Video quality
Color space and bits per pixel
Example of U-V color plane, Y value=0.5
Color model name describes the video color representation. YIQ was used in NTSC television. It corresponds closely to the YUV scheme used in NTSC and PAL television and the YDbDr scheme used by SECAM television.
The number of distinct colours that can be represented by a pixel depends on the number of bits per pixel (bpp). A common way to reduce the number of bits per pixel in digital video is by chroma subsampling (e.g. 4:4:4, 4:2:2, 4:2:0/4:1:1).
Video quality
Video quality can be measured with formal metrics like PSNR or with subjective video quality using expert observation.
The subjective video quality of a video processing system may be evaluated as follows:
* Choose the video sequences (the SRC) to use for testing.
* Choose the settings of the system to evaluate (the HRC).
* Choose a test method for how to present video sequences to experts and to collect their ratings.
* Invite a sufficient number of experts, preferably not fewer than 15.
* Carry out testing.
* Calculate the average marks for each HRC based on the experts' ratings.
Many subjective video quality methods are described in the ITU-T recommendation BT.500. One of the standardized method is the Double Stimulus Impairment Scale (DSIS). In DSIS, each expert views an unimpaired reference video followed by an impaired version of the same video. The expert then rates the impaired video using a scale ranging from "impairments are imperceptible" to "impairments are very annoying".
Video compression method (digital only)
Main article: Video compression
A wide variety of methods are used to compress video streams. Video data contains spatial and temporal redundancy, making uncompressed video streams extremely inefficient. Broadly speaking, spatial redundancy is reduced by registering differences between parts of a single frame; this task is known as intraframe compression and is closely related to image compression. Likewise, temporal redundancy can be reduced by registering differences between frames; this task is known as interframe compression, including motion compensation and other techniques. The most common modern standards are MPEG-2, used for DVD and satellite television, and MPEG-4, used for home video.
Bit rate (digital only)
Bit rate is a measure of the rate of information content in a video stream. It is quantified using the bit per second (bit/s or bps) unit or Megabits per second (Mbit/s). A higher bit rate allows better video quality. For example VideoCD, with a bit rate of about 1 Mbit/s, is lower quality than DVD, with a bit rate of about 5 Mbit/s. HDTV has a still higher quality, with a bit rate of about 20 Mbit/s.
Variable bit rate (VBR) is a strategy to maximize the visual video quality and minimize the bit rate. On fast motion scenes, a variable bit rate uses more bits than it does on slow motion scenes of similar duration yet achieves a consistent visual quality. For real-time and non-buffered video streaming when the available bandwidth is fixed, e.g. in videoconferencing delivered on channels of fixed bandwidth, a constant bit rate (CBR) must be used.
Stereoscopic
Stereoscopic video can be created using several different methods:
* two channels — a right channel for the right eye and a left channel for the left eye. Both channels may be viewed simultaneously by using light-polarizing filters 90 degrees off-axis from each other on two video projectors. These separately polarized channels are viewed wearing eyeglasses with matching polarization filters.
* one channel with two overlayed color coded layers. This left and right layer technique is occasionally used for network broadcast, or recent "anaglyph" releases of 3D movies on DVD. Simple Red/Cyan plastic glasses provide the means to view the images discretely to form a stereoscopic view of the content.
* One channel with alternating left/right frames for each eye, using LCD shutter glasses which read the frame sync from the VGA Display Data Channel to alternately cover each eye, so the appropriate eye sees the correct frame. This method is most common in computer virtual reality applications such as in a Cave Automatic Virtual Environment, but reduces the effective video framerate to one-half of normal (for example, from 120Hz to 60Hz).
Blu-ray Discs greatly improve the sharpness and detail of the two-color 3D effect in color coded stereo programs. See articles Stereoscopy and 3D film.