How can I use OpenCV in Python to draw rectangles around some regions within an image for object detection purposes?
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please don't try with the old cv module, use cv2:
import cv2
cv2.rectangle(img, (x1, y1), (x2, y2), (255,0,0), 2)
x1,y1 ------
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--------x2,y2
[edit] to append the follow-up questions below:
cv2.imwrite("my.png",img)
cv2.imshow("lalala", img)
k = cv2.waitKey(0) # 0==wait forever
berak
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I just tried importing cv2. I get: `DLL load failed: %1 is not a valid Win32 application`. I'm running win7 64-bit. And spyder and scikit libraries and everything's running 64-bit. – user961627 May 18 '14 at 10:42
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@user961627 download 64 bit install binaries from here http://www.lfd.uci.edu/~gohlke/pythonlibs/#opencv – M4rtini May 18 '14 at 11:21
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1Thanks! I've downloaded it... any idea how to make it work with Anaconda's Spyder? – user961627 May 18 '14 at 12:22
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Okay I've got it working on Spyder. @berak, how can I display or save the image? Should I assign it to a numpy array? I tried **ig = ImageViewer( cv2.rectangle(image1, (10, 10), (100, 100), (255,0,0), 2)) ig.show()** but got this error: **TypeError: Layout of the output array img is incompatible with cv::Mat (step[ndims-1] != elemsize or step[1] != elemsize*nchannels)** – user961627 May 18 '14 at 14:28
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how to find these co-ordinates x1,y1 and x2,y2 ? – TheCodeCache Apr 19 '18 at 07:05
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@Manoranjan you'll have to either experiment by starting the rectangle somewhere random on the image and adjusting the coordinates from there, or use another graphics application that you draw the box where you'd like it and pull the coordinates from that. – Eron Lloyd Jun 28 '18 at 11:32
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@berak, I have coordinates in a list called `bbox[0` to `bbox[3]`. When I do `cv2.rectangle()` it works and I know bottom left is (bbox[0], bbox[3]` and top right is `bbox[2], bbox[1]`. But when I go to crop, my y's are reversed so I end up with a crop of shape (0, width, channels). – mLstudent33 Aug 28 '19 at 06:49
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Hi, how do you draw a dotted line instead of a normal line? I tried referring to the documentation adding the parameter of lineType= but it don't seems to work. – babo Jun 04 '20 at 07:42
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1Does the coordinates system of CV start from (0, 0) (Being top left or (1, 1)? – Royi Aug 15 '20 at 07:25
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As the other answers said, the function you need is cv2.rectangle(), but keep in mind that the coordinates for the bounding box vertices need to be integers if they are in a tuple, and they need to be in the order of (left, top) and (right, bottom). Or, equivalently, (xmin, ymin) and (xmax, ymax).
Yanfeng Liu
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You can use cv2.rectangle():
cv2.rectangle(img, pt1, pt2, color, thickness, lineType, shift)
Draws a simple, thick, or filled up-right rectangle.
The function rectangle draws a rectangle outline or a filled rectangle
whose two opposite corners are pt1 and pt2.
Parameters
img Image.
pt1 Vertex of the rectangle.
pt2 Vertex of the rectangle opposite to pt1 .
color Rectangle color or brightness (grayscale image).
thickness Thickness of lines that make up the rectangle. Negative values,
like CV_FILLED , mean that the function has to draw a filled rectangle.
lineType Type of the line. See the line description.
shift Number of fractional bits in the point coordinates.
I have a PIL Image object and I want to draw rectangle on this image. I want to use opencv2 and draw rectangle, and then convert back to PIL Image object. Here is how I do it:
# im is a PIL Image object
im_arr = np.asarray(im)
# convert rgb array to opencv's bgr format
im_arr_bgr = cv2.cvtColor(im_arr, cv2.COLOR_RGB2BGR)
# pts1 and pts2 are the upper left and bottom right coordinates of the rectangle
cv2.rectangle(im_arr_bgr, pts1, pts2,
color=(0, 255, 0), thickness=3)
im_arr = cv2.cvtColor(im_arr_bgr, cv2.COLOR_BGR2RGB)
# convert back to Image object
im = Image.fromarray(im_arr)
jdhao
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