Movement/Motion Detection

Any pixel that is different enough in color or intensity is preserved; pixels that have not changed are set to black (default). To change the sensitivity of this comparison increase or decrease the ‘Difference Amount’ within the movement dialog interface.

To create a movement mask (i.e. set changed pixels to white) click to ‘Set Movement to White’.

If you want to show the pixels that are still uncheck the ‘Set Still to Black’.

To prevent still images from being further processed you can chose to stop processing of the image processing pipeline if the movement detected is below a certain percent of the image size or if no movement is detected above a certain threshold. Setting either will stop image processing and continue with the next frame grabbed from the video source. This is particularly useful when streaming the video source to an AVI file where you only want to record images with movement and not still images.

For global movements or large objects use the ‘Stop below Movement Percent’ as this ensure that small image aberrations do not make it through. Often due to overhead lights or specula effects pixels can abruptly change for an instance to a different color/intensity. However, if small movements need to be detected use the ‘Stop below Pixel Difference’ which will stop processing as long as all pixel differences are below the specified amount. With this condition even a single pixel above the specified value will allow the image to continue thought the processing pipeline.

Unchecking both ‘Set Movement to White’ and ‘Set Still to Black’ is a great way to stabilize a still image and remove the flickering effects due to overhead lighting or poor quality cameras.

Note that using a mean filter before the movement detected is often recommended as another way to reduce any abrupt pixel changes. Spurious pixel values will be more common on lower quality cameras so the values you use may need to be updated when changing cameras.

Interface

Instructions

1. Background Image - Select the reference frame to use when comparing frames for movement. The more frames used the more stable the reference image will be but changes are detected slower.
2. Continuous Update - Specify if the reference frame should be continuously updated or stop after the specified number of frames have occurred.
3. Threshold - Specify the pixel difference amount that needs to be exceeded in order for movement to be detected.
4. Technique - Select which type of comparison you want.

Texture Difference - a small window is used to calculate a measure of texture within the window. This amount is also calculated in the background image. The difference of the two is then used as the threshold value. This allows you to tell when a part of an image's texture (i.e. smooth or not smooth) changes. Note that this is less lighting sensitive as the texture measure is relative to the local window.
5. Anti - Auto Adjustment - if your camera does not provide the option to freeze white balance or auto exposure select this checkbox. The current image's lighting will then be adjusted (reversed) to the previous image's lighting parameters to ensure that comparisons are done within the same lighting conditions.
6. Set Movement to White will set any movement detected to white. This can be used to create a movement mask.
7. Set Still to Black will set any non-movement pixels to black.
8. When specifying the RGB Percent Difference black pixels will contain a lot of noise and therefore should be removed. This threshold specifies at what low level pixel values should be ignored when calculating the percent difference.
9. Stop Below Movement Percent specifies how much movement needs to be detected for the module to signal that something has changed in the image. If nothing is detected then the entire image is set to black.

10. Stop Below Pixel Difference specifies that if all pixel difference values are below the specified amount then the image will be considered still even if the percentage of pixels changed exceeds the above percent.

Notes

Ideally to detect movement your camera needs to maintain the lighting conditions from one image frame to the next. You may be able to turn off auto-exposure or auto-shutter speeds by examining your camera options. Select the options button in the main RoboRealm dialog, then one of the Video Capture or Video Format buttons to see if your camera provides a manual override of setting these parameters. You can test the lighting issue by setting the movement detector to use the last 50 previous frames to calculate the RGB difference. Once these settings have been made the current image will eventually go to black assuming no movement is within the camera's view. If you then hold up a white sheet of paper in front of the camera you will suddenly see the entire background change since the white sheet of paper will have caused your camera to shift intensity values. This changing of pixel values moves the pixel intensity outside the "difference amount" and therefore the entire image will appear to have moved. Switching the camera to manual mode will prevent this intensity shift from happening. If you are not able to switch to manual mode select the 'Anti - Auto Adjustment' checkbox which will attempt to undo these lighting changes by your camera.

When using the movement detector you may notice issues with black streaks or spots in the resulting movement mask. These spots are caused by dark background pixels. Ideally to create an appropriate mask (similar to green screen or blue screening) your background should be as planar as possible and not include the color that the foreground will. For example, using a white sheet as a background and then standing in front of the background with a white shirt on will not allow you to segment your white shirt ... the shirt and white background are considered as the same color and therefore no movement will be detected. This also frequently happens for black areas since the absence of light in shadows will cause confusion if your foreground object has any shadows in it.

Variables

MOVEMENT_PIXELS - number of pixels that have changed from frame to frame
MOVEMENT_PERCENT - percent of pixels that have changed from frame to frame

See Also

Optical Flow
Average

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