At present, I am working on core graphics based iPhone application. 
I have tried to crop center of the image but i can't do that.  

Here,I have an images named **(fig a)** as shown below, and i need to crop it on center of this image. 
So, the resultant image should look like **(fig b)**. 

How can i do this?. 

fig a :

![enter image description here](https://i.stack.imgur.com/EksRz.png)

fig b :

![enter image description here](https://i.stack.imgur.com/Vr9Cp.png)