False in response to CVE-2019-6446.

Thus, the correct solution is to pass allow_pickle=True to the numpy.load function. However, this should be used carefully and ideally only with the files you have previously saved yourself since picking in python can execute arbitrary code thereby compromising the system security.

String Selection Widget for Qt5

Some time back, I developed a data entry application in Qt5. One of the requirements was to let the user select a single string from a predefined list of string. I developed a custom widget called SStringSelector for this purpose. SStringSelector has two views: display and selection. The display view presents the currently selected string (blank if no string is selected), and a push button. To select a string, the user clicks on the button which presents the user with the selection dialog. The selection dialog consists of a list of string in an QListWidget and the user can select one of them by double-clicking a string. If the list of strings are long, the user can filter them using a filter QLineEdit present above the QListWidget.

SStringSelector is distributed as a part of QtUtils repository hosted on Github. The SStringSelector widget is really simple to use. Simple add the SStringSelector.h and SStringSelector.cpp files in your project and add an instance of SStringSelector in the layout of your app.

Below are some screenshots of the widget under Windows:

The SStringSelector Widget.
Selection Dialog of the SStringSelector Widget.
Filtering Strings in the Selection Dialog.

Color Picker Widget for Qt5

Qt5 support standard dialogs such as QFileDialog, QFontDialog, and QColorDialog, however, it does not provide a color picker to allow a user to pick a color. Recently, I need a color picker for one of my projects and I implemented a simple color picker widget.

SColorPicker is available from Github as a part of QtUtils repository. To use SColorPicker, add the header and cpp files directly in your project. Then, simply add an instance of SColorPicker in a layout. SColorPicker will appear as 16×16 pixels colored square in the layout. If you need a different size, change it in the SColorPicker's constructor. When a user double-clicks on the colored square, the system’s color dialog will appear allowing the user to choose a color. The selected color can be obtained from color() function or by connecting to colorPicked() signal.

Below are the screenshots of the SColorPicker_Demo and system color dialog present to the user on Windows 10 computer.

Fill Disk Partition

Recently, I had to give away a computer with couple of disks in it. I wanted to securely erase data on these disks as I stored personal sensitive information on them. Using a program such as DBAN was not an option as I was not allowed to remove the operating system from the computer. My goal was to simply overwrite free space from all the partitions. I couldn’t find anything I liked so I ended up writing a simple tool called FillPartition in python.

FillPartition is hosted on Github at It is really easy to use with just one mandatory argument (the path of the partition) and one optional argument (–outputDir, -od) the directory in the partition where files should be written. FillPartition writes 1GB files filled with 0 bytes until the free space is less than 1GB and then write one final file of the size equal to the remaining free space.

Below is a screenshot of a run of FillPartition on Windows

Search Box using QLineEdit

This week, at work I had to implement a search box for a software I am working on. The search box is to filter some data dynamically as user types a query. I wanted to show a clear (cross) icon at the right side of the search box so that user can clear the results instead of selecting the current query and deleting it manually. Lastly, for clarity I wanted to show a search icon on the left side of search box. The search box looks like this:

Screenshot of the Search box implemented using QLineEdit

After the user enters a query a clear icon appears on the right. The clear icon is in fact a button and clicking it will clear the current search.
Screenshot of the Search box with keywords implemented using QLineEdit

It is really easy to make this search box using QLineEdit. We need only the following three lines of code:

QLineEdit* _lineEdit = new QLineEdit();
_lineEdit->addAction(":/resources/search.ico", QLineEdit::LeadingPosition);

// add _lineEdit to your widget

Line 2 enables the clear button which adds the clear action and cross icon to the right. Line 3 adds another action with a search icon to the left of the QLineEdit. We don’t listen to this action as it is merely decorative. Line 4 adds a placeholder text which is shown in the QLineEdit but is cleared as soon as user starts typing.

We only connect textChanged(const QString&)  signal which is emitted both when a user clicks on the cross icon and when he enters a search query.