CS231n Assignment1总结

lecture3一些关于链式法则的基本知识。
下面是对assignment1的代码一些关键点或者有意思实现的总结
参考答案：https://github.com/sharedeeply/cs231n-assignment-solution/blob/master/assignment1/

数据集读取

对于assigment1中使用的数据集，从其中读出来照片是一张被拉成一维向量的图片，所以输入X_train，X_test的行数为输入数据的个数，列数为图片的像素量。y是一个 $(n,)$ 的矩阵，行数为数据量，每个值为 $class$ 的值。

KNN

部分训练数据显示函数

# Visualize some examples from the dataset.
# We show a few examples of training images from each class.
classes = ['plane', 'car', 'bird', 'cat', 'deer', 'dog', 'frog', 'horse', 'ship', 'truck']
num_classes = len(classes)
samples_per_class = 7
for y, cls in enumerate(classes):
    idxs = np.flatnonzero(y_train == y)# 返回不为0的值所在的坐标
    idxs = np.random.choice(idxs, samples_per_class, replace=False)# 不重复取samples_per_class个数据，按概率取值也是该函数
    for i, idx in enumerate(idxs):
        plt_idx = i * num_classes + y + 1 # 图片的计数从1开始
        plt.subplot(samples_per_class, num_classes, plt_idx)
        plt.imshow(X_train[idx].astype('uint8'))
        plt.axis('off')
        if i == 0:
            plt.title(cls)
plt.show()

关于 $knn$ 对象的调用，在文件中是通过

from cs231n.classifiers import KNearestNeighbor

# Create a kNN classifier instance. 
# Remember that training a kNN classifier is a noop: 
# the Classifier simply remembers the data and does no further processing 
classifier = KNearestNeighbor()
classifier.train(X_train, y_train)

语句实现的，这里需要介绍__iniy__.py文件的作用：

1. Python中package的标识，不能删除

2. 定义__all__用来模糊导入

3. 编写Python代码(不建议在__init__中写python模块，可以在包中在创建另外的模块来写，尽量保证__init__.py简单）

参考：https://www.cnblogs.com/AlwinXu/p/5598543.html
通过classifier文件夹中的__init__.py文件使得上述调用可以实现
$knn$ 对象的实现较为简单，不展开介绍，这里主要介绍一些没见过的语句

# 将train_sq复制为(num_train, num_test)大小的矩阵
train_sq = np.broadcast_to(train_sq, shape=(num_train, num_test))# train_sq需要为一个二维矩阵，不能为(n,)形式的矩阵

交叉验证的实现：

for k in k_choices:
    # 进行交叉验证
    acc = []
    for i in range(num_folds):
        x = X_train_folds[0:i] + X_train_folds[i+1:]
        x = np.concatenate(x, axis=0)  # 使用concatenate将4个训练集拼在一起
        y = y_train_folds[0:i] + y_train_folds[i+1:]
        y = np.concatenate(y)  # 对label使用同样的操作
        test_x = X_train_folds[i]
        test_y = y_train_folds[i]
        
        classifier = KNearestNeighbor()  # 定义model
        classifier.train(x, y)  # 将训练集读入
        dist = classifier.compute_distances_no_loops(test_x)  # 计算距离矩阵
        y_pred = classifier.predict_labels(dist, k)  # 预测结果
        accuracy = np.mean(y_pred == test_y)  # 计算准确率
        acc.append(accuracy)
    k_to_accuracies[k] = acc  # 计算交叉验证的平均准确率

SVM

np.hstack()# 在水平方向平铺
np.vstack()# 在竖直方向堆叠

关于SVM参数更新（本来以为很简单，但在实现过程中还是发现有很多地方不清楚，实现算法果然是最好的学习方法）

def svm_loss_vectorized(W, X, y, reg):
   """
   Structured SVM loss function, vectorized implementation.
   Inputs and outputs are the same as svm_loss_naive.
   """
   loss = 0.0
   dW = np.zeros(W.shape)  # initialize the gradient as zero

   #############################################################################
   # TODO:
   # Implement a vectorized version of the structured SVM loss, storing the    #
   # result in loss.                                                           #
   #############################################################################
   num_train = X.shape[0]  # 得到样本的数目
   scores = np.dot(X, W)  # 计算所有的得分
   y_score = scores[np.arange(num_train), y].reshape((-1, 1))  # 这句话是将每一行数据所属类别（即y的值）处的值取出
   mask = (scores - y_score + 1) > 0  # 有效的score下标
   scores = (scores - y_score + 1) * mask  # 有效的得分
   loss = (np.sum(scores) - num_train * 1) / num_train  # 去掉每个样本多加的对应label得分，然后平均（算有效score的时候应该不算y所属类别的预测值）
   loss += reg * np.sum(W * W)
   #############################################################################
   #                             END OF YOUR CODE                              #
   #############################################################################

   #############################################################################
   # TODO:
   # Implement a vectorized version of the gradient for the structured SVM     #
   # loss, storing the result in dW.                                           #
   #                                                                           #
   # Hint: Instead of computing the gradient from scratch, it may be easier    #
   # to reuse some of the intermediate values that you used to compute the     #
   # loss.                                                                     #
   #############################################################################
   # dw = x.T * dl/ds
   ds = np.ones_like(scores)  # 初始化ds
   ds *= mask  # 有效的score梯度为1，无效的为0（需要更细的值只有max取值不为0位置处的值）
   ds[np.arange(num_train), y] = -1 * (np.sum(mask, axis=1) - 1)  # 每个样本对应label的梯度计算了(有效的score次)，取负号（在求导时，数据原来类别处的导数为每行mask的和减1，减1是因为类别处的score也算一个有效值），其余有效score处的导数为1
   dW = np.dot(X.T, ds) / num_train   # 平均
   dW += 2 * reg * W  # 加上正则项的梯度
   #############################################################################
   #                             END OF YOUR CODE                              #
   #############################################################################

   return loss, dW

Softmax

def softmax_loss_vectorized(W, X, y, reg):
   """
   Softmax loss function, vectorized version.
   Inputs and outputs are the same as softmax_loss_naive.
   """
   # Initialize the loss and gradient to zero.
   loss = 0.0
   dW = np.zeros_like(W)

   #############################################################################
   # TODO: Compute the softmax loss and its gradient using no explicit loops.  #
   # Store the loss in loss and the gradient in dW. If you are not careful     #
   # here, it is easy to run into numeric instability. Don't forget the        #
   # regularization!                                                           #
   #############################################################################
   scores = np.dot(X, W)  # 计算得分
   scores -= np.max(scores, axis=1, keepdims=True)  # 数值稳定性，这里实际上是减去一个常数对求导没有影响
   scores = np.exp(scores)  # 取指数
   scores /= np.sum(scores, axis=1, keepdims=True)  # 计算softmax
   ds = np.copy(scores)  # 初始化loss对scores的梯度
   ds[np.arange(X.shape[0]), y] -= 1  # 求出scores的梯度（对softMax求导之后得到对数据相应类别处的值减1，参考：https://www.jianshu.com/p/c02a1fbffad6）
   dW = np.dot(X.T, ds)  # 求出w的梯度
   loss = scores[np.arange(X.shape[0]), y] # 计算loss
   loss = -np.log(loss).sum()  #求交叉熵
   loss /= X.shape[0]
   dW /= X.shape[0]
   loss += reg * np.sum(W * W)
   dW += 2 * reg * W
   #############################################################################
   #                          END OF YOUR CODE                                 #
   #############################################################################

   return loss, dW

Two-layer network

这个的求导实际上是将softmax与SVM合并，即softmax求导和Max函数的求导

def loss(self, X, y=None, reg=0.0):
       """
       Compute the loss and gradients for a two layer fully connected neural
       network.
       Inputs:
       - X: Input data of shape (N, D). Each X[i] is a training sample.
       - y: Vector of training labels. y[i] is the label for X[i], and each y[i] is
         an integer in the range 0 <= y[i] < C. This parameter is optional; if it
         is not passed then we only return scores, and if it is passed then we
         instead return the loss and gradients.
       - reg: Regularization strength.
       Returns:
       If y is None, return a matrix scores of shape (N, C) where scores[i, c] is
       the score for class c on input X[i].
       If y is not None, instead return a tuple of:
       - loss: Loss (data loss and regularization loss) for this batch of training
         samples.
       - grads: Dictionary mapping parameter names to gradients of those parameters
         with respect to the loss function; has the same keys as self.params.
       """
       # Unpack variables from the params dictionary
       W1, b1 = self.params['W1'], self.params['b1']
       W2, b2 = self.params['W2'], self.params['b2']
       N, D = X.shape

       # Compute the forward pass
       scores = None
       #############################################################################
       # TODO: Perform the forward pass, computing the class scores for the input. #
       # Store the result in the scores variable, which should be an array of      #
       # shape (N, C).                                                             #
       #############################################################################
       s1 = np.dot(X, W1) + b1  # (N, H)
       s1_act = (s1 > 0) * s1
       scores = np.dot(s1_act, W2) + b2  # (N, C)
       #############################################################################
       #                              END OF YOUR CODE                             #
       #############################################################################

       # If the targets are not given then jump out, we're done
       if y is None:
           return scores

       # Compute the loss
       loss = None
       #############################################################################
       # TODO: Finish the forward pass, and compute the loss. This should include  #
       # both the data loss and L2 regularization for W1 and W2. Store the result  #
       # in the variable loss, which should be a scalar. Use the Softmax           #
       # classifier loss.                                                          #
       #############################################################################
       scores -= np.max(scores, axis=1, keepdims=True)  # 数值稳定性
       scores = np.exp(scores)
       scores /= np.sum(scores, axis=1, keepdims=True)  # softmax
       loss = -np.log(scores[np.arange(N), y]).sum()
       loss /= X.shape[0]
       loss += reg * np.sum(W1**2)
       loss += reg * np.sum(W2**2)
       #############################################################################
       #                              END OF YOUR CODE                             #
       #############################################################################
       # Backward pass: compute gradients
       grads = {}
       #############################################################################
       # TODO: Compute the backward pass, computing the derivatives of the weights #
       # and biases. Store the results in the grads dictionary. For example,       #
       # grads['W1'] should store the gradient on W1, and be a matrix of same size #
       #############################################################################
       ds2 = np.copy(scores)  # 计算ds
       ds2[np.arange(X.shape[0]), y] -= 1
       ds2 = ds2 / X.shape[0]
       grads['W2'] = np.dot(s1_act.T, ds2) + 2 * reg * W2
       grads['b2'] = np.sum(ds2, axis=0)

       ds1 = np.dot(ds2, W2.T)
       ds1 = (s1 > 0) * ds1
       grads['W1'] = np.dot(X.T, ds1) + 2 * reg * W1
       grads['b1'] = np.sum(ds1, axis=0)
       #############################################################################
       #                              END OF YOUR CODE                             #
       #############################################################################

       return loss, grads

一些图像上的实现

一张图像HOG的实现

忽略颜色信息提取纹理信息

def hog_feature(im):
 """Compute Histogram of Gradient (HOG) feature for an image
 
      Modified from skimage.feature.hog
      http://pydoc.net/Python/scikits-image/0.4.2/skimage.feature.hog
    
    Reference:
      Histograms of Oriented Gradients for Human Detection
      Navneet Dalal and Bill Triggs, CVPR 2005
    
   Parameters:
     im : an input grayscale or rgb image
     
   Returns:
     feat: Histogram of Gradient (HOG) feature
   
 """
 
 # convert rgb to grayscale if needed
 if im.ndim == 3:
   image = rgb2gray(im)
 else:
   image = np.at_least_2d(im)

 sx, sy = image.shape # image size
 orientations = 9 # number of gradient bins
 cx, cy = (8, 8) # pixels per cell

 gx = np.zeros(image.shape)
 gy = np.zeros(image.shape)
 gx[:, :-1] = np.diff(image, n=1, axis=1) # compute gradient on x-direction 计算y方向的梯度
 gy[:-1, :] = np.diff(image, n=1, axis=0) # compute gradient on y-direction 计算x方向的梯度
 grad_mag = np.sqrt(gx ** 2 + gy ** 2) # gradient magnitude
 grad_ori = np.arctan2(gy, (gx + 1e-15)) * (180 / np.pi) + 90 # gradient orientation

 n_cellsx = int(np.floor(sx / cx))  # number of cells in x
 n_cellsy = int(np.floor(sy / cy))  # number of cells in y
 # compute orientations integral images
 orientation_histogram = np.zeros((n_cellsx, n_cellsy, orientations))
 for i in range(orientations):
   # create new integral image for this orientation
   # isolate orientations in this range
   temp_ori = np.where(grad_ori < 180 / orientations * (i + 1),
                       grad_ori, 0)# np.where是说满足条件的位置取grad_ori处的值，不满足条件的取0值
   temp_ori = np.where(grad_ori >= 180 / orientations * i,
                       temp_ori, 0)
   # select magnitudes for those orientations
   cond2 = temp_ori > 0
   temp_mag = np.where(cond2, grad_mag, 0)
   orientation_histogram[:,:,i] = uniform_filter(temp_mag, size=(cx, cy))[int(cx/2)::cx, int(cy/2)::cy]#uiform_filter均值滤波，以一定大小的kernel，取kernel中所有值和的平均替代中心处的值，输入输出的大小相同，所以0:cx/2的值为填充的值计算的，所以从cx/2开始算。 int(cx/2)::cx是说从int(cx/2)到最后以cx为间隔取值。该方法求histogram很巧妙。
 
 return orientation_histogram.ravel()

HSV

忽略纹理信息提取颜色信息

def color_histogram_hsv(im, nbin=10, xmin=0, xmax=255, normalized=True):
 """
 Compute color histogram for an image using hue.
 Inputs:
 - im: H x W x C array of pixel data for an RGB image.
 - nbin: Number of histogram bins. (default: 10)
 - xmin: Minimum pixel value (default: 0)
 - xmax: Maximum pixel value (default: 255)
 - normalized: Whether to normalize the histogram (default: True)
 Returns:
   1D vector of length nbin giving the color histogram over the hue of the
   input image.
 """
 ndim = im.ndim
 bins = np.linspace(xmin, xmax, nbin+1)
 hsv = matplotlib.colors.rgb_to_hsv(im/xmax) * xmax# rgb转hsv
 imhist, bin_edges = np.histogram(hsv[:,:,0], bins=bins, density=normalized)
 imhist = imhist * np.diff(bin_edges)

 # return histogram
 return imhist