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ORB - object needs to be very close to camera

asked 2015-02-11 08:24:27 -0500

colin747 gravatar image

updated 2015-03-09 04:35:20 -0500

I have a program that takes a video feed from RSTP and checks for an object. The only problem is that the object needs to be about 6" from the camera but when I use a wired webcam the object can be a few feet away. Both camera are transmitting at the same resolution, what is causing this problem?

Camera transmission specs:

Resolution 640 * 480 FPS 20 Bitrate 500000

EDIT: Additional answer here may be of interest to others: http://stackoverflow.com/questions/28...

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answered 2015-02-12 13:47:47 -0500

Doombot gravatar image

Let's do some maths:

Your camera has a 640x480 resolution, is 6 inches away from the scene and has a lens with some focal length (I don't know, look on the spec). So, imagine that, given these parameters, 1 pixel on the image represent 1x1 mm square (or about 0.04'') in reality (this is a random value I chose, it actually depends of the focal length of your lens, the size of the captor, etc.) (If you don't want to calculate, just print a calibration grid with squares of a known dimension and count how many pixels are used to represent a single square...)

Now, you move the exact same camera to a distance of 48'' (4 feet), which is 8 times farther. Then, 1 pixel will represent a region of 8x8mm (about 0.32''). So, your number of pixels per inch has actually decreased! It means that you are no longer able to see fine details. So, when you do an ORB detection on the image, you actually no longer have the same image than in the first setup, or maybe the number of pixels per inch is too low to represent meaningful keypoint.

So, it seems to be the cause of the problem. Now, how to solve it? Well, it is not possible if you keep the same setup. I assume that you cannot place the camera close enough (6''), so you may need to use a lens providing a relevant level of zoom, or choose another camera with a sensor that allow an higher number of pixels per inch. Note that I assume all of this make some sense to you. If not, just ask in a comment and I'll try to point you to the right learning resources...

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That doesn't explain why he gets better results with a different camera (but same resolution)

FooBar gravatar imageFooBar ( 2015-02-12 14:25:59 -0500 )edit
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Oh I completely misread his question!!! I'll let the OP assess if any of this is relevant then remove my answer if this is not usefull at all...

Doombot gravatar imageDoombot ( 2015-02-12 15:12:12 -0500 )edit
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Thanks for an interesting answer, I'd come to a similar conclusion that it was due to the quality of the camera though I'm still not sure exactly what aspect of the camera is letting it down. I'd be reluctant to remove your answer as I still find it relevant to the problem and it may help people in the future. If you would have any resources on how to calculate this out? As it would be helpful to me if I could show on paper why this problem is occurring rather than asking people to take my word for it.

colin747 gravatar imagecolin747 ( 2015-02-13 03:29:38 -0500 )edit

This site (http://www.vision-doctor.co.uk/optics...) has some resources, especially an optical calculator and other tutorials. I would advise to look at various parts of the site and on the internet so you will get more knowledge in order to better explain the situation to your colleagues. Also, I would not immediately put the blame on the "quality" of the cameras. See it as choosing the right tool with the right specs for a given task. Finally, depending of your situation, you might want to get in touch with the distributor of the camera (the "seller"). In my experience, they are valuable assets in these situations because they can help you to select the right camera with the right parameters (lens, resolution, etc.).

Doombot gravatar imageDoombot ( 2015-02-13 09:13:34 -0500 )edit
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Asked: 2015-02-11 08:24:27 -0500

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Last updated: Mar 09 '15