copyediting on python object detection tutorial

clarify some passages, fix grammar errors

doc/py_tutorials/py_objdetect/py_face_detection/py_face_detection.markdown

@@ -20,65 +20,64 @@ other images.
 
 Here we will work with face detection. Initially, the algorithm needs a lot of positive images
 (images of faces) and negative images (images without faces) to train the classifier. Then we need
-to extract features from it. For this, haar features shown in below image are used. They are just
+to extract features from it. For this, Haar features shown in the below image are used. They are just
 like our convolutional kernel. Each feature is a single value obtained by subtracting sum of pixels
-under white rectangle from sum of pixels under black rectangle.
+under the white rectangle from sum of pixels under the black rectangle.
 
 ![image](images/haar_features.jpg)
 
-Now all possible sizes and locations of each kernel is used to calculate plenty of features. (Just
+Now, all possible sizes and locations of each kernel are used to calculate lots of features. (Just
 imagine how much computation it needs? Even a 24x24 window results over 160000 features). For each
-feature calculation, we need to find sum of pixels under white and black rectangles. To solve this,
+feature calculation, we need to find the sum of the pixels under white and black rectangles. To solve
-they introduced the integral images. It simplifies calculation of sum of pixels, how large may be
+this, they introduced the integral image. However large your image, it reduces the calculations for a
-the number of pixels, to an operation involving just four pixels. Nice, isn't it? It makes things
+given pixel to an operation involving just four pixels. Nice, isn't it? It makes things super-fast.
-super-fast.
 
 But among all these features we calculated, most of them are irrelevant. For example, consider the
-image below. Top row shows two good features. The first feature selected seems to focus on the
+image below. The top row shows two good features. The first feature selected seems to focus on the
 property that the region of the eyes is often darker than the region of the nose and cheeks. The
 second feature selected relies on the property that the eyes are darker than the bridge of the nose.
-But the same windows applying on cheeks or any other place is irrelevant. So how do we select the
+But the same windows applied to cheeks or any other place is irrelevant. So how do we select the
 best features out of 160000+ features? It is achieved by **Adaboost**.
 
 ![image](images/haar.png)
 
 For this, we apply each and every feature on all the training images. For each feature, it finds the
-best threshold which will classify the faces to positive and negative. But obviously, there will be
+best threshold which will classify the faces to positive and negative. Obviously, there will be
 errors or misclassifications. We select the features with minimum error rate, which means they are
-the features that best classifies the face and non-face images. (The process is not as simple as
+the features that most accurately classify the face and non-face images. (The process is not as simple as
 this. Each image is given an equal weight in the beginning. After each classification, weights of
-misclassified images are increased. Then again same process is done. New error rates are calculated.
+misclassified images are increased. Then the same process is done. New error rates are calculated.
-Also new weights. The process is continued until required accuracy or error rate is achieved or
+Also new weights. The process is continued until the required accuracy or error rate is achieved or
-required number of features are found).
+the required number of features are found).
 
-Final classifier is a weighted sum of these weak classifiers. It is called weak because it alone
+The final classifier is a weighted sum of these weak classifiers. It is called weak because it alone
 can't classify the image, but together with others forms a strong classifier. The paper says even
 200 features provide detection with 95% accuracy. Their final setup had around 6000 features.
 (Imagine a reduction from 160000+ features to 6000 features. That is a big gain).
 
 So now you take an image. Take each 24x24 window. Apply 6000 features to it. Check if it is face or
-not. Wow.. Wow.. Isn't it a little inefficient and time consuming? Yes, it is. Authors have a good
+not. Wow.. Isn't it a little inefficient and time consuming? Yes, it is. The authors have a good
 solution for that.
 
-In an image, most of the image region is non-face region. So it is a better idea to have a simple
+In an image, most of the image is non-face region. So it is a better idea to have a simple
-method to check if a window is not a face region. If it is not, discard it in a single shot. Don't
+method to check if a window is not a face region. If it is not, discard it in a single shot, and don't
-process it again. Instead focus on region where there can be a face. This way, we can find more time
+process it again. Instead, focus on regions where there can be a face. This way, we spend more time
-to check a possible face region.
+checking possible face regions.
 
-For this they introduced the concept of **Cascade of Classifiers**. Instead of applying all the 6000
+For this they introduced the concept of **Cascade of Classifiers**. Instead of applying all 6000
-features on a window, group the features into different stages of classifiers and apply one-by-one.
+features on a window, the features are grouped into different stages of classifiers and applied one-by-one.
-(Normally first few stages will contain very less number of features). If a window fails the first
+(Normally the first few stages will contain very many fewer features). If a window fails the first
-stage, discard it. We don't consider remaining features on it. If it passes, apply the second stage
+stage, discard it. We don't consider the remaining features on it. If it passes, apply the second stage
 of features and continue the process. The window which passes all stages is a face region. How is
-the plan !!!
+that plan!
 
-Authors' detector had 6000+ features with 38 stages with 1, 10, 25, 25 and 50 features in first five
+The authors' detector had 6000+ features with 38 stages with 1, 10, 25, 25 and 50 features in the first five
-stages. (Two features in the above image is actually obtained as the best two features from
+stages. (The two features in the above image are actually obtained as the best two features from
-Adaboost). According to authors, on an average, 10 features out of 6000+ are evaluated per
+Adaboost). According to the authors, on average 10 features out of 6000+ are evaluated per
 sub-window.
 
-So this is a simple intuitive explanation of how Viola-Jones face detection works. Read paper for
+So this is a simple intuitive explanation of how Viola-Jones face detection works. Read the paper for
-more details or check out the references in Additional Resources section.
+more details or check out the references in the Additional Resources section.
 
 Haar-cascade Detection in OpenCV
 --------------------------------
@@ -88,8 +87,8 @@ object like car, planes etc. you can use OpenCV to create one. Its full details
 [Cascade Classifier Training](@ref tutorial_traincascade).
 
 Here we will deal with detection. OpenCV already contains many pre-trained classifiers for face,
-eyes, smile etc. Those XML files are stored in opencv/data/haarcascades/ folder. Let's create face
+eyes, smiles, etc. Those XML files are stored in the opencv/data/haarcascades/ folder. Let's create a
-and eye detector with OpenCV.
+face and eye detector with OpenCV.
 
 First we need to load the required XML classifiers. Then load our input image (or video) in
 grayscale mode.