实验要求可以参考deeplearning的tutorial,Exercise:Sparse Autoencoder。稀疏自动编码的原理可以参照之前的博文,神经网络, 稀疏自动编码 。
实验是实现三层的稀疏自动编码神经网络,神经网络结构包括输入层64个neuron,隐含层25个neuron(都不包括bias结点),输出层和输入层相同的neuron的个数。
实验中的原始数据是10副512×512大小的灰度图像,存储在“IMAGES.mat”文件中。下图是其中的一张图像。
我们随机中10副图像中挑选一个,然后再从该图像中随机的选一个8*8的图像patch,得到的这个patch作为一个训练数据,重复这个随机过程10000,最后得到10000数据组成的训练集,存储在64×10000 的矩阵,每一列代表一个数据。
下图是总10000个数据中随机挑选出100个进行显示。
稀疏自动编码的损失函数由三部分组成,公式如下:
偏导数的计算是通过BP算法,需要注意是,由于在实验中使用的是batch的优化方法,所以偏导数计算的时候对所有样本的偏导数做了一个求和操作。
有网友说他们在自己的电脑上跑Gradient checking这步的时候用了1个多小时,但不知道是不是我的电脑配置比较高,我才跑了不到4分钟。
因为我们已经有了损失函数和相应参数的偏导数,所以和直接用成熟的L-BFGS算法的包,最后算法到底最大迭代次数400的时候,停止迭代。
下图显示的是第一层到第二层的参数,由于输入层为64个neuron,隐含层为25个neuron,所以下面共有25个patch,每一个patch大小为8*8.
源代码下载
function patches = sampleIMAGES() % sampleIMAGES % Returns 10000 patches for training load IMAGES; % load images from disk patchsize = 8; % we'll use 8x8 patches numpatches = 10000; % Initialize patches with zeros. Your code will fill in this matrix--one % column per patch, 10000 columns. patches = zeros(patchsize*patchsize, numpatches); %% ---------- YOUR CODE HERE -------------------------------------- % Instructions: Fill in the variable called "patches" using data % from IMAGES. % % IMAGES is a 3D array containing 10 images % For instance, IMAGES(:,:,6) is a 512x512 array containing the 6th image, % and you can type "imagesc(IMAGES(:,:,6)), colormap gray;" to visualize % it. (The contrast on these images look a bit off because they have % been preprocessed using using "whitening." See the lecture notes for % more details.) As a second example, IMAGES(21:30,21:30,1) is an image % patch corresponding to the pixels in the block (21,21) to (30,30) of % Image 1 for i = 1:numpatches x = ceil(rand(1)*505); y = ceil(rand(1)*505); z = ceil(rand(1)*10); patch = IMAGES(x : x+patchsize-1, y : y+patchsize-1, z); patches(:,i) = patch(:); end %% --------------------------------------------------------------- % For the autoencoder to work well we need to normalize the data % Specifically, since the output of the network is bounded between [0,1] % (due to the sigmoid activation function), we have to make sure % the range of pixel values is also bounded between [0,1] patches = normalizeData(patches); end %% --------------------------------------------------------------- function patches = normalizeData(patches) % Squash data to [0.1, 0.9] since we use sigmoid as the activation % function in the output layer % Remove DC (mean of images). patches = bsxfun(@minus, patches, mean(patches)); % Truncate to +/-3 standard deviations and scale to -1 to 1 pstd = 3 * std(patches(:)); patches = max(min(patches, pstd), -pstd) / pstd; % Rescale from [-1,1] to [0.1,0.9] patches = (patches + 1) * 0.4 + 0.1; end
function [cost,grad] = sparseAutoencoderCost(theta, visibleSize, hiddenSize, ... lambda, sparsityParam, beta, data) % visibleSize: the number of input units (probably 64) % hiddenSize: the number of hidden units (probably 25) % lambda: weight decay parameter % sparsityParam: The desired average activation for the hidden units (denoted in the lecture % notes by the greek alphabet rho, which looks like a lower-case "p"). % beta: weight of sparsity penalty term % data: Our 64x10000 matrix containing the training data. So, data(:,i) is the i-th training example. % The input theta is a vector (because minFunc expects the parameters to be a vector). % We first convert theta to the (W1, W2, b1, b2) matrix/vector format, so that this % follows the notation convention of the lecture notes. W1 = reshape(theta(1:hiddenSize*visibleSize), hiddenSize, visibleSize); W2 = reshape(theta(hiddenSize*visibleSize+1:2*hiddenSize*visibleSize), visibleSize, hiddenSize); b1 = theta(2*hiddenSize*visibleSize+1:2*hiddenSize*visibleSize+hiddenSize); b2 = theta(2*hiddenSize*visibleSize+hiddenSize+1:end); % Cost and gradient variables (your code needs to compute these values). % Here, we initialize them to zeros. cost = 0; W1grad = zeros(size(W1)); W2grad = zeros(size(W2)); b1grad = zeros(size(b1)); b2grad = zeros(size(b2)); %% ---------- YOUR CODE HERE -------------------------------------- % Instructions: Compute the cost/optimization objective J_sparse(W,b) for the Sparse Autoencoder, % and the corresponding gradients W1grad, W2grad, b1grad, b2grad. % % W1grad, W2grad, b1grad and b2grad should be computed using backpropagation. % Note that W1grad has the same dimensions as W1, b1grad has the same dimensions % as b1, etc. Your code should set W1grad to be the partial derivative of J_sparse(W,b) with % respect to W1. I.e., W1grad(i,j) should be the partial derivative of J_sparse(W,b) % with respect to the input parameter W1(i,j). Thus, W1grad should be equal to the term % [(1/m) \Delta W^{(1)} + \lambda W^{(1)}] in the last block of pseudo-code in Section 2.2 % of the lecture notes (and similarly for W2grad, b1grad, b2grad). % % Stated differently, if we were using batch gradient descent to optimize the parameters, % the gradient descent update to W1 would be W1 := W1 - alpha * W1grad, and similarly for W2, b1, b2. % % data = [ones(1,size(data,2)); data]; [n m] = size(data); z2 = W1* data + repmat(b1,1,m); a2 = sigmoid(z2); z3 = W2 * a2 + repmat(b2,1,m); a3 = sigmoid(z3); phat = mean(a2,2); p = repmat(sparsityParam, size(phat)); sparse = p .* log(p ./ phat) + (1-p) .* log((1-p) ./ (1-phat)); %J = trace((data - a3)' * (data - a3)) / (size(data,2)*2); J = sum(sum((a3-data).^2)) / (m*2); regu = (W1(:)'*W1(:) + W2(:)'*W2(:))/2; cost = J + lambda*regu + beta * sum(sparse); delta3 = -1* (data-a3).*a3.*(1-a3); delta2 = (W2'*delta3+beta*repmat(-p./phat+(1-p)./(1-phat),1,size(data,2))).*a2.*(1-a2); W2grad = delta3*a2'/m; b2grad = mean(delta3,2); W1grad = delta2*data'/m; b1grad = mean(delta2,2); W2grad = W2grad + lambda*W2; W1grad = W1grad + lambda*W1; %------------------------------------------------------------------- % After computing the cost and gradient, we will convert the gradients back % to a vector format (suitable for minFunc). Specifically, we will unroll % your gradient matrices into a vector. grad = [W1grad(:) ; W2grad(:) ; b1grad(:) ; b2grad(:)]; end %------------------------------------------------------------------- % Here's an implementation of the sigmoid function, which you may find useful % in your computation of the costs and the gradients. This inputs a (row or % column) vector (say (z1, z2, z3)) and returns (f(z1), f(z2), f(z3)). function sigm = sigmoid(x) sigm = 1 ./ (1 + exp(-x)); end