Defining Zones

Zone configuration and tuning are important when running in the motion detection modes to avoid storing, sorting through, or being alerted on uninteresting video data. Configuring a zone involves setting some basic parameters, as well as choosing an alarm check method and tuning their associated detection parameters.

The Zone view is split into two main areas, on the left is the options are area and on the right is the zone drawing area. A default or new zone will cover the whole drawing area and will overlay any other zones you already have on there. Unlike the previous zones image, the current zone is coloured green, other zones will be orange regardless of type. The smaller the zone, the less processing time it takes to examine it.

Basic parameters

Name

Each Zone can be named for reference purposes. It is used for logging and debugging. Choose a name that helps you identify your zones.

Type

This is one of the more important concepts in ZoneMinder and there are five to choose from.

• Active

  Triggers an alarm when motion is detected within it. This is the zone type you'll use most often, and which will be set for your default zone. Only Active and Exclusive zones can trigger an alarm.

• Inclusive

  This zone type can be used for any zones that you want to trigger an alarm only if at least one other Active zone has already triggered one. This might be for example to cover an area of the image like a plant or tree which moves a lot and which would trigger lots of alarms. Perhaps this is behind an area you'd like to monitor though, in this case you'd create an active zone covering the non-moving parts and an inclusive zone covering the tree perhaps with less sensitive detection settings also. If something triggered an alarm in the Active zone and also in the Inclusive zone they would both be registered and the resulting alarm would be that much bigger than if you had blanked it out altogether.

• Exclusive

  Triggers an alarm when motion is detected within it, as long as no alarms have already been triggered in an Active zone. This is the most specialized of the zone types. For instance in the camera covering my garden I keep watch for a hedgehog that visits most nights and scoffs the food out of my cats bowls. By creating a sensitive Exclusive zone in that area I can ensure that a hedgehog alarm will only trigger if there is activity in that small area. If something much bigger occurs, like someone walking by it will trigger a regular alarm and not one from the Exclusive zone. Thus I can ensure I get alarms for big events and also special small events but not the noise in between.

• Preclusive

  This zone type is relatively recent. It is called a Preclusive zone because if it is triggered it actually precludes an alarm being generated for that image frame. So motion or other changes that occur in a Preclusive zone will have the effect of ensuring that no alarm occurs at all. The application for this zone type is primarily as a shortcut for detecting general large-scale lighting or other changes. Generally this may be achieved by limiting the maximum number of alarm pixels or other measure in an Active zone. However in some cases that zone may cover an area where the area of variable illumination occurs in different places as the sun and/or shadows move and it thus may be difficult to come up with general values. Additionally, if the sun comes out rapidly then although the initial change may be ignored in this way as the reference image catches up an alarm may ultimately be triggered as the image becomes less different. Using one or more Preclusive zones offers a different approach. Preclusive zones are designed to be fairly small, even just a few pixels across, with quite low alarm thresholds. They should be situated in areas of the image that are less likely to have motion occur such as high on a wall or in a corner. Should a general illumination change occur they would be triggered at least as early as any Active zones and prevent any other zones from generating an alarm. Obviously careful placement is required to ensure that they do not cancel any genuine alarms or that they are not so close together that any motion just hops from one Preclusive zone to another. Preclusive zones may also be used to reduce processing time by situating one over an Active zone. The Preclusive zone is processed first; if it is small, and is triggered, the rest of the zone/image will not be processed.

• Inactive

  Suppresses the detection of motion within it. This can be layered on top of any other zone type, preventing motion within the Inactive zone from being effective for any other zone type. Use inactive zones to cover areas in which nothing notable will ever happen or where you get false alarms that don't relate to what you are trying to monitor. Inactive zones may be overlaid on other zones to blank out areas, and are processed first. As a general practice, you should try and make zones abut each other instead of overlapping to avoid repeated duplicate processing of the same area.

Preset

The preset chooser sets sensible default values based on computational needs (fast v. best) and sensitivity (low, medium, high.) It is not required that you select a preset, and you can alter any of the parameters after choosing a preset. For a small number of monitors with ZoneMinder running on modern equipment, Best, high sensitivity can be chosen as a good starting point.

Units

• Pixels - Selecting this option will allow many of the following values to be entered (or viewed) in units of pixels.

• Percentage - Selecting this option will allow may of the following values to be entered (or viewed) as a percentage. The sense of the percentage values refers to the area of the zone and not the image as a whole. This makes trying to work out necessary sizes rather easier.

Region points

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The sample region shown to the right shows a region defined by 6 control points. The shape of the region causes the check methods to ignore the sidewalk and areas of the porch wall that receive changing sunlight; two conditions that are not of interest in this zone.

A region is a part of the captured image that is of interest for this zone. By default, a region is configured to cover the whole captured image. Depending on the selected type of this zone, the shape of the region can be adjusted to accommodate multiple effects. This can be done by dragging the control points in the reference image around, or by altering the coordinates found in the controls below the reference image. Clicking on a control point in the reference image highlights the coordinates in the table below. Clicking the + button in a point row adds a control point between this point and the next; clicking the - button removes this control point. It is possible to accidentally place a control point outside of the valid coordinates of the image. This will prevent the monitor from working properly. You can make zones almost any shape you like; except that zones may not self-intersect (i.e. edges crossing over each other).

Alarm Colour

These parameters can be used to individually colorize the zone overlay pattern. Alarms in this zone will be highlighted in the alarm colour. This option is irrelevant for Preclusive and Inactive zones and will be disabled.

Alarm Check Methods

There are 3 Alarm Check Methods. They are sequential, and are layered: In AlarmedPixels mode, only the AlarmedPixel analysis is performed. In FilteredPixels mode, the AlarmedPixel analysis is performed first, followed by the AlarmedPixel analysis. In the Blobs mode, all 3 analysis methods are performed in order. An alarm is only triggered if *all* of the enabled analysis modes are triggered. For performance reasons, as soon as the criteria for one of the analysis modes is not met, the alarm checking for the frame is complete. Since the subsequent modes each require progressively more computations, it is a good idea to tune the parameters in each of the activated layers.

For reference purposes, the Zone Area box shows the area of the entire region of interest. In percent mode, this is 100. In Pixels mode, this is the pixel count of the region. All 3 Min/Max Area parameter groups are based on the Zone Area as the maximum sensible value, and all 3 are interpreted in the units specified in the Units input.

AlarmedPixels

Alarmed pixels is the first layer of analysis, and is always enabled. Its recommended that you start with this method and move on to the subsequent methods once the effects of the basic parameters are understood. In the AlarmedPixels mode, 2 parameter categories are available for tuning: Min/Max Pixel Threshold, and Min/Max Alarmed Area.

Min/Max Pixel Threshold (0-255)

In the AlarmedPixel layer of analysis, each individual pixel of the image is compared to the current reference image. Pixels that are different from the reference image are considered alarmed pixels. However, small aberrations in lighting or auto exposure camera adjustments may cause the explicit value of a pixel to vary by small amounts from image to image. This parameter allows you to set the limits of what will be considered a changed pixel. For example, if your camera points to a blank white wall, and you raise a black colored item into view, then the change in any one pixel will be great, indeed, extreme. If however, you raise a white piece of paper, then the change in an individual pixel will be less.

The minimum pixel threshold setting should be high enough to cause minor lighting, imaging, or compression changes to be ignored. Setting the minimum value too high, may allow a white cat to walk undetected across the view of the white wall. A good starting point for the minimum pixel threshold is 40, meaning that the difference in pixel value from must be greater than 40. A good default for the maximum pixel threshold is 0 (indicating that all differences above the minimum threshold are considered a change.)

Min/Max Alarmed Area

The count of alarmed pixels (or percentage of alarmed pixels relative to the pixel area of the region if in percent mode) is used in this first layer of analysis to determine if an alarm is triggered. If the count or percentage is above the minimum alarmed area, but less than the maximum alarmed area, an alarm is triggered. These settings depend on the size of the object you are trying to capture: a value too low may cause false alarms, while a value too high might not detect small objects. A good starting point for both the minimum and maximum are 0 and 0, indicating that any number of alarmed pixels (or any percentage) greater than 0 will trigger an alarm. The frame scores from logged events can then be used to bring the minimum up to a reasonable value. An alternative starting point for the minimum alarmed area (in percent) is 25% of the area that an object of interest takes up in the region. For example, if you approximate that a subject moving through the frame takes up 30% of the frame, then a good starting minimum area is about 7.5%.

FilteredPixels

Selecting the FilteredPixels Alarm Check Method adds an additional layer of analysis to the AlarmedPixels check along with 2 additional parameter categories for tuning. This layer works by analyzing the alarmed pixels identified in the first layer. Alarmed pixels are disregarded, in this and future layers if enabled, if they are not in groups of a minimum small square size. Primarily, this filtering removes isolated alarmed pixels that may be artifacts of the camera, lens, or compression.

Filter Width/Height (pixels)

This parameter is always specified in Pixels, even when Percentages are the selected units. It specifies the size of the group of pixels surrounding a given pixel that must be in alarmed pixels for the pixel itself to be considered an alarmed pixel. The width and height should always be an odd number. 3 x 3 is the default value, and 5 x 5 is also suggested as a sensible alternative. Avoid using large numbers for the width and height of the filter area. When using the Blobs Alarm Check Method, FilteredPixels can be effectively disabled by setting either the width or height to a value less than 1.

Min/Max Filtered Area

Applying the filtering analysis results in an area that is less than or equal to the alarmed area. Thus the minimum and maximum filtered area parameters for alarm should be equal to or less than the corresponding alarm area parameters, or the FilteredPixels analysis will never trigger an alarm. In particular, it is useful to raise the minimum alarmed area parameter until false events from image artifacts disappear, and setting a minimum filtered area parameter less the minimum alarmed area parameter by enough to capture small events of interest.

Blobs

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This captured frame shows an image with 1 identified blob. The blob is outlined in the Alarm Colour specified above.

When two or more Filtered areas touch or share a boundary, it is sensible to evaluate the regions as one contiguous area instead of separate entities. A Blob is a contiguous area made up of multiple filtered areas. Whereas FilteredPixes is useful for excluding parts of the image that are not part of the actual scene, Blob filtering is better suited to disregarding areas of the actual scene that are not of interest. Selecting the Blobs Alarm Check Method opens up all of the available parameters. Enabling Blobs adds one more layer of analysis to the AlarmedPixel and FilteredPixel checks in the determination of a valid alarm along along with 2 additional parameter categories for tuning: the size of the blobs, and the number of blobs. A Blob is not necessarily the whole object that may be of interest. In the example image, the subject is moving, but only a portion of him is marked as a blob. This is because as the subject moves, many pixels of the image do not change in value beyond the set threshold. A pixel that is representing the subject's shoulder in one frame may be representing his back in the next, however, the value of the pixel remains nearly the same.

Min/Max Blob Area

The blob area parameters control the smallest and largest contiguous areas that are to be considered a blob. A good value for the maximum area is the default of 0. (There is no upper bound for the size of a contiguous area that will still be considered a blob.)

Min/Max Blobs

Normally, you would want any positive number of blobs to trigger an event, so the default value of 1 should suffice. In some circumstances, it may benefit to have only one blob NOT trigger an event, in which case, setting this value to 2 or higher may serve some special purpose. A good value for the maximum blobs is the default of 0. (There is no upper bound for the number of blobs that will trigger an event. Use the maximum blobs parameter can be used to tune out events that show a high number of blobs.

Overload Frame Ignore Count

This setting specifies the number of frames to NOT raise an alarm after an overload. In this context, overload is defined as a detected change too big to raise an alarm. Depending on the alarm check method that could be

• Number of alarmed pixels > Max Alarmed Area or
• Number of filtered pixels > Max Filtered Area or
• Number of Blobs > Max Blobs

The idea is that after a change like a light going on that is considered too big to count as an alarm, it could take a couple of frames for things to settle down again.