I have Quickbird/Ikonos/GeoEye image and want to extract all the buildings from the image as an independent layer in QGIS.
How can i achieve that?
There are only two ways to extract information like this. One thing you have to remember is that a "building" in a satellite image is an interpretation by a human. The buildings are a collection of pixels. If you are looking to do this in a batch/automated way, you will want to classify the image. QGIS has plugins to do this, such as Semi-Automatic Classification Plugin. The remaining option are manual. You can heads-up digitize the buildings using a polygon.
The Mahalanobis distance classifier can be good at this sort of thing. Check out the IPLER site from the Rochester Institute of Technology. They did some interesting work identifying blue tarps in satellite imagery as a means to detect IDP (internally displaced persons) camps after the 2010 earthquake.
At the bottom of the IPLER page there is a description of the project.