Opencv ROI fills more than it should

Question

When I try to set frame ROI it fills up empty pixels, from given x to the end of input image. ROI is extracted well, can anyone tell me why isn't it 30 pixels wide as it should be? Image is resized from original size to 640x360.

resize(frame, frame, Size(), 0.5, 0.5, INTER_LINEAR);
Mat roi = img(Rect(460, 0, 30, 300));

Image is shown below. Original image to the left and ROI to the right

Yes. The `cv::Rect` ctor takes: xOffset, yOffset, width and height. You asked for width 30 and height 300. — Adi Shavit, May 19 '15 at 13:44
Actually I thought that it will show me only the area I want @AdiShavit. I expected only 30x300 rectangle, and it shows this additional whitish pixels which I didn't expect — Moirae, May 19 '15 at 13:50
Ah, yes. On some systems there is a minimum window width/height/size. — Adi Shavit, May 19 '15 at 13:51
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/78207/discussion-between-moirae-and-adi-shavit). — Moirae, May 19 '15 at 13:53

Adi Shavit · Accepted Answer · 2015-05-21T19:57:08.353

1

Your ROI looks fine. The cv::Rect ctor takes: xOffset, yOffset, width and height. You asked for width 30 and height 300.

However, on some UI systems there is a minimum window width/height/size. Displaying images smaller than this minimum will add an extra blank region on the side which is not part of the actual cv::Mat.

edited May 21 '15 at 19:57

answered May 19 '15 at 13:57

Adi Shavit

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I agree, I have seen this before, also in OSX – Drodbar May 19 '15 at 14:46
@Drodbar actually I am using OSX at the moment. Last time I was working with OpenCV it was on Linux and I can't remember this happening. – Moirae May 21 '15 at 19:56

Opencv ROI fills more than it should

1 Answers1