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The goal is to find the line that represents the distance between the "hole" and the outer edge. Is Canny edge detection the best approach?

The goal is to find the line that represents the distance between the "hole" and the outer edge (transition from black to white). I was able to successfully binarize this photo and get a very clean black and white image. The next step would be to find the (almost) vertical line on it and calculate the perpendicular distance to the midpoint of this vertical line and the hole.
original picture
hole - zoomed in
ps: what I call "hole" is a shadow. I am shooting a laser into a hole. So the lines we can see is a steel and the black part without a line is a hole. The 2 white lines serve as a reference to measure the distance.
Is Canny edge detection the best approach? If so, what are good values for the A, B and C parameters? I can't tune it. I'm getting too much noise.

This is not complete; You have to take the time to reach the final result. But this idea might help you.
Preprocessors:
import os
import cv2
import numpy as np
Main code:
# Read original image
dir = os.path.abspath(os.path.dirname(__file__))
im = cv2.imread(dir+'/'+'im.jpg')
h, w = im.shape[:2]
print(w, h)
# Convert image to Grayscale
imGray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
cv2.imwrite(dir+'/im_1_grayscale.jpg', imGray)
# Eliminate noise and display laser light better
imHLine = imGray.copy()
imHLine = cv2.GaussianBlur(imHLine, (0, 9), 21) # 5, 51
cv2.imwrite(dir+'/im_2_h_line.jpg', imHLine)
# Make a BW mask to find the ROI of laser array
imHLineBW = cv2.threshold(imHLine, 22, 255, cv2.THRESH_BINARY)[1]
cv2.imwrite(dir+'/im_3_h_line_bw.jpg', imHLineBW)
# Remove noise with mask and extract just needed area
imHLineROI = imGray.copy()
imHLineROI[np.where(imHLineBW == 0)] = 0
imHLineROI = cv2.GaussianBlur(imHLineROI, (0, 3), 6)
imHLineROI = cv2.threshold(imHLineROI, 25, 255, cv2.THRESH_BINARY)[1] # 22
cv2.imwrite(dir+'/im_4_ROI.jpg', imHLineROI)
# Found laser array and draw box around it
cnts, _ = cv2.findContours(
imHLineROI, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts.sort(key=lambda x: cv2.boundingRect(x)[0])
pts = []
for cnt in cnts:
x2, y2, w2, h2 = cv2.boundingRect(cnt)
if h2 < h/10:
cv2.rectangle(im, (x2, y2), (x2+w2, y2+h2), (0, 255, 0), 1)
pts.append({'x': x2, 'y': y2, 'w': w2, 'h': h2})
circle = {
'left': (pts[0]['x']+pts[0]['w'], pts[0]['y']+pts[0]['h']/2),
'right': (pts[1]['x'], pts[1]['y']+pts[1]['h']/2),
}
circle['center'] = calculateMiddlePint(circle['left'], circle['right'])
circle['radius'] = (circle['right'][0]-circle['left'][0])//2
# Draw line and Circle inside it
im = drawLine(im, circle['left'], circle['right'], color=(27, 50, 120))
im = cv2.circle(im, circle['center'], circle['radius'], (255, 25, 25), 3)
# Remove pepper/salt noise to find metal edge
imVLine = imGray.copy()
imVLine = cv2.medianBlur(imVLine, 17)
cv2.imwrite(dir+'/im_6_v_line.jpg', imVLine)
# Remove remove the shadows to find metal edge
imVLineBW = cv2.threshold(imVLine, 50, 255, cv2.THRESH_BINARY)[1]
cv2.imwrite(dir+'/im_7_v_bw.jpg', imVLineBW)
# Finding the right vertical edge of metal
y1, y2 = h/5, h-h/5
x1 = horizantalDistance(imVLineBW, y1)
x2 = horizantalDistance(imVLineBW, y2)
pt1, pt2 = (x1, y1), (x2, y2)
imVLineBW = drawLine(imVLineBW, pt1, pt2)
cv2.imwrite(dir+'/im_8_v_bw.jpg', imVLineBW)
# Draw lines
im = drawLine(im, pt1, pt2)
im = drawLine(im, calculateMiddlePint(pt1, pt2), circle['center'])
# Draw final image
cv2.imwrite(dir+'/im_8_output.jpg', im)
Extra functions:
Find the first white pixel in one line of picture:
# This function only processes on a horizontal line of the image
# Its job is to examine the pixels one by one from the right and
# report the distance of the first white pixel from the right of
# the image.
def horizantalDistance(im, y):
y = int(y)
h, w = im.shape[:2]
for i in range(0, w):
x = w-i-1
if im[y][x] == 255:
return x
return -1
To draw a line in opencv:
def drawLine(im, pt1, pt2, color=(128, 0, 200), thickness=2):
return cv2.line(
im,
pt1=(int(pt1[0]), int(pt1[1])),
pt2=(int(pt2[0]), int(pt2[1])),
color=color,
thickness=thickness,
lineType=cv2.LINE_AA # Anti-Aliased
)
To calculate middle point of two 2d points:
def calculateMiddlePint(p1, p2):
return (int((p1[0]+p2[0])/2), int((p1[1]+p2[1])/2))
Output:
Original image:
Eliminate noise and process to see the laser array better:
Find the laser area to extract the hole:
Work on another copy of image to find the right side of metal object:
Remove the shadows to better see the right edge:
The final output:
I first defined an ROI area. I changed the code later but did not change the names of the variables. If you were asked.

You can try one of binarized Canny, or straight binarized image (Otsu). In both cases, you will obtain a quasi-straight line from which you can extract the right-most white pixel on every row or so.
Then use line fitting to these points (depending on image quality the fitting needs to be robust or not).
It would be wise to calibrate the method against ground truth, because the edge your are looking after borders a gradient area, and the location of the edge might be offset by several pixels.

Remove and measure a line openCV

Links to all images at the bottom
I have drawn a line over an arrow which captures the angle of that arrow. I would like to then remove the arrow, keep only the line, and use cv2.minAreaRect to determine the angle. So far I've got everything to work except removing the original arrow, which results in an incorrect angle generated by the cv2.minAreaRect bounding box.
Really, I just want the bold black line running through the arrow to use to measure the angle, not the arrow itself. if anyone has an idea to make this work, or a simpler way, please let me know. Thanks
Code:
import numpy as np
import cv2
image = cv2.imread("templates/a_15.png")
image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(image, 127, 255, 0)
contours, hierarchy = cv2.findContours(thresh, 1, 2)
cont = contours[0]
rows,cols = image.shape[:2]
[vx,vy,x,y] = cv2.fitLine(cont, cv2.DIST_L2,0,0.01,0.01)
leftish = int((-x*vy/vx) + y)
rightish = int(((cols-x)*vy/vx)+y)
line = cv2.line(image,(cols-1,rightish),(0,leftish),(0,255,0),10)
# thresholding
thresh = cv2.threshold(line, 0, 255, cv2.THRESH_BINARY | cv2.THRESH_OTSU)[1]
# compute rotated bounding box based on all pixel values > 0 and
# use coordinates to compute a rotated bounding box of those coordinates
coordinates = np.column_stack(np.where(thresh > 0))
w = coordinates[0]
h = coordinates[1]
# Compute minimum rotated angle that contains entire image.
# Return angle values in the range [-90, 0).
# As the rectangle rotates clockwise, angle values increase towards 0.
# Once 0 is reached, angle is set back to -90 degrees.
angle = cv2.minAreaRect(coordinates)[-1]
# for angles less than -45 degrees, add 90 degrees to angle to take the inverse.
if angle < - 45:
angle = -(90 + angle)
else:
angle = -angle
# rotate image
(h, w) = image.shape[:2]
center = (w // 2, h // 2) # image center
RM = cv2.getRotationMatrix2D(center, angle, 1.0)
rotated = cv2.warpAffine(image, RM, (w, h),
flags=cv2.INTER_CUBIC, borderMode=cv2.BORDER_REPLICATE)
# correction angle for validation
cv2.putText(rotated, "Angle {:.2f} degrees".format(angle),
(10, 30), cv2.FONT_HERSHEY_DUPLEX, 0.9, (0, 255, 0), 2)
# output
print("[INFO] angle: {:.3f}".format(angle))
cv2.imshow("Line", line)
cv2.imshow("Input", image)
cv2.imshow("Rotated", rotated)
cv2.waitKey(0)
Images
original
current results
goal

Here's a possible solution. The main idea is to identify de "tip" and the "tail" of the arrow approximating some key points. After you have identified both ends, you can draw a line joining both points. It is also an advantage to know which of the endpoints is the tip, because that way you can measure the angle from a constant point.
There's more than one way to achieve this. I choose something that I have applied in the past: I will use this approach to identify the endpoints of the overall shape. My assumption is that the tip will yield more points than the tail. After that, I'll cluster all the endpoints in two groups: tip and tail. I can use K-Means for that, as it will return the mean centers for both clusters. After that, we have our tip and tail points that can be joined easily with a line. These are the steps:
Convert the image to grayscale
Get the skeleton of the image, to normalize the shape to a width of 1 pixel
Apply the method described in the link to get the arrow's endpoints
Divide the endpoints in two clusters and use K-Means to get their centers
Join both endpoints with a line
Let's see the code:
# imports:
import cv2
import numpy as np
# image path
path = "D://opencvImages//"
fileName = "CoXeb.png"
# Reading an image in default mode:
inputImage = cv2.imread(path + fileName)
# Grayscale conversion:
grayscaleImage = cv2.cvtColor(inputImage, cv2.COLOR_BGR2GRAY)
grayscaleImage = 255 - grayscaleImage
# Extend the borders for the skeleton:
extendedImg = cv2.copyMakeBorder(grayscaleImage, 5, 5, 5, 5, cv2.BORDER_CONSTANT)
# Store a deep copy of the crop for results:
grayscaleImageCopy = cv2.cvtColor(extendedImg, cv2.COLOR_GRAY2BGR)
# Compute the skeleton:
skeleton = cv2.ximgproc.thinning(extendedImg, None, 1)
The first step is to get the skeleton of the arrow. As I said, this step is needed prior to the convolution-based method that identifies the endpoints of a shape. Computing the skeleton normalizes the shape to a one pixel width. However, sometimes, if the shape is too close to the "canvas" borders, the skeleton could show some artifacts. This is avoided with a border extension. The skeleton of the arrow is this:
Check that image out. If we identify the endpoints, the tip will exhibit at least 3 points, while the tail at least 1. That's handy - the tip will always have more points than the tail. If only we could detect those points... Luckily, we can:
# Threshold the image so that white pixels get a value of 0 and
# black pixels a value of 10:
_, binaryImage = cv2.threshold(skeleton, 128, 10, cv2.THRESH_BINARY)
# Set the end-points kernel:
h = np.array([[1, 1, 1],
[1, 10, 1],
[1, 1, 1]])
# Convolve the image with the kernel:
imgFiltered = cv2.filter2D(binaryImage, -1, h)
# Extract only the end-points pixels, those with
# an intensity value of 110:
binaryImage = np.where(imgFiltered == 110, 255, 0)
# The above operation converted the image to 32-bit float,
# convert back to 8-bit uint
binaryImage = binaryImage.astype(np.uint8)
This endpoint detecting method convolves the skeleton with a special kernel that identifies endpoints. It returns a binary image where all the endpoints have the value 110. After thresholding this mid-result, we get this image, which represents the arrow endpoints:
Nice, as you see, we can group the points in two clusters and get their cluster centers. Sounds like a job for K-Means, because that's exactly what it does. We first need to treat our data, though, because K-Means operates on defined-shaped arrays of float data:
# Find the X, Y location of all the end-points
# pixels:
Y, X = binaryImage.nonzero()
# Reshape the arrays for K-means
Y = Y.reshape(-1,1)
X = X.reshape(-1,1)
Z = np.hstack((X, Y))
# K-means operates on 32-bit float data:
floatPoints = np.float32(Z)
# Set the convergence criteria and call K-means:
criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 10, 1.0)
ret, label, center = cv2.kmeans(floatPoints, 2, None, criteria, 10, cv2.KMEANS_RANDOM_CENTERS)
# Set the cluster count, find the points belonging
# to cluster 0 and cluster 1:
cluster1Count = np.count_nonzero(label)
cluster0Count = np.shape(label)[0] - cluster1Count
print("Elements of Cluster 0: "+str(cluster0Count))
print("Elements of Cluster 1: " + str(cluster1Count))
The last two lines prints the endpoints that are assigned to Cluster 0 Cluster 1, respectively. That outputs this:
Elements of Cluster 0: 3
Elements of Cluster 1: 2
Just as expected - well, kinda. Seems that Cluster 0 is the tip and cluster 2 the tail! But the tail actually got 2 points. If you look the image of the skeleton closely, you'll see there's a small bifurcation at the tail. That's why we, in reality, got two points instead of just one. Alright, let's get the center points and draw them on the original input:
# Look for the cluster of max number of points
# That cluster will be the tip of the arrow:
maxCluster = 0
if cluster1Count > cluster0Count:
maxCluster = 1
# Check out the centers of each cluster:
matRows, matCols = center.shape
# Store the ordered end-points here:
orderedPoints = [None] * 2
# Let's identify and draw the two end-points
# of the arrow:
for b in range(matRows):
# Get cluster center:
pointX = int(center[b][0])
pointY = int(center[b][1])
# Get the "tip"
if b == maxCluster:
color = (0, 0, 255)
orderedPoints[0] = (pointX, pointY)
# Get the "tail"
else:
color = (255, 0, 0)
orderedPoints[1] = (pointX, pointY)
# Draw it:
cv2.circle(grayscaleImageCopy, (pointX, pointY), 3, color, -1)
cv2.imshow("End-Points", grayscaleImageCopy)
cv2.waitKey(0)
This is the resulting image:
The tip always gets drawn in red while the tail is drawn in blue. Very cool, let's store these points in the orderedPoints list and draw the final line in a new "canvas", with dimension same as the original image:
# Store the tip and tail points:
p0x = orderedPoints[1][0]
p0y = orderedPoints[1][1]
p1x = orderedPoints[0][0]
p1y = orderedPoints[0][1]
# Create a new "canvas" (image) using the input dimensions:
imageHeight, imageWidth = binaryImage.shape[:2]
newImage = np.zeros((imageHeight, imageWidth), np.uint8)
newImage = 255 - newImage
# Draw a line using the detected points:
(x1, y1) = orderedPoints[0]
(x2, y2) = orderedPoints[1]
lineColor = (0, 0, 0)
cv2.line(newImage , (x1, y1), (x2, y2), lineColor, thickness=2)
cv2.imshow("Detected Line", newImage)
cv2.waitKey(0)
The line overlaid on the original image and the new image containing only the line:

It sounds like you want to measure the angle of the line but because you are measuring a line you drew in the original image, you must now filter out the original image to get an accurate measure of the line...which you drew with coordinates you know the endpoints of?
I guess:
make a better filter?
draw the line in a blank image and detect angle there?
determine the angle from the known coordinates?
Since you were asking for just a line, I tried that...just made a blank image, drew your detected line on it and then used that downstream...
blankIm = np.ones((height, width, channels), dtype=np.uint8)
blankIm.fill(255)
line = cv2.line(blankIm,(cols-1,rightish),(0,leftish),(0,255,0),10)

How to detect a precise rectangle in a photo opencv

I was using this code to detect the rectangle on the photo, at first it was working well, untill i realized that i would have an object that is also a square in the middle :
Question:
How can i properly detect the 4 corners like on the first result picture without detecting the corner of the thing in the middle of the square. Thanks a lot.
Code:
import numpy as np
import cv2
img = cv2.imread('Photos/lastBoard.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
canny = cv2.Canny(gray, 100, 200)
corners = cv2.goodFeaturesToTrack(gray, 25, 0.01, 50)
corner_list = []
for corner in corners:
x, y = corner.ravel()
if(y < 700 and (50 < x < 800 )):
corner_list.append([int(x), int(y)])
cv2.circle(img, (x, y), 5, (36, 255, 12), -1)
cv2.imshow("yo", img)
cv2.waitKey(0)

My man, it breaks my heart you aren't using the techniques and processing we covered in your last question. You have already plenty of functions you could re-use. The rectangle you are trying to segment has a unique color (kind of green) and has a defined area and aspect ratio! Look all the things you have on the table, they are smaller than the rectangle! Plus, the rectangle is almost a square! That means that its aspect ratio is close to 1.0. If you somehow segment the rectangle, approximating its corners should be relativity easy.
This is valuable info, because it allows you to trace your action plan. I see you are using cv2.goodFeaturesToTrack to detect the corners of everything. That's OK, but it could be simplified. I propose a plan of action very similar to last time:
Try to segment the rectangle using its color, let's compute an
HSV-based mask
Let's clean the mask from noise using an area filter and some morphology
Find contours - we are looking for the biggest green contour, the rectangle.
The contour of interest has defined features. Use the area and aspect ratio to filter garbage contours.
Once you have the contour/blob of interest, approximate its corners.
Let's see the code:
# imports:
import numpy as np
import cv2
# image path
path = "D://opencvImages//"
fileName = "table1.jpg"
# Reading an image in default mode:
inputImage = cv2.imread(path + fileName)
inputCopy = inputImage.copy()
# The HSV mask values:
lowerValues = np.array([58, 151, 25])
upperValues = np.array([86, 255, 75])
# Convert the image to HSV:
hsvImage = cv2.cvtColor(inputImage, cv2.COLOR_BGR2HSV)
# Create the HSV mask
mask = cv2.inRange(hsvImage, lowerValues, upperValues)
The first steps aim to create the HSV mask. Very similar to last time, I've defined the HSV range of interest already and applied exactly the same stuff as before. You could (and should) explore more exotic techniques latter, but let's stick with what we know works for the time being, as the project surely is due soon. This is the result:
You see how the mask is pretty nice already? Only the green puck and the rectangle survived the thresholding. It doesn't matter that the rectangle is not complete, because we're gonna approximate its contour with a bounding rectangle! Alright, let's clean this bad boy a little bit better. Use a filterArea (this is exactly the same function we saw last time) and then a closing (dilate followed by erode) just to get a nice mask:
# Run a minimum area filter:
minArea = 50
mask = areaFilter(minArea, mask)
# Pre-process mask:
kernelSize = 3
structuringElement = cv2.getStructuringElement(cv2.MORPH_RECT, (kernelSize, kernelSize))
iterations = 2
mask = cv2.morphologyEx(mask, cv2.MORPH_DILATE, structuringElement, None, None, iterations, cv2.BORDER_REFLECT101)
mask = cv2.morphologyEx(mask, cv2.MORPH_ERODE, structuringElement, None, None, iterations, cv2.BORDER_REFLECT101)
This is the filtered mask, the noise is mostly gone:
Now, let's find contours and filtered based on area and aspect ratio, just like last time. The parameters, however, are different, because our target is not the plucks, but the rectangle:
# Find the big contours/blobs on the filtered image:
contours, hierarchy = cv2.findContours(mask, cv2.RETR_CCOMP, cv2.CHAIN_APPROX_SIMPLE)
# Store the poly approximation and bound
contoursPoly = [None] * len(contours)
# Store the corners of the square here:
detectedCorners = []
# Look for the outer bounding boxes:
for _, c in enumerate(contours):
# Approximate the contour to a polygon:
contoursPoly = cv2.approxPolyDP(c, 3, True)
# Convert the polygon to a bounding rectangle:
boundRect = cv2.boundingRect(contoursPoly)
# Get the bounding rect's data:
rectX = boundRect[0]
rectY = boundRect[1]
rectWidth = boundRect[2]
rectHeight = boundRect[3]
# Calculate the rect's area:
rectArea = rectWidth * rectHeight
# Calculate the aspect ratio:
aspectRatio = rectWidth / rectHeight
delta = abs(1.0 - aspectRatio)
# Set the min threshold values to identify the
# blob of interest:
minArea = 2500
epsilon = 0.2
Alright, so far so good, I hope. As you see I approximated the contour to a 4-vertex polygon and then computed its bounding rectangle. This approximation should fit very nicely to our blob of interest. Now, apply the contour filter and use the bounding rectangle data to approximate the corners. I approximate each corner, one by one, and store them in the
detectedCorners array. Then, we can draw 'em. Here, still inside the for loop:
# Is this bounding rectangle we
# are looking for?
if rectArea > minArea and delta < epsilon:
# Compute the corners/vertices:
# Corner 1 (top left)
corner1 = (rectX, rectY)
detectedCorners.append(corner1)
# Corner 2 (top right)
corner2 = (rectX + rectWidth, rectY)
detectedCorners.append(corner2)
# Corner 3 (bottom left)
corner3 = (rectX, rectY + rectHeight)
detectedCorners.append(corner3)
# Corner 4 (bottom right)
corner4 = (rectX + rectWidth, rectY + rectHeight)
detectedCorners.append(corner4)
# Draw the corner points:
for p in detectedCorners:
color = (0, 0, 255)
cv2.circle(inputCopy, (p[0], p[1]), 5, color, -1)
cv2.imshow("Square Corners", inputCopy)
cv2.waitKey(0)
Here are the results for both images. The approximated corners are the red dots:
Here's the definition and implementation of the areaFilter function:
def areaFilter(minArea, inputImage):
# Perform an area filter on the binary blobs:
componentsNumber, labeledImage, componentStats, componentCentroids = \
cv2.connectedComponentsWithStats(inputImage, connectivity=4)
# Get the indices/labels of the remaining components based on the area stat
# (skip the background component at index 0)
remainingComponentLabels = [i for i in range(1, componentsNumber) if componentStats[i][4] >= minArea]
# Filter the labeled pixels based on the remaining labels,
# assign pixel intensity to 255 (uint8) for the remaining pixels
filteredImage = np.where(np.isin(labeledImage, remainingComponentLabels) == True, 255, 0).astype('uint8')
return filteredImage

Bulk removing unwanted parts of images

I have downloaded a number of images (1000) from a website but they each have a black and white ruler running along 1 or 2 edges and some have these catalogue number tickets. I need these elements removed, the ruler at the very least.
Example images of coins:
The images all have the ruler in slightly different places so i cant just preform the same crop on them.
So I tried to remove the black and replace it with white using this code
from PIL import Image
import numpy as np
import matplotlib.pyplot as plt
im = Image.open('image-0.jpg')
im = im.convert('RGBA')
data = np.array(im) # "data" is a height x width x 4 numpy array
red, green, blue, alpha = data.T # Temporarily unpack the bands for readability
# Replace black with white
black_areas = (red < 150) & (blue < 150) & (green < 150)
data[..., :-1][black_areas.T] = (255, 255, 255) # Transpose back needed
im2 = Image.fromarray(data)
im2.show()
but it pretty much just removed half the coin as well:
I was having a read of some posts on opencv but though I'd see if there was a simpler way I'd missed first.

So I have taken a look at your problem and I have found a solution for your two images you provided, I hope it works for you other images as well but it is always hard to tell as it can be different on an individual basis. This solution is using OpenCV for preprocessing and contour detection to get the 2nd and 3rd largest elements in your picture (largest is the bounding box around the edges) which should be your coins. Then I create a box around those two items and add some padding before I crop to size.
So we start off with preprocessing:
import numpy as np
import cv2
img = cv2.imread(r'<PATH TO YOUR IMAGE>')
img = cv2.resize(img, None, fx=3, fy=3)
imgray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(imgray, (5, 5), 0)
ret, thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)
Still rather basic, we make the image bigger so it is easier to detect contours, then we turn it into grayscale, blur it and apply thresholding to it so we turn all grey values either white or black. This then gives us the following image:
We now do contour detection, get the areas around our contours and sort them by the biggest area. Then we drop the biggest one as it is the box around the whole image and take the 2nd and 3rd biggest. And then get the x,y,w,h values we are interested in.
contours, hierarchy = cv2.findContours(
thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
areas = []
for cnt in contours:
area = cv2.contourArea(cnt)
areas.append((area, cnt))
areas.sort(key=lambda x: x[0], reverse=True)
areas.pop(0)
x, y, w, h = cv2.boundingRect(areas[0][1])
x2, y2, w2, h2 = cv2.boundingRect(areas[1][1])
If we draw a rectangle around those contours:
Now we take those coordinates and create a box around both of them. This might need some minor adjusting as I just quickly took the bigger width of the two and not the corresponding one for the right coin but since I added extra padding it should be fine in most cases. And finally crop to size:
pad = 15
img = img[(min(y, y2) - pad) : (max(y, y2) + max(h, h2) + pad),
(min(x, x2) - pad) : (max(x, x2) + max(w, w2) + pad)]
I hope this helps you to understand how you could achieve what you want, I tried it on both your images and it worked well for them. It might need some adjustments and depending on how your other images look the simple approach of taking the two biggest objects (apart from image bounding box) might be turned into something more sophisticated to detect the cricular shapes or something along those lines. Alternatively you could try to detect the rulers and crop from their position inwards. You will have to decide after you have done this on more example images in your dataset.

If you're looking for a robust solution, you should try something like Max Kaha's response, since it'll provide you with greater fine tuning.
Since the rulers tend to be left with just a little bit of text after your "black to white" filter, a quick solution is to use erosion followed by a dilation to create a mask for your images, and then apply the mask to the original image.
Pillow offers that with the ImageFilter class. Here's your code with a few modifications that'll achieve that:
from PIL import Image, ImageFilter
import numpy as np
import matplotlib.pyplot as plt
WHITE = 255, 255, 255
input_image = Image.open('image.png')
input_image = input_image.convert('RGBA')
input_data = np.array(input_image) # "data" is a height x width x 4 numpy array
red, green, blue, alpha = input_data.T # Temporarily unpack the bands for readability
# Replace black with white
thresh = 30
black_areas = (red < thresh) & (blue < thresh) & (green < thresh)
input_data[..., :-1][black_areas.T] = WHITE # Transpose back needed
erosion_factor = 5
# dilation is bigger to avoid cropping the objects of interest
dilation_factor = 11
erosion_filter = ImageFilter.MaxFilter(erosion_factor)
dilation_filter = ImageFilter.MinFilter(dilation_factor)
eroded = Image.fromarray(input_data).filter(erosion_filter)
dilated = eroded.filter(dilation_filter)
mask_threshold = 220
# the mask is black on regions to be hidden
mask = dilated.convert('L').point(lambda x: 255 if x < mask_threshold else 0)
# create base image
output_image = Image.new('RGBA', input_image.size, WHITE)
# paste only the desired regions
output_image.paste(input_image, mask=mask)
output_image.show()
You should also play around with the black to white threshold and the erosion/dilation factors to try and find the best fit for most of your images.

How to close contour over outline rather than edge - OpenCV

Tl;DR: How to measure area enclosed by contour rather than just the contour line itself
I want to find the outline of the object in the below image and have a code that works for most cases.
Thresholding and adpative thresholding do not work reliably as the ligthing changes. I use a Canny edge detection and check the area to ensure I found the proper contour. However, once in a while, when there is a gap that cannot be closed by morphological closing, the shape is correct but the area is of the contour line instead of the whole object.
What I usually do is use convexHull, as it returns a contour around the object. However, in this case the object curves inwards along the top and convexHull isn't a good approximation to the area anymore.
I tried using approxPolyDP but the area that gets returned is of the contour line rather than the object.
How can I get the approxPolyDP to return a similar closed contour around the object, just like the convexHull function does?
Code illustrating this using the above picture:
import cv2
img = cv2.imread('Img_0.jpg',0)
cv2.imshow('Original', img)
edges = cv2.Canny(img,50,150)
cv2.imshow('Canny', edges)
contours, hierarchy = cv2.findContours(edges,cv2.cv.CV_RETR_EXTERNAL,cv2.cv.CV_CHAIN_APPROX_NONE)
cnt = contours[1] #I have a function to do this but for simplicity here by hand
M = cv2.moments(cnt)
print('Area = %f \t' %M['m00'], end="")
cntHull = cv2.convexHull(cnt, returnPoints=True)
cntPoly=cv2.approxPolyDP(cnt, epsilon=1, closed=True)
MHull = cv2.moments(cntHull)
MPoly = cv2.moments(cntPoly)
print('Area after Convec Hull = %f \t Area after apporxPoly = %f \n' %(MHull['m00'], MPoly['m00']), end="")
x, y =img.shape
size = (w, h, channels) = (x, y, 1)
canvas = np.zeros(size, np.uint8)
cv2.drawContours(canvas, cnt, -1, 255)
cv2.imshow('Contour', canvas)
canvas = np.zeros(size, np.uint8)
cv2.drawContours(canvas, cntHull, -1, 255)
cv2.imshow('Hull', canvas)
canvas = np.zeros(size, np.uint8)
cv2.drawContours(canvas, cntPoly, -1, 255)
cv2.imshow('Poly', canvas)
The output from the code is
Area = 24.500000 Area after Convec Hull = 3960.500000 Area after apporxPoly = 29.500000

Here's a very promising ppt from geosensor.net that discusses several algorithms. My recommendation would be to use the swing arm method with a limited radius.
Another completely un-tested, off the wall idea I have is to scan across the image by row and column (more directions increase accuracy) and color in the regions between line intersections:
_______
/-------\
/---------\
--------+---------+------ (fill between 2 intersections)
| |
|
--------+---------------- (no fill between single intersection)
\
-------
the maximum error would then decrease as the number of line directions scanned increases (more than 90 and 45 degrees). Getting a final area would then be as simple as a pixel count.