Tuesday, March 12, 2013

Setting up FTP server vsftpd in Fedora 16

The file transfer protocol (FTP) can be used to transfer files across different systems. In this article, I set up a secure ftp server vsftpd in my fedora machine and access the files on this server from my friend Niroj Karki’s client computer. To set up the file server on fedora you need to first install the ftp server using following command

$yum –y install vsftpd.x86_64

Open the file /etc/vsftpd/vsftpd.conf and uncomment/add the following line

Setting up SSH server in Fedora 16 : Theory and Configuration Details

rlogin and ssh are used to login to remote server. They are very useful tool to login to the remote machine and access the resources available. Rlogin and ssh both are used for this purpose. The only difference between them is in security aspect. In rlogin, all information, including passwords, is transmitted unencrypted (making it vulnerable to interception). So now-a-days ssh (secured shell) is used most often.

The original Berkeley package which provides rlogin also features rcp (remote-copy, allowing files to be copied over the network) and rsh (remote-shell, allowing commands to be run on a remote machine without the user logging into it). These share the hosts.equiv and .rhosts access-control scheme (although they connect to a different daemon, rshd), and as such suffer from the same security problems. The ssh suite contains suitable replacements for both: scp replaces rcp, and ssh itself replaces both rlogin and rsh.

Steps needed to configure SSH server in Fedora 16

Monday, March 4, 2013

Sobel and Prewitt edge detector in C++: Image Processing

Sobel and Prewitt are used extensively for detecting edges in image processing.

Sobel Operator

The operator calculates the gradient of the image intensity at each point, giving the direction of the largest possible increase from light to dark and the rate of change in that direction. The result therefore shows how "abruptly" or "smoothly" the image changes at that point, and therefore how likely it is that part of the image represents an edge, as well as how that edge is likely to be oriented. The Sobel kernels are given by

Here the kernel hx is sensitive to changes in the x direction, i.e., edges that

Saturday, March 2, 2013

Gaussian blurring using separable kernel in C++

Gaussian blurring is used to reduce the noise and details of the image. It reduces the image's high frequency components and thus it is type of low pass filter. Gaussian blurring is obtained by convolving the image with Gaussian function. For more information about Gaussian function see the Wikipedia page.  Since 2D Gaussian function can be obtained by multiplying two 1D Gaussian functions, the blurring can be obtained by using separable kernel.

Use of Separable Kernel