# How to create a custom Caffe layer in Python?
This tutorial will guide through the steps to create a simple custom layer for Caffe using python. By the end of it, there are some examples of custom layers.

###- Why would I want to do that?
Usually you would create a custom layer to implement a funcionality that isn't available in Caffe, tuning it for your requirements.

###- What will I need?
Python and Caffe instaled and a sample layer template (provided bellow) so you can customize it.

##Layer Template
 
```python
import caffe

class My_Custom_Layer(caffe.Layer):
    def setup(self, bottom, top):
        pass
        
    def forward(self, bottom, top):
        pass
        
    def reshape(self, bottom, top):
        pass

    def backward(self, bottom, top):
        pass
```

So important things to remember:
- Your custom layer has to inherit from **caffe.Layer** (so don't forget to *import caffe*);
- You must define the four following methods: **setup**, **forward**, **reshape** and **backward**;
- All methods have a *top* and a *bottom* parameters, which are the blobs that store the input and the output passed to your layer. You can access it using *top[i].data* or *bottom[i].data*, where *i* is the index of the blob in case you have more than one upper or lower blob.


###- Setup method
The Setup method is called once during the lifetime of the execution, when Caffe is instantiating all layers. This is where you will read parameters, instantiate fixed-size buffers. 

###- Reshape method
Use the reshape method for initialization/setup that depends on the bottom blob (layer input) size. It is called once for each batch.

###- Forward method
The Forward method is called for each input batch and is where most of your logic will be.

###- Backward method
The Backward method is called during the backward pass of the network. For example, in a convolution-like layer, this would be where you would calculate the gradients. This is optional (a layer can be forward-only). 

##Prototxt Template
Ok, so now you have your layer designed! This is how you define it in your *.prototxt* file:

```
layer {
  name: "LayerName"
  type: "Python"
  top: "TopBlobName"
  bottom: "BottomBlobName"
  python_param {
    module: "My_Custom_Layer_File"
    layer: "My_Custom_Layer_Class"
    param_str: '{"param1": 1,"param2":True, "param3":"some string"}'
  }
  include{
        phase: TRAIN
  }
}
```

Important remarks:
- **type** must be **Python**;
- You must have a **python_param** dictionary with at least the **module** and **layer** parameters;
- **module** refers to the file where you implemented your layer;
- **layer** refers to the name of your class;
- You can pass parameters to the layer using **param_str** (more on accessing them bellow);
- Just like any other layer, you can define in which phase you want it to be active (see the examples to see how you can check the current phase);

##Passing Parameters to the layer
You can define the layer parameters in the prototxt by using *param_str*. Once you've done it, here is an example on how you access these paremeters inside the layer class:

```python
def setup(self, bottom, top):
    params = eval(self.param_str)
    param1 = params["param1"]
    param2 = params.get('param2', False) #I usually use this when fetching a bool
    param3 = params["param3"]
    
    #Continue with the setup
    # ...
```