Glossary / Terminology

Alpha channel

An alpha channel is a part of the image data that describes each pixel’s level of opacity or translucency. Contrast that with the other data, which describes light and color. Many people think of an alpha channel as a transparency mask. However, that can lead one to think of it as rather binary - opaque or transparent - whereas most often an alpha channel has at least 256 levels.

Aspect ratio

Aspect ratio is how the width of a rectangle compares with its height. In other words, wide or tall, and by how much? This is typically expressed as a fraction of width over height. For example, if a rectangle is twice as wide as it is high, we can say it i has a 2:1 aspect ratio. This can get complicated in video where pixels themselves can have an aspect ratio that is distinct from the display aspect ratio. For example, SD NTSC video has a resolution of 720x480 but can be either 4:3 or 16:9 display aspect ratio neither of which are square pixels! Modern video standards such as ATSC and UHD avoid using non-square pixels such that the common HD resolution 1920x1080 reduces mathematically to 16:9, which is also the most common video aspect ratio in use today.

B frames

Bitrate

Codec

Colorspace

Color space is about how color and light is represented especially numerically. In video and computer images, the two most popular systems of organization are RGB and YUV (or Y’CbCr). This is a complicated subject area; you can read more on Wikipedia.

Deinterlacer

A deinterlacer is an algorithm to convert interlace video to progressive scan. See below for definitions on these two terms.

Field order

Interlace video consists of two fields per frame. This term describes which field appears before the other in storage and/or display.

GOP

Interlace

Interlace is a simple form of video compression that uses two half vertical resolution frames to represent a full frame. Basically, you can double the refresh rate for the same data rate. Each half vertical resolution image is called a field. Typically the fields are interleaved in storage and then displayed one after the other on play back by skipping every other line.

Interpolation

Interpolation is the computation of values based on neighboring values. With respect to Settings, it easiest to think of this as the quality level when changing the size of an image. Interpolation is also a term used for animating parameters in Keyframes.

Keyframe

A keyframe defines a specific value or set of values at a specific point in time. The term is used when talking about animating parameter values in Keyframes. It is also used in temporal video compression (so-called delta or P- or B-frames).

Metadata

Metadata is data about another data. In the context of multimedia, the media data (audio/video) is the core data, and all other data in the file is metadata. There can be metadata about the media attributes such as resolution or number of audio channels. And there can be metadata about the context of the media file such as its creator, creation data, title, etc.

MLT

MLT is another open source software project that is the engine of Shotcut. Shotcut is primarily the user interface running on top of this engine. This engine provides some effects of its own, but it also uses other libraries such as FFmpeg, Qt, WebKit, frei0r, lads, etc.

Progressive

In video, this refers to a scan mode where (“scan” refers to old tube-based TV technology where a cathode ray draws video by drawing lines) each frame of video is a whole picture from a single point in time. This is the opposite of interlace.

Resolution

Display Resolution is the actual pixel size of video and images. Example sizes: 3840x2160 (4k), 1920x1080 (HD), 1280x720 (HD), 720 × 480 (SD), 1080x1380 (Vertical)

Ripple

Ripple means that an operation can affect the clips on the timeline that are later or after the clip being changed. For example, a ripple delete not only removes the clip but also the space it occupies. This requires changing the start time of all of the following clips.

Safe area

Sample rate

Scrubbing

Scrubbing is seeking by clicking some object and dragging it. Typically this is the play head in the player or timeline. But it can also refer to simply rewind and fast forward playback through media.

Skimming

Skimming is seeking based on the horizontal position of the mouse over the video image or timeline. You press and hold the Shift key in Shotcut to enable skimming.

Snapping

Snapping is a setting on the timeline. Aids in sliding two clips together on the same track with no gap in between them.
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Scan mode

Scan mode indicates whether video is progressive or interlaced - see related definitions above.

Threads

Threads are a software programming mechanism to let multiple things occur at the same time. Most CPUs now consist of multiple execution units typically called “cores.” Often, these cores support a CPU-based “thread,” which you can think of as a light core (not completely parallel). While it is important that your operating system let multiple things run at the same time to use these CPU cores and threads, it also important that Shotcut run things (i.e. parallel processing) at the same time because media decompression, processing, and compression is very computationally heavy. You can learn more about how Shotcut uses multiple cores and threads in the FAQ.

Timecode

Timecode is a way to represent time numerically. Shotcut uses a standard called SMPTE from the Society of Motion Picture and Television Engineers. It is a display of the running time of video that is frame-accurate yet easier for humans to understand than pure frame count. It shows hours, minutes, seconds, and frames in the format HH:MM:SS:FF. On many video modes that use a non-integer frame rate (e.g. 29.970030 or 30000/1001 fps), the semicolon (;) is the delimiter between seconds and frames to indicate that it is using drop-frame timecode. Drop-frame is a technique to make the timecode follow the real time over long durations. For 30000/1001 fps, drop-frame subtracts two frames every minute except every tenth minute.

XML

XML is a text format that is designed to be both human and machine readable and writable. It is standardize, structured, and extensible - the X in eXtensible Markup Language. There are many dialects of XML, and when one video editor says it reads “XML” it does not mean it can read the XML that another video editor can export. They need to be the same kind of XML. Shotcut reads and writes MLT XML, but at this time it can only fully understand the MLT XML that it writes.