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Can anyone suggest me a fast way of getting the foreground image?

Currently I am using BackgroundSubtractorMOG2 class to do this. it is very slow. and my task doesn't need that much complex algorithm.

I can get a image of the background in the binging. camera position will not change. so I believe that there is a easy way to do this.

I need to capture a blob of the object moving in front of the camera. and there will be only one object always.

Cœur
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user2389323
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1 Answers1

5

I suggest to do as following, simple solution:

  1. Compute difference matrix:

    • cv::absdiff(frame, background, absDiff);

    This makes each pixel (i,j) in absDiff set to |frame(i,j) - background(i.j)|. Each channel (e.g. R,G,B) is procesed independently.

  2. Convert result to single-channeled monocolor image:

    • cv::cvtColor(absDiff, absDiffGray, cv::COLOR_BGR2GRAY);

  3. Apply binary filter:

    • cv::threshold(absDiffGray, absDiffGrayThres, 0, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);

    Here we used Ots'u Method to determine appriopriate threshold level. If there was any noise from step 2, binary filter would remove it.

  4. Apply blob detection in absDiffGrayThres image. This can be one of built-in opencv method's or manually written code which look for pixels positions which vale are 255 (remember about fast opencv pixel retrieval operations)

Such process is enough fast to manage with 640x480 RGB images with frame rate at least 30 fps on quite old Core 2 Duo 2.1 GHz, 4 GB RAM without GPU support.

Hardware remark: be sure that your camera lense aperture is not set to auto-adjust. Imagine following situation: you computed a background image on the beginning. Then, some object appears and covers bigger part of camera view. Less light comes to the lense and, beacause of auto light adjustment, camera increases aperture, background color changes, difference gives a blob in place where actually there is not any object.

marol
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  • Hi! I tried this method too before i post the question... But it was not successful at all. probably due to the automatic aperture adjusting... how can i turn it off?? – user2389323 Jun 19 '13 at 15:11
  • Not successfull it terms of runtime speed or given results? – marol Jun 19 '13 at 15:23
  • Turning off aperture adjustment depends on camera directly. You'd better find if camera's drivers have such possibility. For example, if you have exterior webcam, there should be such option in webcam control panel. – marol Jun 19 '13 at 15:29