# Image Processing Class from Scratch on Python

**Contents**

# 1 Writing a Image Processing Codes from Python on Scratch

What will you do when you suddenly think about `Convolutional Neural Networks from Scratch`

while serving cows? For me, I wrote some codes for image processing before thinking about those codes. Once again I am not going to write another `OpenCV`

here.

## 1.1 What am I using?

`Numpy`

for array operations`imageio`

builtin library for reading image`warnings`

to show warning`matplotlib`

for visualizing

## 1.2 What this blog includes?

- Converting an image into Grayscale from RGB.
- Convolution of an image using different kernels.

# 2 Steps

- Initializing a
`ImageProcessing`

class. - Adding a read method
- Adding a show method
- Adding color converison method
- Adding a convolution method

## Initializing a `ImageProcessing`

class

```
class ImageProcessing:
def __init__(self):
self.readmode = {1 : "RGB", 0 : "Grayscale"} # this dictionary will hold readmode values
```

## Adding a read method

```
def read_image(self, location = "", mode = 1):
"""
Uses imageio on back.
location: Directory of image file.
mode: Image readmode{1 : RGB, 0 : Grayscale}.
"""
img = imageio.imread(location)
if mode == 1:
img = img
elif mode == 0:
img = 0.21 * img[:,:,0] + 0.72 * img[:,:,1] + 0.07 * img[:,:,2]
elif mode == 2:
pass
else:
raise ValueError(f"Readmode not understood. Choose from {self.readmode}.")
return img
```

- This method only wraps the
`imageio`

, but I am applying a concept of`RGB`

to`GRAYSCALE`

conversion. - By default, imageio reads on RGB format.
- A typical
`RGB`

to`GRAYSCALE`

can be done on below concepts (taken from):-- Average Method: \begin{equation} Grayscale = \frac{R + G + B}{3} \end{equation} All channels are given 33% contribution.
- Weighted Method of luminosity method
\begin{equation}
Grayscale = 0.3
*R + 0.59*G + 0.11*B \end{equation} Red channel have 30%, Green have 59 and Blue have 11% contribution.

But I am using different version of method (taken from).

- If user enter different mode, then raise error.

## Adding a show method

```
def show(self, image, figsize=(5, 5)):
"""
Uses Matplotlib.pyplot.
image: A image to be shown.
figsize: How big image to show. From plt.figure()
"""
fig = plt.figure(figsize=figsize)
im = image
plt.imshow(im, cmap='gray')
plt.show()
```

Nothing to say here, docstring is enough.

## Color conversion

```
def convert_color(self, img, to=0):
if to==0:
return 0.21 * img[:,:,0] + 0.72 * img[:,:,1] + 0.07 * img[:,:,2]
else:
raise ValueError("Color conversion can not understood.")
```

I have still have not thought about grayscale to RGB conversion. But even using `OpenCV`

`cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)`

, we can not get complete BGR image.

## Adding a convolution method

```
def convolve(self, image, kernel = None, padding = "zero", stride=(1, 1), show=False, bias = 0):
"""
image: A image to be convolved.
kernel: A filter/window of odd shape for convolution. Used Sobel(3, 3) default.
padding: Border operation. Available from zero, same, none.
stride: How frequently do convolution?
show: whether to show result
bias: a bias term(used on Convolutional NN)
"""
if len(image.shape) > 3:
raise ValueError("Only 2 and 3 channel image supported.")
if type(kernel) == type(None):
warnings.warn("No kernel provided, trying to apply Sobel(3, 3).")
kernel = np.array([[1, 0, -1],
[1, 0, -1],
[1, 0, -1]])
kernel += kernel.T
kshape = kernel.shape
if kshape[0] % 2 != 1 or kshape[1] % 2 != 1:
raise ValueError("Please provide odd length of 2d kernel.")
if type(stride) == int:
stride = (stride, stride)
shape = image.shape
if padding == "zero":
zeros_h = np.zeros(shape[1]).reshape(-1, shape[1])
zeros_v = np.zeros(shape[0]+2).reshape(shape[0]+2, -1)
padded_img = np.vstack((zeros_h, image, zeros_h)) # add rows
padded_img = np.hstack((zeros_v, padded_img, zeros_v)) # add cols
image = padded_img
shape = image.shape
elif padding == "same":
h1 = image[0].reshape(-1, shape[1])
h2 = image[-1].reshape(-1, shape[1])
padded_img = np.vstack((h1, image, h2)) # add rows
v1 = padded_img[:, 0].reshape(padded_img.shape[0], -1)
v2 = padded_img[:, -1].reshape(padded_img.shape[0], -1)
padded_img = np.hstack((v1, padded_img, v2)) # add cols
image = padded_img
shape = image.shape
elif padding == None:
pass
rv = 0
cimg = []
for r in range(kshape[0], shape[0]+1, stride[0]):
cv = 0
for c in range(kshape[1], shape[1]+1, stride[1]):
chunk = image[rv:r, cv:c]
soma = (np.multiply(chunk, kernel)+bias).sum()
try:
chunk = int(soma)
except:
chunk = int(0)
if chunk < 0:
chunk = 0
if chunk > 255:
chunk = 255
cimg.append(chunk)
cv+=stride[1]
rv+=stride[0]
cimg = np.array(cimg, dtype=np.uint8).reshape(int(rv/stride[0]), int(cv/stride[1]))
if show:
print(f"Image convolved with \nKernel:{kernel}, \nPadding: {padding}, \nStride: {stride}")
return cimg
```

What is happening above?

- First the kernel is checked, if not given, used from sobel 3 by 3
- If the given kernel shape is not odd, error is raised.
- For padding,
`numpy`

stack methods are used. - Initialize an empty list to store convoluted values
- For convolution,
- we loop through every rows in step of kernel’s row upto total img rows
- loop through every cols in step of kernel’s col up to total img cols
- get a current chunk of image and multiply its elements with the kernel’s elements
- if current sum is geater than 255, set it 255
- append sum to list

- Finally convert the list into array then into right shape.

### Recall the mathematics of Convolution Operation

\begin{equation}
g(x, y) = f(x,y) * h(x,y)
\end{equation}
Where `f`

is a image function and `h`

is a kernel or mask or filter.

What happens on convolution can be clear from the matrix form of operation.
Lets take a image of `5X5`

and kernel of `3X3`

sobel y.

We have to move the kernel over the each and every pixels of the image from top left to bottom. Placing a kernel over a image and taking a elementwise matrix multiplication of the kernel and chunk of image of the kernel shape. For most cases, we use odd shaped kernel. By using odd shaped kernel, we can place a center of kernel to the center of image chunk.

Now we try to start from the top right pixel, but since our kernel is 3 by 3, we don’t have any pixels that will be facing the 1st row of kernel. So we have to work with the concept of `padding`

or we will loose those pixels of the border. For the sake of simplicity, lets take a `zero padding`

.

Now the first chunk of image will be:

\[\begin{pmatrix} 0 & 0 & 0 \\ 0 & 1 & 10 \\ 0 & 12 & 200 \\ \end{pmatrix}\]Now the convolution operation:

\[\begin{equation} \begin{pmatrix} 0 & 0 & 0 \\ 0 & 1 & 10 \\ 0 & 12 & 200 \\ \end{pmatrix} * \begin{pmatrix} -1 & 0 & 1\\ -1 & 0 & 1\\ -1 & 0 & 1\\ \end{pmatrix}= 0*-1+0*0+0*1+0*- 1+1*0+10*1+0*-1+12*0+200*1 = 210 \end{equation}\]Similarly, the final image will be like below after sliding through row then column:

\[\begin{pmatrix} 210 & 150 & 213 & 0 & 0\\ 400 & 61 & 43 & 0 & 0 \\ 597 & 51 & 0 & 0 & 0\\ 506 & 10 & 0 & 0 & 0 \\ 316 & 99 & 107 & 0 & 0\\ \end{pmatrix}\]But we will set 255 to all values which exceeds 255.

A better visualisation of a convolution operation can be seen by below gif(i don’t own this gif):-

Finally, visualizing our convolutated image:-

```
ip = ImageProcessing()
img = np.array([1, 10, 11, 200, 30, 12, 200, 152, 223, 60, 100,
190, 11, 20, 10, 102, 207, 102, 223, 50, 18, 109, 117, 200, 30]).reshape(5, 5)
cv = ip.convolve(img)
ip.show(cv)
```

If we printed the output of this code, i.e. `cv`

, then we will see the array just like above.

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