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blogContent/posts/open-source/shallow-dive-into-open-cv.md

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+This blog post going over the basic image manipulation things you can
+do with Open CV. [Open CV](https://opencv.org/) is an open-source
+library of computer vision tools. Open CV is written to be used in
+conjunction with deep learning frameworks like
+[TensorFlow](https://www.tensorflow.org/). This tutorial is going to
+be using Python3, although you can also use Open CV with C++, Java,
+and [Matlab](https://www.mathworks.com/products/matlab.html) 
+
+# Reading and Displaying Images
+
+The first thing that you want to do when you start playing around with
+open cv is to import the dependencies required. Most basic computer
+vision projects with OpenCV will use NumPy and matplotlib. All images
+in Open CV are represented as NumPy matrices with shape (x, y, 3),
+with the data type uint8. This essentially means that every image is a
+2d matrix with three color channels for BGR where each pixel can have
+an intensity between 0 and 255. Zero is black where 255 is white in
+grayscale. 
+
+
+```python
+# Open cv library
+import cv2
+
+# numpy library for matrix manipulation
+import numpy as np
+
+# matplotlib for displaying the images 
+from matplotlib import pyplot as plt
+```
+
+Reading an image is as easy as using the "cv2.imread" function.  If
+you simply try to print the image with Python's print function, you
+will flood your terminal with a massive matrix. In this post, we are
+going to be using the infamous
+[Lenna](https://en.wikipedia.org/wiki/Lenna) image which has been used
+in the Computer Vision field since 1973. 
+
+
+```python
+lenna = cv2.imread('lenna.jpg')
+
+# Prints a single pixel value
+print(lenna[50][50])
+
+# Prints the image dimensions
+# (width, height, 3 -- BRG)
+print(lenna.shape)
+```
+
+    [ 89 104 220]
+    (440, 440, 3)
+
+
+By now you might have noticed that I am saying "BRG" instead of "RGB";
+in Open CV colors are in the order of "BRG" instead of "RGB". This
+makes it particularly difficult when printing the images using a
+different library like matplotlib because they expect images to be in
+the form "RGB". Thankfully for us we can use some functions in the
+Open CV library to convert the color scheme. 
+
+
+```python
+def printI(img):
+    rgb = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)
+    plt.imshow(rgb)
+
+printI(lenna)
+```
+
+
+![png](media/cv1/output_6_0.png)
+
+
+Going a step further with image visualization, we can use matplotlib
+to view images side by side to each other. This makes it easier to
+make comparisons when running different algorithms on the same image. 
+
+
+```python
+def printI3(i1, i2, i3):
+    fig = plt.figure()
+    ax1 = fig.add_subplot(1,3,1)
+    ax1.imshow(cv2.cvtColor(i1, cv2.COLOR_BGR2RGB))
+    ax2 = fig.add_subplot(1,3,2)
+    ax2.imshow(cv2.cvtColor(i2, cv2.COLOR_BGR2RGB))
+    ax3 = fig.add_subplot(1,3,3)
+    ax3.imshow(cv2.cvtColor(i3, cv2.COLOR_BGR2RGB))
+    
+    
+def printI2(i1, i2):
+    fig = plt.figure()
+    ax1 = fig.add_subplot(1,2,1)
+    ax1.imshow(cv2.cvtColor(i1, cv2.COLOR_BGR2RGB))
+    ax2 = fig.add_subplot(1,2,2)
+    ax2.imshow(cv2.cvtColor(i2, cv2.COLOR_BGR2RGB))
+    
+```
+
+If we zero out the other colored layers and only left one channel, we
+can visualize each channel individually. In the following example
+notice that image.copy() generates a deep-copy of the image matrix --
+this is a useful NumPy function. 
+
+
+```python
+def generateBlueImage(image):
+    b = image.copy()
+    # set the green and red channels to 0
+    # note images are in BGR
+    b[:, :, 1] = 0
+    b[:, :, 2] = 0
+    return b
+
+
+def generateGreenImage(image):
+    g = image.copy()
+    # sets the blue and red channels to 0
+    g[:, :, 0] = 0
+    g[:, :, 2] = 0
+    return g
+
+def generateRedImage(image):
+    r = image.copy()
+    # sets the blue and green channels to 0
+    r[:, :, 0] = 0
+    r[:, :, 1] = 0
+    return r
+
+def visualizeRGB(image):
+    printI3(generateRedImage(image), generateGreenImage(image), generateBlueImage(image))
+```
+
+
+```python
+visualizeRGB(lenna)
+```
+
+
+![png](media/cv1/output_11_0.png)
+
+
+# Grayscale Images
+
+Converting a color image to grayscale reduces the dimensionality
+because you are squishing each color layer into one channel. Open CV
+has a built-in function to do this. 
+
+
+```python
+glenna = cv2.cvtColor(lenna, cv2.COLOR_BGR2GRAY)
+printI(glenna)
+```
+
+
+![png](media/cv1/output_14_0.png)
+
+
+The builtin function works in most applications, however, you
+sometimes want more control in which color layers are weighted more in
+generating the grayscale image. To do that you can  
+
+
+```python
+def generateGrayScale(image, rw = 0.25, gw = 0.5, bw = 0.25):
+    """
+    Image is the open cv image
+    w = weight to apply to each color layer
+    """
+    w = np.array([[[ bw, gw,  rw]]])
+    gray2 = cv2.convertScaleAbs(np.sum(image*w, axis=2))
+    return gray2
+```
+
+
+```python
+printI(generateGrayScale(lenna))
+```
+
+
+![png](media/cv1/output_17_0.png)
+
+
+Notice that the sum of the weights is equal to 1 if it above 1, it
+would brighten the image but if it was below 1, it would darken the
+image. 
+
+
+```python
+printI2(generateGrayScale(lenna, 0.1, 0.3, 0.1), generateGrayScale(lenna, 0.5, 0.6, 0.5))
+```
+
+
+![png](media/cv1/output_19_0.png)
+
+
+We could also use our function to display the grayscale output of each
+color layer. 
+
+
+```python
+printI3(generateGrayScale(lenna, 1.0, 0.0, 0.0), generateGrayScale(lenna, 0.0, 1.0, 0.0), generateGrayScale(lenna, 0.0, 0.0, 1.0))
+```
+
+
+![png](media/cv1/output_21_0.png)
+
+
+Based on this output, the red layer is the brightest which makes sense
+because the majority of the image is in a pinkish/red tone.  
+
+# Pixel Operations
+
+Pixel operations are simply things that you do to every pixel in the
+image. 
+
+## Negative
+
+To take the negative of an image, you simply invert the image. Ie: if
+the pixel was 0, it would now be 255, if the pixel was 0 it would now
+be 255. Since all the images are unsigned ints of length 8, right
+once, a pixel hits a boundary, it would automatically wrap over which
+is convenient for us. With NumPy, if you subtract a number from a
+matrix, it would do that for every element in that matrix -- neat.
+Therefore if we wanted to invert an image we could just take 255 and
+subtract it from the image. 
+
+
+```python
+invert_lenna = 255 - lenna
+printI(invert_lenna)
+```
+
+
+![png](media/cv1/output_25_0.png)
+
+
+## Darken And Lighten
+
+To brighten and darken an image you can add constants to the image
+because that would push the image closer twords 0 and 255 which is
+black and white. 
+
+
+```python
+bright_bad_lenna = lenna + 25
+
+printI(bright_bad_lenna)
+```
+
+
+![png](media/cv1/output_28_0.png)
+
+
+Notice that the image got brighter but in some parts the image got
+inverted. This is because when we add two images, and we don't want to
+wrap, we have to set a clipping threshold to be the 0 and 255. IE:
+when we add a constant to the image at pixel 240, we don't want it to
+wrap back to 0, we just want it to retain a value of 255. Open CV has
+built-in functions for this. 
+
+
+```python
+def brightenImg(img, num):
+    a = np.zeros(img.shape, dtype=np.uint8)
+    a[:] = num
+    return cv2.add(img, a)
+
+def darkenImg(img, num):
+    a = np.zeros(img.shape, dtype=np.uint8)
+    a[:] = num
+    return cv2.subtract(img, a)
+
+brighten_lenna = brightenImg(lenna, 50)
+darken_lenna = darkenImg(lenna, 50)
+
+printI2(brighten_lenna, darken_lenna)
+```
+
+
+![png](media/cv1/output_30_0.png)
+
+
+## Contrast
+
+Adjusting the contrast of an image is a matter of multiplying the
+image by a constant. Multiplying by a number greater than 1 would
+increase the contrast and multiplying by a number lower than 1 would
+decrease the contrast. 
+
+
+```python
+def adjustContrast(img, amount):
+    """
+    changes the data type to float32 so we can adjust the contrast by
+    more than integers, then we need to clip the values and 
+    convert data types at the end.
+    """
+    a = np.zeros(img.shape, dtype=np.float32)
+    a[:] = amount
+    b = img.astype(float)
+    c = np.multiply(a, b)
+    np.clip(c, 0, 255, out=c) # clips between 0 and 255
+    return c.astype(np.uint8)
+```
+
+
+```python
+printI2(adjustContrast(lenna, 0.8) ,adjustContrast(lenna, 1.3))
+```
+
+
+![png](media/cv1/output_33_0.png)
+
+
+# Noise
+
+I most cases you don't want to add random noise to your image,
+however, in some algorithms, it becomes necessary to do for testing.
+Noise is anything that makes the image imperfect. In the "real world"
+this is usually in the form of dead pixels on your camera lens or
+other things distorting your view.  
+
+## Salt and Pepper
+
+Salt and pepper noise is adding random black and white pixels to your
+image. 
+
+
+```python
+import random
+
+def uniformNoise(image, num):
+    img = image.copy()
+    h, w, c = img.shape
+    x = np.random.uniform(0,w,num)
+    y = np.random.uniform(0,h,num)
+
+    for i in range(0, num):
+        r = 0 if random.randrange(0,2) == 0 else 255
+        img[int(x[i])][int(y[i])] = np.asarray([r, r, r])
+        
+    return img
+printI2(uniformNoise(lenna, 1000), uniformNoise(lenna, 7000))
+```
+
+
+![png](media/cv1/output_36_0.png)
+
+
+# Image Denoising
+
+It is possible to remove the salt and pepper noise from an image to
+clean it up. Unlike how my professor worded it, this is not
+"enhancing" the image, this is merely using filters that remove the
+noise from the image by blurring it.  
+
+## Moving Average
+
+The moving average technique sets each pixel equal to the average of
+its neighborhood. The bigger your neighborhood the more the image is
+blurred. 
+
+
+```python
+bad_lenna = uniformNoise(lenna, 6000)
+
+blur_lenna = cv2.blur(bad_lenna,(3,3))
+
+printI2(bad_lenna, blur_lenna)
+```
+
+
+![png](media/cv1/output_39_0.png)
+
+
+As you can see, most of the noise was removed from the image but,
+imperfections were left. To see the effects of the filter size, you
+can play around with it. 
+
+
+```python
+blur_lenna_3 = cv2.blur(bad_lenna,(3,3))
+blur_lenna_8 = cv2.blur(bad_lenna,(8,8))
+printI2(blur_lenna_3, blur_lenna_8)
+```
+
+
+![png](media/cv1/output_41_0.png)
+
+
+## Median Filtering
+
+Median filters transform every pixel by taking the median value of its
+neighborhood. This is a lot better than average filters for noise
+reduction because it has less of a blurring effect and it is extremely
+well at removing outliers like salt and pepper noise.  
+
+
+```python
+median_lenna = cv2.medianBlur(bad_lenna,3)
+
+printI2(bad_lenna, median_lenna)
+```
+
+
+![png](media/cv1/output_43_0.png)
+
+
+# Remarks
+
+Open CV is a vastly powerful framework for image manipulation. This
+post only covered some of the more basic applications of Open CV.
+Future posts might explore some of the more advanced techniques in
+computer vision like filters, Canny edge detection, template matching,
+and Harris Corner detection.