December 17, 2014

Don't update VirtualBox to v4.3.18 or v4.3.20

If you have Windows 7 64bit, don't update VirtualBox to v4.3.18 or v4.3.20.  Most of your VMs won't start and instead you'll get an error message about can't load some DLL.  It's a known problem and no solution but going back to the prior version.  v4.3.12 Works fine.

I had to restore the system to make sure it's clean state and recover the VB.

December 10, 2014

Error: cannot retrieve metalink for repository epel. please verify its path and try again

I got that error message when I tried to use YUM on CentOS 6.

Two possible issues and solutions:

1. system time is incorrect.

2. ca-cert needs to be updated.


For #1, install NTP and set date/time:

  1. yum install ntp npdate
  2. chkconfig ntpd on
  3. ntpdate
  4. service ntpd start

For #2, update CA-cert: yum update -y ca-certificates

February 8, 2014

Installing SimpleCV on Windows 7 64bit

SimpleCV ( is the easiest way to use OpenCV (  There is a python module for OpenCV, but SimpleCV really simplifies the use of it.
On SimpleCV's site, there is "SimpleCV Superpack" that includes everything to use SimpleCV, but I wanted to use my existing Python 64-bit installation and don't want to manage another copy of Python install.

When I tried to install SimpleCV after checking out from its Git repository, of course it didn't work -- I have to install many dependent python modules.  This is a bit too much -- I do not want to spend my time installing all the dependent modules, which requires sometimes C/C++ compilation.  Here is a simple tip/trick to avoid the all the hassle:
  1. Make sure you already installed OpenCV and configured properly.
  2. Install OpenCV module for Windows 64bit from this site, -- it shows what modules are dependent and all the dependent pre-compiled modules are available on the same site.  And install PILLOWS module from the site.
  3. Make sure your Python for OpenCV works.
  4. Check out SimpleCV from its Git repository (
  5. Modify "" -- remove "PIL" requirement from "requires=" line in that file.  Make sure you have installed "Pillows" module from step #2.  Pillows module replaces PIL module.
  6. Run setup.
This does the trick.  Now my one Python installation has all the modules I want to use together in one place.

Picture thumbnails but not for videos

I'm using Windows 7 64bit and sometimes opening folder becomes very slow and copying/moving video files hang in the middle of the copying/moving process.  I noticed that if I check off "Always show icons, never thumbnails" option in Folder Options this problem doesn't occur.  I searched on this issue and read many posts -- indeed one of the solutions is to disable showing thumbnails, and another solution is in the properties of the folder containing videos, click on the tab "customize" -> then change "optimize this folder" to "general."
Tried both but I found that changing "customize" in properties is a real hassle to change all the folders that contains video files, and not seeing thumbnails is inconvenient as there are lots of pictures without meaningful file names that my camera created.
I save video files from Courera online courses for later viewing, and the file names has meaningful descriptions so I don't need thumbnails for videos, but for pictures, I want to have thumbnails view.  Unfortunately, the file explorer's option is either turn on or off thumbnails view for both pictures and videos.
One solution I found and settled on is:
  1. Turn off thumbnails view in folder's view options.
  2. Use 3rd party software, SageThumbs (
This combination gives me exactly what I want -- no thumbnails for videos, but thumbnails for pictures.
I hope this post is helpful for anyone experiencing  the same issue and having the same needs.