def custom_activation(x, alpha=0.01, beta=0.01):
    if (x < 0):
        return alpha * x
    elif (x <= 1):
        return x
    else:
        return 1 + beta * (x - 1) 

model = Sequential()
model.add(Flatten(input_shape=(width, heigth, 3)))
model.add(Dense(500, activation = custom_activation))

from keras import backend as K