Detect if the bottom ofdiv is in the viewport and change css position of another div

JavaScript
1

this is my code http://jsfiddle.net/Salmen/uHKGw/

my idea is, when i scroll at the 50% of the image i fix the block text , and when i reach the bottom of each image, i change its css position to scroll up .

it is possibile to make it ?

thanks