GLOSSARY - ENCODING

 

  Aliasing
  Alpha Channel
  Artefacts
  Bandwidth
  Codec
  Colour Bit Depth
  Compression
  Data Rate
  Drop-Frame
  Lossless
  Lossy
  Raw
  Redundancy
  Sampling
  Subsampling
  Wavelet Compression
COLOUR
LOGIC
OPTICS
PERSONNEL

 

 

ALIASING
  • ALIASING is a phenomenon that can occur during any form of SAMPLING that in digital image processing can lead to picture ARTIFACTS.


The diagram shows the sampling of a high frequency signal at a high sampling rate and a low sampling rate. There are enough sample points at he high sampling rate give to give an accurate representation of the high frequency signal. However at the lower sampling rate, there is not only no representation of the original signal, there is a clear but erroneous representation of a much lower frequency signal, the frequency of which is a function of the periodicities of the sampling rate and the original signal.

Sampling in digital images can be both spatial or temporal. Spatial sampling is defined by the resolution of the picture - an HD image has 1920 horizontal samples. Temporal sampling depends on the frame rate - in cine film a full frame is recorded every 1/24 second.

Aliasing in spatial sampling occurs particularly when trying to resolve patterns and can result in the distinctive moire-pattern seen on the Siemens star to the right. The curved patterns are caused by the computer screen, with its limited number of pixels arranged in a grid arranged horizontally and vertically, failing to fully resolve the fine detail especially the diagonal lines at the centre of the star.

Temporal aliasing can be seen in the familiar effect of car or bicycle wheels appearing to rotate in the wrong direction when seen on film. This is particularly noticeable when a wheel is filmed accelerating from a standstill. Up to a certain speed we see the wheel rotating faster and faster. At a certain speed the film camera's frame rate cannot resolve the incremental turning of the wheel and an illusion occurs where the eye interprets a spoke which has not quite completed a full revolution between frames as actually moving backwards. This effect is greater at short exposure times - MOTION BLUR can hide aliasing ARTIFACTS.



Another example of aliasing: if you've ever been in a club with a crowd bouncing up and down to upbeat music when the strobe comes on, you will have noticed that people look like they're dancing in slow motion even though you know they are moving rapidly, or "having it". This is an example of under sampling causing aliasing - the frequency of the strobe is not high enough to resolve the crowd's rapid, repetitive movements.

If you scroll this page up and down and look at the Siemens star while scrolling, the moire-patterning increases and changes shape. This is due to a combination of spatial aliasing and temporal aliasing as the screen refreshes.

 

 

COLOUR BIT DEPTH
  • "x-BIT" when used to describe a camera system refers to the colour BIT-DEPTH of either the internal processing or the output video format - they are not necessarily or even usually the same.


For instance most DSLRs have 14-BIT internal colour processing when grabbing image information from the sensor; this 14-BIT image is then compressed to an 8-BIT format that is saved as a video file by the camera.

The higher the BIT-DEPTH the greater the number of gradations between the 0% and 100% levels on a colour channel and hence the more subtle the tones.The human eye can detect approximately 10million distinct colours - fewer than that encoded even by 8-BIT colour systems (which, with 256 values for each colour channel, can encode 256x256x256 colours, more than 16million).

Despite this higher BIT-DEPTH systems are advantageous. The human eye is not equally sensitive throughout its dynamic range. The high BIT-DEPTH, linear, colour data from the sensor can be intelligently encoded in-camera or in post production to preserve the colour detail to which the eye is most sensitive- often by encoding in a logarithmic COLOUR SPACE, and using COLOUR SUBSAMPLING, and/or a BEYER MASK.

 

 

SUBSAMPLING
  • or "Chroma Subsampling"
  • A means of reducing the amount of data required to store or transmit a video picture by sampling CHROMA less frequently than LUMA.

Cinematography is about creating beautiful moving pictures, and that means storing immense amounts of information about the sequence of individual still images that form the movie clip. When compared with stills imaging, the importance of using the available storage capacity as efficiently as possible is much greater in moving picture systems due to the sheer number of individual "frames" involved in the process. At 24 frames per second, a 60 minute film will display 86,400 single frames - that's a big bill at snappy snaps.

Subsampling is a process used in both analogue and digital camera systems, and CODECs, in order to get the most out of a limited DATA RATE. It relies on the human eye's tendency to be less responsive to CHROMA information than it is to LUMA. This is due to the eye having a high proportion of "rods" (which are highly sensitive to LUMA but play little part in resolving CHROMA) to the CHROMA sensitive "cones". The result is that when sampling an image it is possible to record less CHROMA data than LUMA data without necessarily reducing the perceived quality of the resulting image.

Subsampling works on a given array of photosites or pixels by taking fewer CHROMA samples than LUMA samples. The nomenclature used to describe different types of SUBSAMPLING takes the form:

    "J:a:b" where:
    • J = number of horizontal luma (Y) samples
    • a = the number of CrCb "colour pair" (U+V) samples on the first line
    • b = the number of CrCb "colour pair" (U+V) samples on the second line

    • *Please note: This nomenclature has evolved in a haphazard fashion and can be confusing! J originally described the LUMA sampling as a multiple of 3 3/8 MHz, and the "b" figure described the Cr  subsampling only. HDTV would have been shown as 22:11:11. The nomenclature has been simplified to reflect the systems generally in use and, as described here, is used by manufacturers and developers to roughly describe those systems. It cannot accurately describe all subsampling systems and has particular problems with vertical subsampling.The nomenclature refers to subsampling applied to an 8 PIXEL or PHOTOSITE array of 4 across by 2 down. Each of the 8 units can potentially have a sample for each of LUMA, Cb and Cr, giving 24 "units" of information. In the nomenclature "a" and "b" refer to CbCr pairs, which have twice the amount of information as the LUMA samples referred to by "J".


  • 4:4:4 or no sampling - each line has full LUMA sampling (8 units of data), the first line has full CrCb sampling (8 units of data), the second line has full CrCb sampling (8 units of data). The full picture - 24 units of data. (Used in DUAL-LINK SDI).

  • 4:2:2 subsampling - each line has full LUMA sampling (8 units of data), the first line has 2 CrCb subsamples (4 units of data), the second line has 2 CrCb subsamples (4 units of data). Total units of data is 16 so a 33% saving compared to 4:4:4. (Used in Digi-Beta, XDCAM and ProRes amongst others).

  • 4:2:0 subsampling - each line has full LUMA sampling (8 units of data), the first line has 2 CrCb subsamples (4 units of data), the second line has no colour subsampling at all. Total units of data is 12 representing a 50% saving compared to 4:4:4. (Used in MPEG CODECs, common JPEG settings, and PAL DVCAM amongst others).
  • 4:1:1 subsampling - each line has full LUMA sampling (8 units of data), the second line has 1 CbCr subsample (2 units of data), the second line has 1 CbCr subsample (2 units of data). Total units of data is also 12 representing a 50% saving compared to 4:4:4. (Used in DVC Pro and NTSC DVCAM amongst others).

Where there is a fourth figure in the ratio i.e. J:a:b:x most often in 4:4:4:4, the last figure refers to the ALPHA channel, a measure of transparency used in CGI.There are many details specific to proprietary subsampling systems which are not expressed by this nomenclature, including at which PIXELS or PHOTOSITES the subsampling takes place or whether an average is taken across more than one; the timing of the subsampling, particularly with regard to interlaced signals, and so on. These can all impact on the suitability of different systems for different tasks.Subsampling is a form of LOSSY COMPRESSION and visual information is therefore lost. It is ubiquitous and used in most forms of displayed video systems and all but the highest end camera systems. While the result may be imperceptible when used correctly there is always a danger of losing too much visual information and the potential to generate ARTEFACTS. Especially so when converting from one subsampled format to another.

 


CREDITS: Thanks to Roger Bowles ACO for his continued work in compiling this glossary.



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