Segnet is deep fully convolutional neural network architecture for semantic pixel-wise segmentation. This is implementation of http://arxiv.org/pdf/1511.00561v2.pdf (Except for the Upsampling layer where paper uses indices based upsampling which is not implemented in keras yet( I am working on it), but that shouldnt make a lot of difference). You can directly download the code from https://github.com/preddy5/segnet. This post is a explaination of what is happening in the code.

http://preddy5.github.io/2016/03/08/segnet-post.html