The perception of an image or a video on a mobile device depends on the resolution of the video, the resolution of the device, the size of display screen, and the human eye.
Due to the size of the screen size on current smart phones, no matter how high the resolution of the phone, or a video file, there is only so much the average human eye can perceive.
In this Best Practice we are going to discuss delivering a video file with a resolution that matches the capability of a mobile device to display it and the human eye to perceive it.
As mentioned, perception of an image or a video depends on resolution, screen size and the human eye.
Video resolution measurement is a complicated subject, and the defining characteristics have changed over time. Also, there is the resolution of the video file and the resolution capability of a device to display it. Here are a few details:
1. Historically, with analog broadcast and older television sets, video resolution was measured by scan lines. But with the advent of digital recording and playback, High Definition, and newer LCD, Plasma, OLED television sets, the video resolution began to be measured in pixels.
2. Another factor to consider with resolution is aspect ratio, which measures the ratio of the width to the height of an image or screen. It defines the overall shape of the image, and it is usually shown as W:H (W is the width and H is the height).
The original film aspect ratio was 4:3 (also known as 1.33:1) originated with Thomas Edison’s lab, early Hollywood movies, and all television until the rise of high definition. Next came the Academy Ratio of 1.37:1 in 1932, which was designed to accommodate sound striping on 35mm film. Eventually with HDTV came 1.78:1 (also known as 16:9), which was engineered so that an HDTV set could display different kinds of video without much adjustment. Standard television production, DVDs, Blu-ray, and 4K all use this ratio.
3. Video resolution was traditionally measured by height, but starting with 2K video that has changed to it being measured by width.
4. As mobile device came along, because of its flexibility, users could watch and record video in either portrait or landscape mode, which affects the resolution and the aspect ratio of the video, and it affects the viewing experience.
5. Resolution on a mobile device is further complicated by the matter of screen size, which can be measured in pixels per inch (PPI) or dots per inch (DPI), which can be determined by dividing the number of pixels, or dots, across, by the size of the screen in inches.
For most of the 20th Century, broadcasters transferred video via an analog signal, which is now referred to as Standard-Definition Television (SD or SDTV). In the Americas, the National Television System Committee (NTSC) system was based on 480 scanned lines.
With the creation of High Definition Television (HD or HDTV) digital video in the 1980’s, substantially higher resolution became possible. The first HD format was 720p, or 720 vertical lines of resolution, at a progressive (the "P") rate of either 24 or 30 frames per second (fps) in the United States.
Below is a table with the ongoing advancements in video resolution, which identifies measurements by height and width.
480p video resolution
= 852 pixels wide x 480 pixels high
720p video resolution
= 1280 pixels wide x 720 pixels high
HD or HD Ready resolution
1080p video resolution
= 1920 pixels wide x 1080 pixels high
Full HD resolution
2K video resolution
= 2048 pixels wide x 1080 pixels high
Called 2K because the width is over 2000 pixels
1440p video resolution
= 2560 pixels wide x 1440 pixels high
QHD or Quad HD resolution, seen on high-end smartphones - 1440p is 4 times the pixels of 720p
4K video resolution
= 3840 pixels wide x 2160 pixels high
Ultra HD or UHD, it’s called 4K because the width is close to 4000 pixels – 2160 is 4 times the pixels of 1080p
8K video resolution
= 7680 pixels wide x 4320 pixels high
Also, Ultra HD or UHD, and called 8K because the width is close to 8000 pixels - 7680 is 16 times pixels of 1080p
The letter "p" in 720p or 1080p stands for progressive scan, which is the common standard today.
In the past you may have seen see the letter "i" after the pixel height, such as 720i or 1080i, which indicated interlaced scanning, which was a different way of “painting’ the electronic image, or delivering the video lines to a display, common from broadcast television and CRT monitors, but out of use today.
2K 4K and 8K Video
Resolution changed further with 2K, 4K, and 8K.
These terms don't refer to specific resolutions. They are resolution categories and approximations. They are also measured differently.
It's called 2K because it's approximately 2,000 pixels horizontally, 4K is approximately 4,000 pixels horizontally, and 8K is approximately 8,000 pixels horizontally.
The purpose for the change in resolution has to do with projection in movie theaters, which require a slightly different aspect ratio from consumer HDTVs and computer monitors, and when the 2048 x 1080 resolution was termed 2K for movie projection, the name carried over to consumer applications like television and mobile devices.
While resolutions have continued explode in size, the screen size on the average smart phone has grown slowly. Screen size is measured diagonally, from a top corner to the lower corner on the opposite side. The average screen size of smartphones runs from under 5 inches to 5.5 inches, generally topping out at 6.5 inches.
As mentioned above, due to the size of the screen size on current smart phones, no matter how high the resolution of a video file, or the phone, there is only so much detail the average human eye can perceive.
Just because you can send a 4K or 8K video file to a mobile device with a 5.5 inch screen size, it doesn’t mean it’s a good idea.
In a usability study conducted by AT&T, test subjects on small screens detected minimal difference in quality between video above 480p, and they may not have perceived any improvement in quality for video sent at resolutions between 720p to 1080p. Higher resolutions than 1080p were not specifically studied but are not expected to increase quality of experience on current generation of small screens.
Since there is no statistical difference in usability ratings for resolutions above 720p, it makes no sense to send video with a higher resolution to smartphones.
And if you simply increase the resolution of a video, the quality will not improve but the video file size will grow. And since the average human cannot tell the difference, that data is all wasted.
Best Practice Recommendation
When preparing a video file for streaming distribution, you need to make some decisions about the resolution.
Sending video at a higher resolution then 720p makes complete sense if the target is a tablet, or a headset, or it is being transferred to a large monitor, but not if it is being sent to a smartphone.
Unless there is a specific reason for doing so, it is a recommended practice to verify the resolution of a video file sent to a smartphone does not exceed the perception of the average user.
Video Optimizer conducts a test that flags video files with resolutions greater than 720p that are being sent to a smartphone.
Video and other types of rich media are on the rise in mobile apps and we continue to work on more guidance on streaming and other video technology in our Mobile Development Best Practices recommendations.
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