{"id":6831,"date":"2023-02-17T14:22:08","date_gmt":"2023-02-17T19:22:08","guid":{"rendered":"https:\/\/blogs.mathworks.com\/steve\/?p=6831"},"modified":"2023-02-17T14:22:08","modified_gmt":"2023-02-17T19:22:08","slug":"objects-touching-border-imclearborder-regionprops","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/steve\/2023\/02\/17\/objects-touching-border-imclearborder-regionprops\/","title":{"rendered":"Identifying Border-Touching Objects Using imclearborder or regionprops"},"content":{"rendered":"\r\n
I have seen some requests and questions related to identifying objects in a binary image that are touching the image border. Sometimes the question relates to the use of <\/span>imclearborder<\/span>, and sometimes the question is about <\/span>regionprops<\/span>. Today, I'll show you how to tackle the problem both ways.<\/span><\/div>

Using imclearborder<\/span><\/h2>
I'll be using this binary version of the rice.png sample image from the Image Processing Toolbox.<\/span><\/div>
url = <\/span>\"https:\/\/blogs.mathworks.com\/steve\/files\/rice-bw-1.png\"<\/span>;<\/span><\/span><\/div><\/div>
A = imread(url);<\/span><\/span><\/div><\/div>
imshow(A)<\/span><\/span><\/div>
<\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
The function <\/span>imclearborder<\/span> removes all objects touching the border.<\/span><\/div>
B = imclearborder(A);<\/span><\/span><\/div><\/div>
imshow(B)<\/span><\/span><\/div>
<\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
That seems to be the opposite of what we want. We can, however, convert this into the desired result by using the MATLAB element-wise logical operators, such as <\/span>&<\/span> (and), <\/span>|<\/span> (or), and <\/span>~<\/span> (not). In words, we want the foreground pixels that are in <\/span>A<\/span> and are not in <\/span>B<\/span>. As a MATLAB expression, it looks like this:<\/span><\/div>
C = A & ~B;<\/span><\/span><\/div><\/div>
imshow(C)<\/span><\/span><\/div>
<\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>

Using regionprops<\/span><\/h2>
The function <\/span>regionprops<\/span> can compute all sorts of properties of binary image objects. Here is a simple example that computes the area and centroid of each object in our sample image. I'm using the form of <\/span>regionprops<\/span> that returns a table.<\/span><\/div>
props = regionprops(<\/span>\"table\"<\/span>,A,[<\/span>\"Area\" \"Centroid\"<\/span>])<\/span><\/span><\/div>
t = <\/span>97\u00d72 table <\/span><\/div>
 <\/span><\/th>Area<\/span><\/th>Centroid<\/span><\/th><\/tr><\/thead>
1<\/span><\/th>138<\/span><\/td>5.9855<\/span><\/td>35.0870<\/span><\/td><\/tr>
2<\/span><\/th>121<\/span><\/td>6.4463<\/span><\/td>62.0579<\/span><\/td><\/tr>
3<\/span><\/th>67<\/span><\/td>2.1045<\/span><\/td>85.1940<\/span><\/td><\/tr>
4<\/span><\/th>178<\/span><\/td>4.5056<\/span><\/td>117.5674<\/span><\/td><\/tr>
5<\/span><\/th>157<\/span><\/td>9.4650<\/span><\/td>141.2675<\/span><\/td><\/tr>
6<\/span><\/th>286<\/span><\/td>11.8007<\/span><\/td>168.5350<\/span><\/td><\/tr>
7<\/span><\/th>195<\/span><\/td>12.0872<\/span><\/td>205.2974<\/span><\/td><\/tr>
8<\/span><\/th>135<\/span><\/td>9.4296<\/span><\/td>223.0296<\/span><\/td><\/tr>
9<\/span><\/th>176<\/span><\/td>21.3693<\/span><\/td>95.9716<\/span><\/td><\/tr>
10<\/span><\/th>208<\/span><\/td>24.1971<\/span><\/td>26.3125<\/span><\/td><\/tr>
11<\/span><\/th>199<\/span><\/td>28.1859<\/span><\/td>48.2563<\/span><\/td><\/tr>
12<\/span><\/th>188<\/span><\/td>22.1011<\/span><\/td>122.6011<\/span><\/td><\/tr>
13<\/span><\/th>216<\/span><\/td>28.2778<\/span><\/td>179.4630<\/span><\/td><\/tr>
14<\/span><\/th>143<\/span><\/td>28.3427<\/span><\/td>251.9720<\/span><\/td><\/tr>
\u22ee<\/th><\/tr><\/tbody><\/table><\/div><\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
My technique for finding border-touching objects with <\/span>regionprops<\/span> uses the <\/span>BoundingBox<\/span> property, so include that property along with any other properties that you want to measure.<\/span><\/div>
props = regionprops(<\/span>\"table\"<\/span>,A,[<\/span>\"Area\" \"Centroid\" \"BoundingBox\"<\/span>])<\/span><\/span><\/div>
t = <\/span>97\u00d73 table <\/span><\/div>
 <\/span><\/th>Area<\/span><\/th>Centroid<\/span><\/th>BoundingBox<\/span><\/th><\/tr><\/thead>
1<\/span><\/th>138<\/span><\/td>5.9855<\/span><\/td>35.0870<\/span><\/td>0.5000<\/span><\/td>22.5000<\/span><\/td>13<\/span><\/td>22<\/span><\/td><\/tr>
2<\/span><\/th>121<\/span><\/td>6.4463<\/span><\/td>62.0579<\/span><\/td>0.5000<\/span><\/td>53.5000<\/span><\/td>13<\/span><\/td>16<\/span><\/td><\/tr>
3<\/span><\/th>67<\/span><\/td>2.1045<\/span><\/td>85.1940<\/span><\/td>0.5000<\/span><\/td>73.5000<\/span><\/td>4<\/span><\/td>23<\/span><\/td><\/tr>
4<\/span><\/th>178<\/span><\/td>4.5056<\/span><\/td>117.5674<\/span><\/td>0.5000<\/span><\/td>102.5000<\/span><\/td>10<\/span><\/td>28<\/span><\/td><\/tr>
5<\/span><\/th>157<\/span><\/td>9.4650<\/span><\/td>141.2675<\/span><\/td>0.5000<\/span><\/td>132.5000<\/span><\/td>20<\/span><\/td>16<\/span><\/td><\/tr>
6<\/span><\/th>286<\/span><\/td>11.8007<\/span><\/td>168.5350<\/span><\/td>0.5000<\/span><\/td>152.5000<\/span><\/td>26<\/span><\/td>33<\/span><\/td><\/tr>
7<\/span><\/th>195<\/span><\/td>12.0872<\/span><\/td>205.2974<\/span><\/td>0.5000<\/span><\/td>199.5000<\/span><\/td>24<\/span><\/td>12<\/span><\/td><\/tr>
8<\/span><\/th>135<\/span><\/td>9.4296<\/span><\/td>223.0296<\/span><\/td>0.5000<\/span><\/td>218.5000<\/span><\/td>20<\/span><\/td>10<\/span><\/td><\/tr>
9<\/span><\/th>176<\/span><\/td>21.3693<\/span><\/td>95.9716<\/span><\/td>9.5000<\/span><\/td>86.5000<\/span><\/td>25<\/span><\/td>17<\/span><\/td><\/tr>
10<\/span><\/th>208<\/span><\/td>24.1971<\/span><\/td>26.3125<\/span><\/td>10.5000<\/span><\/td>18.5000<\/span><\/td>26<\/span><\/td>16<\/span><\/td><\/tr>
11<\/span><\/th>199<\/span><\/td>28.1859<\/span><\/td>48.2563<\/span><\/td>17.5000<\/span><\/td>37.5000<\/span><\/td>19<\/span><\/td>23<\/span><\/td><\/tr>
12<\/span><\/th>188<\/span><\/td>22.1011<\/span><\/td>122.6011<\/span><\/td>17.5000<\/span><\/td>108.5000<\/span><\/td>10<\/span><\/td>29<\/span><\/td><\/tr>
13<\/span><\/th>216<\/span><\/td>28.2778<\/span><\/td>179.4630<\/span><\/td>17.5000<\/span><\/td>166.5000<\/span><\/td>20<\/span><\/td>24<\/span><\/td><\/tr>
14<\/span><\/th>143<\/span><\/td>28.3427<\/span><\/td>251.9720<\/span><\/td>18.5000<\/span><\/td>245.5000<\/span><\/td>23<\/span><\/td>11<\/span><\/td><\/tr>
\u22ee<\/th><\/tr><\/tbody><\/table><\/div><\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
For any particular object, <\/span>BoundingBox<\/span> is a four-element vector containing the left, top, width, and height of the bounding box. For example, here is the bounding box of the 20th object:<\/span><\/div>
props.BoundingBox(20,:)<\/span><\/span><\/div>
ans = <\/span>1\u00d74<\/span><\/div><\/div>
35.5000 113.5000 9.0000 28.0000\r\n<\/div>
<\/div>
<\/div><\/div><\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
By comparing these values to the size of the image, we can identify which objects touch the image border.<\/span><\/div>
Start by determining the objects that touch specific borders.<\/span><\/div>
left_coordinate = props.BoundingBox(:,1);<\/span><\/span><\/div><\/div>
props.TouchesLeftBorder = (left_coordinate == 0.5);<\/span><\/span><\/div><\/div>
top_coordinate = props.BoundingBox(:,2);<\/span><\/span><\/div><\/div>
props.TouchesTopBorder = (top_coordinate == 0.5);<\/span><\/span><\/div><\/div>
right_coordinate = left_coordinate + props.BoundingBox(:,3);<\/span><\/span><\/div><\/div>
bottom_coordinate = top_coordinate + props.BoundingBox(:,4);<\/span><\/span><\/div><\/div>
[M,N] = size(A);<\/span><\/span><\/div><\/div>
props.TouchesRightBorder = (right_coordinate == (N+0.5));<\/span><\/span><\/div><\/div>
props.TouchesBottomBorder = (bottom_coordinate == (M+0.5))<\/span><\/span><\/div>
t = <\/span>97\u00d77 table <\/span><\/div>
 <\/span><\/th>Area<\/span><\/th>Centroid<\/span><\/th>BoundingBox<\/span><\/th>TouchesLeftBorder<\/span><\/th>TouchesTopBorder<\/span><\/th>TouchesRightBorder<\/span><\/th>TouchesBottomBorder<\/span><\/th><\/tr><\/thead>
1<\/span><\/th>138<\/span><\/td>5.9855<\/span><\/td>35.0870<\/span><\/td>0.5000<\/span><\/td>22.5000<\/span><\/td>13<\/span><\/td>22<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
2<\/span><\/th>121<\/span><\/td>6.4463<\/span><\/td>62.0579<\/span><\/td>0.5000<\/span><\/td>53.5000<\/span><\/td>13<\/span><\/td>16<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
3<\/span><\/th>67<\/span><\/td>2.1045<\/span><\/td>85.1940<\/span><\/td>0.5000<\/span><\/td>73.5000<\/span><\/td>4<\/span><\/td>23<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
4<\/span><\/th>178<\/span><\/td>4.5056<\/span><\/td>117.5674<\/span><\/td>0.5000<\/span><\/td>102.5000<\/span><\/td>10<\/span><\/td>28<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
5<\/span><\/th>157<\/span><\/td>9.4650<\/span><\/td>141.2675<\/span><\/td>0.5000<\/span><\/td>132.5000<\/span><\/td>20<\/span><\/td>16<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
6<\/span><\/th>286<\/span><\/td>11.8007<\/span><\/td>168.5350<\/span><\/td>0.5000<\/span><\/td>152.5000<\/span><\/td>26<\/span><\/td>33<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
7<\/span><\/th>195<\/span><\/td>12.0872<\/span><\/td>205.2974<\/span><\/td>0.5000<\/span><\/td>199.5000<\/span><\/td>24<\/span><\/td>12<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
8<\/span><\/th>135<\/span><\/td>9.4296<\/span><\/td>223.0296<\/span><\/td>0.5000<\/span><\/td>218.5000<\/span><\/td>20<\/span><\/td>10<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
9<\/span><\/th>176<\/span><\/td>21.3693<\/span><\/td>95.9716<\/span><\/td>9.5000<\/span><\/td>86.5000<\/span><\/td>25<\/span><\/td>17<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
10<\/span><\/th>208<\/span><\/td>24.1971<\/span><\/td>26.3125<\/span><\/td>10.5000<\/span><\/td>18.5000<\/span><\/td>26<\/span><\/td>16<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
11<\/span><\/th>199<\/span><\/td>28.1859<\/span><\/td>48.2563<\/span><\/td>17.5000<\/span><\/td>37.5000<\/span><\/td>19<\/span><\/td>23<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
12<\/span><\/th>188<\/span><\/td>22.1011<\/span><\/td>122.6011<\/span><\/td>17.5000<\/span><\/td>108.5000<\/span><\/td>10<\/span><\/td>29<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
13<\/span><\/th>216<\/span><\/td>28.2778<\/span><\/td>179.4630<\/span><\/td>17.5000<\/span><\/td>166.5000<\/span><\/td>20<\/span><\/td>24<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
14<\/span><\/th>143<\/span><\/td>28.3427<\/span><\/td>251.9720<\/span><\/td>18.5000<\/span><\/td>245.5000<\/span><\/td>23<\/span><\/td>11<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
\u22ee<\/th><\/tr><\/tbody><\/table><\/div><\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
Finally, compute whether objects touch any border using <\/span>|<\/span>, the element-wise OR operator.<\/span><\/div>
props.TouchesAnyBorder = props.TouchesLeftBorder | <\/span>...<\/span><\/span><\/div><\/div>
props.TouchesTopBorder | <\/span>...<\/span><\/span><\/div><\/div>
props.TouchesRightBorder | <\/span>...<\/span><\/span><\/div><\/div>
props.TouchesBottomBorder<\/span><\/span><\/div>
t = <\/span>97\u00d78 table <\/span><\/div>
 <\/span><\/th>Area<\/span><\/th>Centroid<\/span><\/th>BoundingBox<\/span><\/th>TouchesLeftBorder<\/span><\/th>TouchesTopBorder<\/span><\/th>TouchesRightBorder<\/span><\/th>TouchesBottomBorder<\/span><\/th>TouchesAnyBorder<\/span><\/th><\/tr><\/thead>
1<\/span><\/th>138<\/span><\/td>5.9855<\/span><\/td>35.0870<\/span><\/td>0.5000<\/span><\/td>22.5000<\/span><\/td>13<\/span><\/td>22<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
2<\/span><\/th>121<\/span><\/td>6.4463<\/span><\/td>62.0579<\/span><\/td>0.5000<\/span><\/td>53.5000<\/span><\/td>13<\/span><\/td>16<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
3<\/span><\/th>67<\/span><\/td>2.1045<\/span><\/td>85.1940<\/span><\/td>0.5000<\/span><\/td>73.5000<\/span><\/td>4<\/span><\/td>23<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
4<\/span><\/th>178<\/span><\/td>4.5056<\/span><\/td>117.5674<\/span><\/td>0.5000<\/span><\/td>102.5000<\/span><\/td>10<\/span><\/td>28<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
5<\/span><\/th>157<\/span><\/td>9.4650<\/span><\/td>141.2675<\/span><\/td>0.5000<\/span><\/td>132.5000<\/span><\/td>20<\/span><\/td>16<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
6<\/span><\/th>286<\/span><\/td>11.8007<\/span><\/td>168.5350<\/span><\/td>0.5000<\/span><\/td>152.5000<\/span><\/td>26<\/span><\/td>33<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
7<\/span><\/th>195<\/span><\/td>12.0872<\/span><\/td>205.2974<\/span><\/td>0.5000<\/span><\/td>199.5000<\/span><\/td>24<\/span><\/td>12<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
8<\/span><\/th>135<\/span><\/td>9.4296<\/span><\/td>223.0296<\/span><\/td>0.5000<\/span><\/td>218.5000<\/span><\/td>20<\/span><\/td>10<\/span><\/td>1<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td><\/tr>
9<\/span><\/th>176<\/span><\/td>21.3693<\/span><\/td>95.9716<\/span><\/td>9.5000<\/span><\/td>86.5000<\/span><\/td>25<\/span><\/td>17<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
10<\/span><\/th>208<\/span><\/td>24.1971<\/span><\/td>26.3125<\/span><\/td>10.5000<\/span><\/td>18.5000<\/span><\/td>26<\/span><\/td>16<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
11<\/span><\/th>199<\/span><\/td>28.1859<\/span><\/td>48.2563<\/span><\/td>17.5000<\/span><\/td>37.5000<\/span><\/td>19<\/span><\/td>23<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
12<\/span><\/th>188<\/span><\/td>22.1011<\/span><\/td>122.6011<\/span><\/td>17.5000<\/span><\/td>108.5000<\/span><\/td>10<\/span><\/td>29<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
13<\/span><\/th>216<\/span><\/td>28.2778<\/span><\/td>179.4630<\/span><\/td>17.5000<\/span><\/td>166.5000<\/span><\/td>20<\/span><\/td>24<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td><\/tr>
14<\/span><\/th>143<\/span><\/td>28.3427<\/span><\/td>251.9720<\/span><\/td>18.5000<\/span><\/td>245.5000<\/span><\/td>23<\/span><\/td>11<\/span><\/td>0<\/span><\/td>0<\/span><\/td>0<\/span><\/td>1<\/span><\/td>1<\/span><\/td><\/tr>
\u22ee<\/th><\/tr><\/tbody><\/table><\/div><\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>
I'll finish with a quick visual sanity check of the results.<\/span><\/div>
L = bwlabel(A);<\/span><\/span><\/div><\/div>
L_touches_border = ismember(L,find(props.TouchesAnyBorder));<\/span><\/span><\/div><\/div>
imshowpair(A,L_touches_border)<\/span><\/span><\/div>
<\/div>
<\/div>
<\/div>
<\/div><\/div><\/div><\/div><\/div>\r\n