本文结合Deep learning的一个应用，Convolution Neural Network 进行一些基本应用，参考Lecun的Document 0.1进行部分拓展，与结果展示（in python）。
分为以下几部分：
1. Convolution（卷积）
2. Pooling（降采样过程）
3. CNN结构
4. 跑实验
下面分别介绍。
PS：本篇blog为ese机器学习短期班参考资料（20140516课程），本文只是简要讲最naive最simple的思想，重在实践部分，原理课上详述。
1. Convolution（卷积）
类似于高斯卷积，对imagebatch中的所有image进行卷积。对于一张图，其所有feature map用一个filter卷成一张feature map。 如下面的代码，对一个imagebatch（含两张图）进行操作，每个图初始有3张feature map(R,G,B), 用两个9*9的filter进行卷积，，结果是，每张图得到两个feature map。
卷积操作由theano的conv.conv2d实现，这里我们用随机参数W，b。结果有点像edge detector是不是？
Code: （详见注释）
# -*- coding: utf-8 -*-"""Created on Sat May 10 18:55:26 2014@author: rachelFunction: convolution option of two pictures with same size (width,height)input: 3 feature maps (3 channels <RGB> of a picture)convolution: two 9*9 convolutional filters"""from theano.tensor.nnet import convimport theano.tensor as Timport numpy, theanorng = numpy.random.RandomState(23455)# symbol variableinput = T.tensor4(name = ‘input’)# initial weightsw_shape = (2,3,9,9) #2 convolutional filters, 3 channels, filter shape: 9*9w_bound = numpy.sqrt(3*9*9)W = theano.shared(numpy.asarray(rng.uniform(low = -1.0/w_bound, high = 1.0/w_bound,size = w_shape),dtype = input.dtype),name = ‘W’)b_shape = (2,)b = theano.shared(numpy.asarray(rng.uniform(low = -.5, high = .5, size = b_shape),dtype = input.dtype),name = ‘b’)conv_out = conv.conv2d(input,W)#T.TensorVariable.dimshuffle() can reshape or broadcast (add dimension)#dimshuffle(self,*pattern)# >>>b1 = b.dimshuffle(‘x’,0,’x’,’x’)# >>>b1.shape.eval()# array([1,2,1,1])output = T.nnet.sigmoid(conv_out + b.dimshuffle(‘x’,0,’x’,’x’))f = theano.function([input],output)# demoimport pylabfrom PIL import Image#minibatch_img = T.tensor4(name = ‘minibatch_img’)#————-img1—————img1 = Image.open(open(‘//home//rachel//Documents//ZJU_Projects//DL//Dataset//rachel.jpg’))width1,height1 = img1.sizeimg1 = numpy.asarray(img1, dtype = ‘float32’)/256. # (height, width, 3)# put image in 4D tensor of shape (1,3,height,width)img1_rgb = img1.swapaxes(0,2).swapaxes(1,2).reshape(1,3,height1,width1) #(3,height,width)#————-img2—————img2 = Image.open(open(‘//home//rachel//Documents//ZJU_Projects//DL//Dataset//rachel1.jpg’))width2,height2 = img2.sizeimg2 = numpy.asarray(img2,dtype = ‘float32’)/256.img2_rgb = img2.swapaxes(0,2).swapaxes(1,2).reshape(1,3,height2,width2) #(3,height,width)#minibatch_img = T.join(0,img1_rgb,img2_rgb)minibatch_img = numpy.concatenate((img1_rgb,img2_rgb),axis = 0)filtered_img = f(minibatch_img)# plot original image and two convoluted resultspylab.subplot(2,3,1);pylab.axis(‘off’);pylab.imshow(img1)pylab.subplot(2,3,4);pylab.axis(‘off’);pylab.imshow(img2)pylab.gray()pylab.subplot(2,3,2); pylab.axis("off")pylab.imshow(filtered_img[0,0,:,:]) #0:minibatch_index; 0:1-st filterpylab.subplot(2,3,3); pylab.axis("off")pylab.imshow(filtered_img[0,1,:,:]) #0:minibatch_index; 1:1-st filterpylab.subplot(2,3,5); pylab.axis("off")pylab.imshow(filtered_img[1,0,:,:]) #0:minibatch_index; 0:1-st filterpylab.subplot(2,3,6); pylab.axis("off")pylab.imshow(filtered_img[1,1,:,:]) #0:minibatch_index; 1:1-st filterpylab.show()
2. Pooling（降采样过程）
最常用的Maxpooling. 解决了两个问题：
1. 减少计算量
2. 旋转不变性 （原因自己悟）
PS：对于旋转不变性，回忆下SIFT，LBP：采用主方向；HOG：选择不同方向的模版
Maxpooling的降采样过程会将feature map的长宽各减半。（下面结果图中没有体现出来，python自动给拉到一样大了，但实际上像素数是减半的）
Code: （详见注释）
# -*- coding: utf-8 -*-"""Created on Sat May 10 18:55:26 2014@author: rachelFunction: convolution option input: 3 feature maps (3 channels <RGB> of a picture)convolution: two 9*9 convolutional filters"""from theano.tensor.nnet import convimport theano.tensor as Timport numpy, theanorng = numpy.random.RandomState(23455)# symbol variableinput = T.tensor4(name = ‘input’)# initial weightsw_shape = (2,3,9,9) #2 convolutional filters, 3 channels, filter shape: 9*9w_bound = numpy.sqrt(3*9*9)W = theano.shared(numpy.asarray(rng.uniform(low = -1.0/w_bound, high = 1.0/w_bound,size = w_shape),dtype = input.dtype),name = ‘W’)b_shape = (2,)b = theano.shared(numpy.asarray(rng.uniform(low = -.5, high = .5, size = b_shape),dtype = input.dtype),name = ‘b’)conv_out = conv.conv2d(input,W)#T.TensorVariable.dimshuffle() can reshape or broadcast (add dimension)#dimshuffle(self,*pattern)# >>>b1 = b.dimshuffle(‘x’,0,’x’,’x’)# >>>b1.shape.eval()# array([1,2,1,1])output = T.nnet.sigmoid(conv_out + b.dimshuffle(‘x’,0,’x’,’x’))f = theano.function([input],output)# demoimport pylabfrom PIL import Imagefrom matplotlib.pyplot import *#open random imageimg = Image.open(open(‘//home//rachel//Documents//ZJU_Projects//DL//Dataset//rachel.jpg’))width,height = img.sizeimg = numpy.asarray(img, dtype = ‘float32’)/256. # (height, width, 3)# put image in 4D tensor of shape (1,3,height,width)img_rgb = img.swapaxes(0,2).swapaxes(1,2) #(3,height,width)minibatch_img = img_rgb.reshape(1,3,height,width)filtered_img = f(minibatch_img)# plot original image and two convoluted resultspylab.figure(1)pylab.subplot(1,3,1);pylab.axis(‘off’);pylab.imshow(img)title(‘origin image’)pylab.gray()pylab.subplot(2,3,2); pylab.axis("off")pylab.imshow(filtered_img[0,0,:,:]) #0:minibatch_index; 0:1-st filtertitle(‘convolution 1’)pylab.subplot(2,3,3); pylab.axis("off")pylab.imshow(filtered_img[0,1,:,:]) #0:minibatch_index; 1:1-st filtertitle(‘convolution 2’)#pylab.show()# maxpoolingfrom theano.tensor.signal import downsampleinput = T.tensor4(‘input’)maxpool_shape = (2,2)pooled_img = downsample.max_pool_2d(input,maxpool_shape,ignore_border = False)maxpool = theano.function(inputs = [input],outputs = [pooled_img])pooled_res = numpy.squeeze(maxpool(filtered_img))#pylab.figure(2)pylab.subplot(235);pylab.axis(‘off’);pylab.imshow(pooled_res[0,:,:])title(‘down sampled 1’)pylab.subplot(236);pylab.axis(‘off’);pylab.imshow(pooled_res[1,:,:])title(‘down sampled 2’)pylab.show()
3. CNN结构
想必大家随便google下CNN的图都滥大街了，这里拖出来那时候学CNN的时候一张图，自认为陪上讲解的话画得还易懂（<!–囧–>）
废话不多说了，直接上Lenet结构图：（从下往上顺着箭头看，最下面为底层original input）
将来靠自己双掌;愿你用双掌开拓出美好的梦想。
