Resolution TutorialBy IPVM Team, Published Dec 28, 2017, 09:04am EST
Understanding video surveillance resolution can be surprisingly difficult and complex. While the word 'resolution' seems self-explanatory, its use in surveillance is far from it.
In this tutorial, we will explain 5 critical elements:
- What resolution traditionally means – seeing details - and the constraints of this approach
- What resolution usually means in surveillance – pixels – and the limits of using this metric
- How sensor and stream resolutions may vary
- How compression impacts resolution greatly
- What limits resolution's value
Resolution: Seeing Details
In normal English and general usage, resolution means the ability to resolve details – to see or make them out. For example, can you read the lowest line on an eye chart? Can the camera clearly display multiple lines side by side on a monitor? It is a performance metric focusing on results.
Historically, video surveillance used a similar test chart approach. Analog camera resolution was measured with line counts, literally the camera's ability to display more lines side by side in a given area on a monitor.
If you could see more lines, it meant you theoretically could see more real world details: facial features, characters, license plates, etc.
For example, in our testing, we found 1080p cameras to be equivalent to roughly 900 'lines', 5MP equivalent to 1,000, etc. as shown in the image below:
Problem: TVL Only Tested In Ideal Conditions
However, TVL tested is only intended to be done in even lighting conditions. With direct sunlight, backlight, or low light, TVL measurements dropped significantly. Because of this, TVL measurements do not account for overexposure, encoding noise and artifacting, IR illumination effects, or other issues present in the real world.
Resolution: Pixel Count
In marketing IP cameras, manufacturers do not even attempt to measure performance. Instead, resolution has been redefined as counting the number of physical pixels that an image sensor has.
For example, a 1080p resolution camera is commonly described as having 2MP (million pixel) resolution because the sensor used has ~2 million pixels on it (technically 2,073,600 pixels as that is the product of 1920 horizontal x 1080 vertical pixels). The image of an imager below shows this example:
For more details on image sensors, see our Surveillance Camera Imager Tutorial.
Limitations On Resolution
The presumption is that more pixels, much like higher line counts, delivers higher ‘quality’. However, this is far from certain.
Just like with classic resolution measurements that used only ideal lighting conditions, measuring pixels alone ignores the impact of common real world surveillance lighting challenges. Often, but not always, having many more pixels can result in poorer resolving power in low light conditions. Additionally, cameras with lower pixel counts but superior image processing can deliver higher quality images in bright sunlight / WDR scenes.
Pixels Determine Potential, Not Quality
Nonetheless, pixels are a cornerstone of specifying IP video surveillance. Without a minimum number of pixels for a given area / target, it is impossible. See our tutorial on why Pixels Determine Potential, Not Quality.
Common Surveillance Resolutions
The table below summarizes the most common resolutions used in production video surveillance deployments today. Note that VGA is no longer common except in thermal cameras, but is included here for reference of what 'standard definition' refers to.
Changes For 2021
Compared to past years, there were few notable changes in camera resolution in 2020. 1080p remains most common, but is beginning to decline, with 4MP, 5MP, and higher growing. See Camera Resolution Usage Statistics 2020 for full details.
Camera max resolution advanced moderately in 2020, but releases remain limited. Hanwha released their 8K / 33MP camera (see our test) and Avigilon has now announced their H5 Pro series with resolution up to 61MP (shipping early 2021).
Aside from these releases, there has been limited movement to 8K and higher resolutions. Even if more models were released in 2021, they are likely to remain niche due to the larger size and high cost for the next few years.
Resolution Vs. Cost
Everything else equal, higher resolution cameras generally cost more than lower cost models, though pricing for higher resolutions has decreased significantly from where it once was, formerly limited to higher end models for 5MP and 4K resolution. These resolutions are now common across product lines, from low cost fixed lens models to high end cameras with advanced features.
Additionally, note that higher cost for higher resolution does not always result in higher performance, as advanced features such as super low light and true WDR are not always supported or as high performing in higher resolution models, or requiring a significant increase in cost. For example, 1080p cameras most commonly offer strong WDR and super low light options, while higher resolution models may lack one or more of these features.
Sensor Resolution vs. Stream Resolution
While manufacturers typically specify cameras based on the resolution (i.e. pixel count) of the sensor, sometimes, the resolution of the stream sent can be less. This happens in multiple cases:
- Limited camera capabilities: In some cases, manufacturers may use readily available sensors of one resolution but crop the sensor to a lower pixel count due to limitations in processing at full resolution. For example, a 6MP sensor may be cropped to 5MP in order to stream at higher frame rates or apply WDR or higher gain levels.
- Panoramic cameras: Second, manufacturers often crop unused portions of the sensor from panoramic camera streams, so a "12MP" fisheye model may actually stream at 8-9MP. See our report Beware Imager vs Stream Resolution for more information on this issue.
- Reduced in software: Finally, an installer may explicitly or mistakenly set a camera to a lower resolution. Some times this is done to save bandwidth but other times it is simply an error or glitch in the VMS default resolution configuration. Either way, many times an HD resolution may look ‘terrible’ but the issue is simply that it is not set to its max stream resolution (i.e., a 3MP camera set to 640 x 480).
Because of these issues, users should be sure to check not only the resolution of the sensor but the stream resolutions supported and used, typically found lower down the camera's datasheet:
Compression Impact On Resolution
Because resolution most often simply means pixel count, no consideration is given to how much pixels are compressed. Each pixel is assigned a value to represent its color, typically out of a range of ~16 million (24 bits), creating a huge amount of data. For instance, a 1080p/30fps uncompressed stream is over 1Gb/s. However, surveillance video is compressed, with that 1080p/30fps stream more typically recorded at 1Mb/s to 8Mb/s — 1/100th to 1/1000th less than the uncompressed stream. The only question — and it is a big one — how much is the video compressed?
Just because two cameras have the same resolution (i.e. pixel counts), the visible image quality could vary substantially because of differences in compression levels chosen. Here is an example:
Choosing Compression Levels
Picking the right compression level can be tricky. How much compression loss can be tolerated often depends on subjective preferences of viewers or the details of the scene being captured.
Equally important, increasing compression can result in great savings on hard drive costs (less storage required for similar durations), server configuration (less CPU required is required to store less bandwidth), and switches (copper gigabit switches may be used instead of fiber 10GbE).
For full coverage of these details, see our video quality / compression tutorial.
Also important for considering compression is that manufacturers default compression settings vary significantly, for more see: IP Camera Manufacturer Compression Comparison.
Angle Of Incidence Is Key
Regardless of how high quality an image is, it needs to be at a proper angle to 'see' details of a subject, referred to as 'angle of incidence'.
For instance, the image below shows a subject at the same distance in an indoor warehouse test scene, at a high angle of incidence on the left and low/zero angle on the right:
Even if the image on the left had 10x the pixels as the one on the right, the left one is incapable of seeing the full facial details of the subject as he is simply not facing the camera.
This is frequently a practical problem in trying to cover wide areas with one high-resolution camera. Even if you can get the 'right' number of pixels, if a person is not facing the camera or a car is driving perpendicular to the camera, you may not have any chance of capturing details.
Historically, surveillance has been starved for resolution, with almost always too little for its needs. Anyone familiar with suspect photos on their local news can see this:
However, as the amount of pixels has increased to 1080p and beyond, the opposite issue presents itself: unnecessarily high resolution for the scene.
Once you have enough to capture facial and license plates details, most users get little practical benefit from more pixels.
The image might look 'nicer' but the evidentiary quality remains the same. This is a major consideration when looking at PPF calculations and ensuring that you do not 'waste' pixels.
Additional Factors Impacting Resolution
Finally, note that beyond issues discussed above, many other factors impact surveillance resolution beyond pixels, including:
- Low light performance
- WDR performance
- Integrated IR performance
- Compression settings
- Camera angle / downtilt
- Lens selection
- Lens focus
Do not accept specified resolution (i.e. pixel count) as the one and only quality metric as it will result in great problems. Understand and factor in all of these drivers to obtain the highest quality for your applications.
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[Note: The tutorial was originally written in 2012 but was revised to show more images / examples and updated information on the state of the market]
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