The Basics of Video Compression

Why Do We Compress the video?

• An uncompressed 1080p file requires about 237 MB/second for playback.  
• It is far exceeding any internal connection speed mere mortals can afford.  
• Most compressed movies playback from the web at less than 1.5 MB/s
• The higher the quality of the source file, the higher the potential quality of the compressed files derived from it.
• You can't make a compressed file look better than the source file; with the possible exception of making the frame size smaller.
• When compressing a file, you apply settings to determine what you want the source file to become.
• The size and format of the source file have almost no relevance.
• all compression software automatically determines the specs of the source file and creates the transcoded file based on your settings.

Definitions

Codec (Compressor/Decompressor)
The mathematics used to convert light and sound into numbers that can be stored and played back from a computer.  Codecs are the heart and soul of compression.  There is no single codec that is perfect for capture, editing, and distribution.

Bit rate
The speed, in bits per seconds (bps), of a media file during playback.  Controlling bit rate is a key element of compression.  Also called data rate or bandwidth.  This also determines how smoothly a web video will stream.
Our Goal, when creating videos for the web, is to get the highest possible image quality from the smallest possible bit rate.

Frame size
The size, in pixels, of a video frame

Frame rate
The number of frames playing per second; the web supports an infinite variety of frame rates.

Artifacts
Small, ugly, rectangular blocks in compressed video caused by bit rates that are set too low.

I-frame
A method of compression where each frame is compressed individually.  Yield excellent image quality, with very large file sizes.  Best format for editing.

GOP (Group of Pictures)
A method of compression where groups of frames are compressed together.  Yields good image quality with very small file sizes. Best for web videos.

What Determines Quality?

• Image quality is balance of five factors:
• Codec.  Different codecs require different settings for similar quality.
• Compressed frame size.  Larger frame sizes require faster bit rates
• Compress frame rate.  Faster frame rates require faster bit rates.
• Movement between frames.  More movement between frames requires faster bit rates
• Bit fate of the compressed file.  Higher bit rates yield higher quality, but much larger file sizes.

Bit Rate Isn't Everything

• Files encoded at the same bit rate will generally have the same file size, but they may not have the same image quality, unless they also have the same specifications.

Frame Size Makes a Difference

• Bit rates that work for one format, don't work for others.
• NTSC DV frame contains 345,600 pixels
• PAL DV frame contains 414,720 pixels
• 720p frame contains 921,600 pixels
• 1080p frame contains 2,073,600 pixels
• 2K frame contains about 2.21 million pixels
• 4D frame contains about 8.29 million pixels

Minimize Movement

• Video compression yields the smallest file sizes when there is very little movement between frames.
• The more movement between frames in the master file - from camera, transitions, effects, or talent - the higher the bit rate that is needed to properly compress it.
• If small file sizes are key, minimize on-screen movement; such as hand-held cameras, file noise effects, video noise due to low light, or leaves waving in the background behind your actors.