Disable OpenCV Logging

When we use OpenCV from command line process, sometimes OpenCV will print internal logging messages such as:

[ INFO:0] global .\opencv-master\sources\modules\core\src\parallel\registry_parallel.impl.hpp (90) cv::parallel::ParallelBackendRegistry::ParallelBackendRegistry core(parallel): Enabled backends(2, sorted by priority): TBB(1000); OPENMP(990)
[ INFO:0] global .\opencv-master\sources\modules\core\include\opencv2/core/parallel/backend/parallel_for.tbb.hpp (54) cv::parallel::tbb::ParallelForBackend::ParallelForBackend Initializing TBB parallel backend: TBB_INTERFACE_VERSION=12010
[ INFO:0] global .\opencv-master\sources\modules\core\src\parallel\parallel.cpp (73) cv::parallel::createParallelForAPI core(parallel): using backend: TBB (priority=1000)

This might be useful while debugging OpenCV problems but it is a noise if we want to print our own messages to the console. In this case, it is desirable to disable the internal OpenCV logging using the following code:

#include "opencv2/core/utils/logger.hpp"

int main()

    return 1;

History of All Visual C++ Versions

ProductRelease dateC++ version_MSC_VER
Visual Studio 2019 16.1110-Aug-2114.291929
Visual Studio 2019 16.1025-May-2114.291929
Visual Studio 2019 16.902-Mar-2114.281928
Visual Studio 2019 16.910-Nov-2014.281928
Visual Studio 2019 16.705-Aug-2014.271927
Visual Studio 2019 16.619-May-2014.261926
Visual Studio 2019 16.516-Mar-2014.251925
Visual Studio 2019 16.403-Dec-1914.241924
Visual Studio 2019 16.323-Sep-1914.231923
Visual Studio 2019 16.224-Jul-1914.221922
Visual Studio 2019 16.121-May-1914.211921
Visual Studio 2019 16.002-Apr-1914.21920
Visual Studio 2017 15.913-Nov-1814.161916
Visual Studio 2017 15.814-Aug-1814.151915
Visual Studio 2017 15.707-May-1814.141914
Visual Studio 2017 15.605-Mar-1814.131913
Visual Studio 2017 15.504-Dec-1714.121912
Visual Studio 2017 15.409-Oct-1714.111911
Visual Studio 2017 15.314-Aug-1714.111911
Visual Studio 2017 15.210-May-1714.11910
Visual Studio 2017 15.105-Apr-1714.11910
Visual Studio 201707-Mar-1714.11910
Visual Studio 201520-Jul-15141900
Visual Studio 201317-Oct-13121800
Visual Studio 201215-Aug-12111700
Visual Studio 201002-Jul-05101600
Visual Studio 200830-Jun-0591500
Visual Studio 200527-Jun-0581400

Git Command Reference

Git Config

# Set name and email you want attached to your commit transactions
git config --global user.name "[name]"
git config --global user.email "[email address]"

# Enables helpful colorization of command line output
git config --global color.ui auto

# Configure kdiff3 as the merge tool on Windows.
git config --global merge.tool kdiff3
git config --global mergetool.kdiff3.path "C:/Program Files/KDiff3/kdiff3.exe"
git config --global mergetool.kdiff3.trustExitCode false

Git Branch

git branch -d localBranchName               # Delete branch locally
git push origin --delete remoteBranchName   # Delete branch remotely
git remote prune origin                     # Delete all local branches not on remote
git push --set-upstream origin BRANCH_NAME  # Push local branch to remote; will create a pull request.
git branch --sort=-committerdate            # Print branches ordered by the commit dates.
git branch -vv                              # Show the last local and remote commit on each branch.

Git Checkout

git checkout @{-N}  # Go back N branches.
git checkout -      # Go to the last branch (shorthand to  for the previous command)

Git Diff

git diff                                   # changed but not staged.
git diff --staged                          # staged and ignore local changes.
git diff --cached                          # same as staged.
git difftool                               # use external diff tool.
git diff $startCommit..$endCommit -- FILE  # Compare changes in one file from startCommit to endCommit.
git diff $startCommit..$endCommit          # Compare two commits.

Git Commit

git commit                  # open external editor for commit message
git commit -m "message"
git commit -a -m "message"  # ignore staging and commit all modified files
git commit --amend          # Fix last commit message.

# Add a file missed in the last commit.
git add missed_file.txt
git commit --amend

# Remove a file after committing:
git reset --soft HEAD~1
git reset accidently_added_file.jpg
rm accidently_added_file.jpg
git commit

Git Stash

git stash        # Store the current changes in stash.
git stash pop    # Apply the stashed changes and then drop it.
git stash drop   # Remove the stashed changes.
git stash apply  # Apply the stashed changes and leave them in the stash.

Git Log

git log
git log -p -2 (--patch)
git log --stat
git log --pretty=oneline
git log --pretty=format:"%h - %an, %ar : %s"

Git Reset

git reset --hard HEAD        # Reset all files to the HEAD of the branch:
git checkout HEAD -- FILE    # reset a single file
git reset --soft HEAD~1      # Revert committed changes.

Git Ignore Patterns

The rules for the patterns you can put in the .gitignore file are as follows:


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 https://pypi.org/project/vtk/. 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 https://www.learnpyqt.com/blog/pyqt5-vs-pyside2/ 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 MainWindow.py, 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: https://vtk.org/Wiki/VTK/Examples/Python/Widgets/EmbedPyQt
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 MainWindow.py 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 https://github.com/saurabhg17/SLogLib.

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.