The Math Behind Megapixels

Jan 1, 2008 12:00 PM, By Paul Bodell

The advantage of this technique is that users can save a lot of bandwidth and storage when there is little motion or activity by sending only specific changes to the key frame. One drawback is that only the key frame is a true “legal” picture. Another drawback is that motion results in a significant increase in bandwidth consumption, thereby reducing the bandwidth advantage over frame-by-frame compression.

In reality, compression is not significant for the following analysis because the examples below will hold true as long as you are not “mixing apples and oranges.” This guide is comparing megapixel cameras with low-resolution cameras to determine if and when deploying megapixel cameras make sense. Essentially, it is simply comparing different sized apples. For the following analysis, assume the user wants the highest quality images available and will, therefore, use a frame-by-frame compression such as MJPEG.

When does megapixel make sense? Let's use the parking lot example, but add depth of coverage. A user wants to cover a parking lot with a forensic detail installation of 40 pixels-per-foot, and the lot is 100-ft. wide. The user also needs to cover multiple rows of cars at a depth of 60 ft. To recognize faces or license plates, the user would need 40 pixels-per-foot × 100 ft. or 4,000 pixels across by 40 pixels-per-foot × 60 ft. or 2,400 pixels-deep. The total number of pixels required is determined by multiplying the number of pixels across by the number of pixels-deep. The total is 9,600,000 or 9.6 megapixels.

To determine the amount of bandwidth required for each option, start by calculating the size of each image while keeping the compression for all cameras the same. Then multiply that image size by the images-per-second to get the bandwidth requirements. For our purposes, we are using a compression guideline found at Milestone's Web site: http://www.milestonesys.com/?cid=419. According to Milestone, a 640 × 480 resolution image with medium-low compression has a file size of approximately 50 kB. For higher resolution images we are using IQinVision's comparison matrix for megapixel cameras at http://www.iqeye.com/iqeye/images/uploads/File/Sales_Datasheets/Product_Specs.pdf, which shows that a typical 3.1-megapixel image will have a 225 kB file size.

An “apples-to-apples” comparison reveals that megapixel cameras can deliver the same image quality for about half of the bandwidth. However, most people will not employ cameras to cover the entire area. To save money, users are more likely to use cameras only at critical “choke points,” or areas in which something a user wants to record is likely to happen. Therefore, the 35 to four camera comparison is not practical.

In a choke point scenario, a single megapixel camera would use 33 percent less bandwidth than using 4 CIF, 640 × 480 resolution cameras.

The case for mechanical PTZ cameras

Mechanical pan/tilt/zoom (PTZ) cameras consist of motors, slip rings, gears and/or belts and wheels. Traditionally, the cameras were controlled by proprietary keyboards. Now, they can be controlled by software. These devices are available as integrated units — most commonly as domes — or they can purchased separately and mounted on a pan/tilt motor. The amount of zoom, (e.g‥ 25x) depends on the ratio of the telephoto setting to the wide-angle setting. So, a lens that zooms from 4mm to 100mm will have a 25x zoom. A lens that zooms from 50mm to 150mm will only provide 3x zoom but will be much better at zooming in farther away because of the 150-mm telephoto setting. It won't, however, have a wide-angle setting (50mm) comparable to the 25x lens (4mm).

Today, most mechanical PTZ cameras employ low-resolution cameras, typically around 704 × 480 resolution or 1/3 of a megapixel. In order to get high-detail (80 pixels-per-foot) with that camera, a user would need to zoom into an area that is no wider than 8 ft. If the user zoomed out all the way, he or she would now be spreading those pixels out over a wide area and would lose all detail.

If the user's goal is to give security staff the ability to monitor general activity and then zoom in over a long distance with good detail, mechanical PTZ cameras with high-zoom telephoto lenses are the way to go.

Mechanical PTZ cameras should be thought of in an “either/or” context. The user either gets wide-area coverage with low-resolution or high resolution in a very narrow area. The user cannot have both. This means if the camera is “zoomed in,” the user will miss everything else. Alternatively, if the camera is “zoomed out,” the user will not have any detail. Users who record “zoomed out” mechanical PTZ images can always “digitally zoom” after the fact, but the results are not always reliable. This limitation places a great deal of responsibility on the guard who controls the camera, especially if there are simultaneous incidents, in which case the guard must decide which is more important.

Another point to consider with mechanical PTZ cameras is the cost of round-the-clock guards dedicated to controlling the cameras. It can be a substantial amount of money. Moreover, if a guard controls the cameras over a TCP/IP network, there is often a long delay between when he or she tells a camera to pan, tilt and zoom and when it actually responds. Mechanical PTZ cameras also have lots of moving parts that wear and require periodic maintenance and repair.

The case for digital PTZ cameras

Megapixel network cameras are ideal for applications where a user does not have the resources to conduct round-the-clock live monitoring and, therefore, has to rely on the forensic value of recorded video. When used with the right lens, megapixel network cameras ensure that there will always be enough forensic detail for users to go back after an incident to determine what happened.

Users using megapixel network cameras only have to make sure they have selected the proper resolution camera and lens to provide the desired detail (pixels-per-foot). Once configured, a user can digitally pan, tilt and zoom around live images without affecting what is recorded. In addition, multiple people can connect to the same camera at the same time and independently pan, tilt and zoom. Regardless of where someone may be digitally panning, tilting and zooming, the user can always go back to recorded video and look at other areas with no loss of detail. Additionally, since these cameras have no moving parts, there are virtually no maintenance requirements.

The benefits of using megapixel cameras seem to be multiplying, and the decision to switch to megapixel technology depends on a number of factors. Most importantly, users should determine the goals of the application and weigh the costs associated with installing and supporting a megapixel camera installation over time. Nevertheless, new storage capabilities, opportunities to save bandwidth and the increasing benefits of digital technology all make the move to megapixel increasingly attractive.

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Paul Bodell is chief marketing officer of IQinVision, San Clemente, Calif., a provider of high-performance network cameras, intelligent video processors and network video recording equipment. He joined IQinVision in 2002 with more than 17 years of international sales management, marketing and product marketing experience. He has been in the electronic security industry since 1994.

PRICE COMPARISON
Megapixel is cheaper

ITEM	PRICE	QUANTITY	TOTAL
640 × 480 camera	$350	7	$2450.00
Housing	$225	7	$1575.00
Cable	$20	7	$140.00
Labor	$100	7	$700.00
TOTAL			$4865.00

ITEM	PRICE	QUANTITY	TOTAL
2048 × 1536 camera	$1249	2	$2498.00
Housing	$225	2	$450.00
Cable	$20	2	$40.00
Labor	$100	2	$200.00
TOTAL			$3188.00

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© 2008 Penton Media Inc.

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