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Using C++ or Python for OpenCV Programs

by Jack Simpson
written by Jack Simpson

When I first started learning OpenCV, I was working exclusively with Python. While I am still a huge fan of the language, today all of my OpenCV programs are written in C++. Why?

  1. Some of the deeper functionality of OpenCV has not been completely ported to Python (although hopefully the release of OpenCV 3.0 will fix most of these issues).
  2. Most in-depth textbooks on image processing and computer vision that cover OpenCV use C++ as their primary language. It was therefore easier to learn from these resources by adopting the language.
  3. A lot of the computer vision techniques I use (SIFT, machine learning, etc), are better documented in C++.
  4. Passing images back and forth between NumPy arrays has overhead that C++ doesn’t have to worry about.
  5. As has been suggested to me by Carl Bell, Python struggles to perform well with overloaded functions.
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OpenCVProgrammingTips/Bugs

Sublime Text OpenCV C++ Build System

by Jack Simpson
written by Jack Simpson

If you want to compile and run a C++ program using OpenCV in Sublime Text, then copy and paste the code below into a build system file. If you’re not interested in explicitly using C++11 you can delete the “-std=c++0x” section.


{
"shell_cmd": "g++ -std=c++0x ${file} -o ${file_base_name} `pkg-config --cflags --libs opencv` && ./${file_base_name}"
}
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ProgrammingTips/Bugs

Parsing CSV Files in Python

by Jack Simpson
written by Jack Simpson

Almost anyone doing research at some point will have to input or extract data from an Excel spreadsheet. However, it is possible to export an Excel file as a .csv file (comma separated file) and then use Python to grab all the data instantly. The Python csv module makes all this incredibly simple:

import csv
column1 = []
column2 = []
column3 = []
openfile = csv.reader(open('data.csv'))
for row in openfile:
    first, second, third = row
    column1.append(first)
    column2.append(second)
    column3.append(third)

This script will open your .csv file, and then use the ‘.next()’ method to create an array of all the lines in your file, which you can parse through with the for loop.

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ProgrammingTips/Bugs

Running an R script through Python

by Jack Simpson
written by Jack Simpson

Before I made the switch to developing on a Linux machine, I noticed that the Python module for calling R (RPy2) seemed to be having some problems on Windows. This gave me an excuse to play around with writing my own Python script to create and run an R script. As you’ll see in the code below, I’ve used the subprocess module to execute the R script I created and then pipe the results back into the terminal.

import subprocess

# rscript is the content of the R file we'll create
rscript = '''#!/usr/bin/Rscript

cat('This is a simple program') # same as c()
NumOfIterations for (i in 1:NumOfIterations) { # 1:10 is range
cat(i, 'Hello world!')
cat(' ')
}
'''
r_file = open("example.r", "w")
r_file.write(rscript)
r_file.close()

r_file_name = "example.r"
ppath = r'C:Program FilesRR-2.15.2bini386Rscript.exe'
proc = subprocess.Popen("%s %s" % (ppath, r_file_name), stdout=subprocess.PIPE)
output = proc.stdout.read()
print output
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