What are frames in an animation

What do keyframes, intraframes and keyframes mean in digital image processing?

The keyframe animation wonderfully illustrates how digitalization has radically changed film production. Keyframes have not only changed in terms of content. Today the term encompasses far more aspects than in the past.

Keyframes are also known as keyframes. This article does not refer to video recordings or symbolic images. In this article, we focus on the importance of keyframing and intraframes in video production and distribution (streaming).

You have to know that

  • In digital video production, there are different definitions of keyframes: usually one means either a frame with a special function on a timeline or so-called intraframes, which are important for video compression.
  • In animated films, the keyframe is a keyframe that serves as a template for the intervening images.
  • In video compression, the key frame defines the basis for compressing pure image sequences. The changes are still saved in the subsequent images, not the entire amount of data.

4 types of keyframes / keyframes

Today we can distinguish four types of keyframes:

  1. First the classic analog keyframe animation (Key = key, frame = picture). They have been around in analog animation film since the dawn of time.
  2. Second the digital computer animationn with keyframing. It is the standard in animation today.
  3. Third, keyframes are nowadays defined in a broader sense than Events on a timeline understood that marks the start and end of a change. This change can be a dissolve on the image plane. Or define an effect on the audio level.
  4. Fourth, keyframing comes in the form of Intraframes in video compression for use. Although this is the most common type of application, it often happens unnoticed in the background. Editor or user do not notice anything.

Let's take a closer look at the differences and similarities between these four categories:

1Keyframes in the classic animation of images

In classic animation, a number of drawings are strung together as individual images. This creates the illusion of movement. Now when a draftsman sits down to create an action for a character in an animated film, he has to work it out frame by frame. This takes a while. And that costs.

That is why Hollywood, even before Snow White and the Seven Dwarfs (1937), considered how this process could be simplified and made more efficient. The result is the keyframe.

Instead of having a draftsman create all of the images in a sequence from A to Z, they chose the greatest talent among the animators. As a keyframe artist, he has now drawn the first and the last image of a shot. Because it defines the beginning and the end and determines everything in between, this function was associated with as much prestige as good pay.

The individual images between the first and last key frame were called "Inkbetween" (somewhat loosely translated: the ink in between). Unlike the key images, these drawings were made by the less talented, pedestrian drawing people.

Keyframe as start and end point

A keyframe is a single drawing that defines the start and end points of a smooth transition. The drawings are called frames because their position in time is measured in frames on a film strip.

A sequence of keyframes defines what movement the viewer will see, while the position of the keyframes in the film or video determines the point in time of the movement. Since only two or three keyframes over a span of a second do not create the impression of movement, the remaining frames are filled with inkbetweens.

The more intermediate frames there are between the key frames, the more detailed and smooth the movements in the animation.

2Keyframes in computer animation

Because you can't make films without outstanding artists, keyframe artists still exist today. The intermediate images, however, are no longer drawn by a human hand in computer animation today: algorithms create these individual images automatically. In animation programs such as After Effects or Cinema 4D, the keyframe only tells the computer where and what to fill in on the time axis between two points.

A simple example to illustrate:

A ball is supposed to move across the screen. The keyframes define what the ball looks like and determine the start position and end point. The software calculates what happens in between based on the entry and exit points. This process is called keyframing.

Keyframing works in a very similar way to hand-drawn keyframes. Except that the computer does the work here, instead of hand-creating the frames between the keyframes.

3Keyframes in media production

In media production, a keyframe is a position on a timeline that marks the beginning or the end of an action. It contains special information that defines where an action should begin or end. As in computer animation, software interpolates the actions between the start point and the end point.

Keyframing is used in every time-based media production process today, including audio production. In post-production practice, they are nothing more or less than a marker that is placed on a timeline with a mouse click and linked to an action. It doesn't take more than two clicks of the mouse to do this.

Because the function of keyframing in video editing goes far beyond the interpolation of drawings, one rarely speaks of keyframes in this context, even in German, but only of keyframes.

Professional software for image processing or sound processing supports an almost infinite number of parameters that can be set for each individual object. An example are image transitions, shifts in the plane of focus or light. They are all unthinkable without keyframes.

Key frames in 3D animation

3D animations (including graphics) treat light sources in a similar way to real recordings. The animated objects cast shadows or create reflections. Assuming an animator wants the intensity of the light to change continuously from one value to another within a predefined period of time, he uses keyframes to do this.

At the beginning of the sequence, a keyframe is set that determines the initial strength. For the end, a stronger value is set with a second keyframe. The software program then automatically interpolates the two values ​​and creates a smooth increase in light intensity.

Keyframes can also change the properties of an audio recording over a period of time. During the creation of an audio recording or the sound mix, start and end marks are set here as well. As always in digital video production, keyframes can also be set, changed and corrected afterwards when editing the sound.

4Keyframes and intraframes in compression

Keyframes also play an important role in the compression of data. Small file sizes and thus fast streaming are nowadays an important criterion for business success for many online platforms such as Netflix.

Keyframes are also known as intraframes in compression. This is interesting because intraframes are just now no Inkbetweens are. They are the cornerstone of compression.

Video compression only saves changes in the data stream that occur from one keyframe to the next. This greatly reduces the amount of information to be stored.

This is based on the fact that most individual images show only minor changes from one frame to the next. Whenever there is a drastic change in the frame, e.g. B. when changing from one setting to another or when changing scenes, an intraframe is required. Therefore, the same applies here (as with animation): the higher the density of keyframes (intraframes), the better the quality. But this also increases the file size.

Consequences for video streams

Because video compression only saves the smallest changes between individual images (except for the intraframe), it is impossible to fast forward or rewind to any point in the video stream. This is why in practice an intraframe is often output once every 10 seconds of video, even if the video image does not change optically. This enables a search within the video stream at intervals of at least 10 seconds.

The downside of the coin is that the resulting video stream becomes larger because many intraframes are now added that are actually not required for the visual representation. However, this disadvantage does not result in a significant loss of compression if the bit rate is already set to a high value for better quality (such as in the DVD MPEG-2 format).