Contrast Algorithm

def change_contrast(A, B, C, D, out_image_name):
    img = Image.open("Ocean.bmp").convert('L')
    gray_img = img.copy()
    for i in range (0,img.size[0]):
        for j in range (0,img.size[1]):
            currPixel= img.getpixel((i, j))
            m = 0
            b = 0
            if(currPixel <= A):
                m = (B-0)/(A-0)
                b = B - int(m * A)
            elif(currPixel <= C):
                m = (D-B)/(C-A)
                b = D - int(m * C)
                #print(b)
            else:
                m = (255-D)//(255-C)
                b = 255 - int(m * 255)
            resultedPixel = int(m * (currPixel)) + b
            gray_img.putpixel((i,j),(resultedPixel))
    plt.imshow(gray_img, cmap="gray")
    gray_img.save(out_image_name)