Video Compression
Making BIG files small
What is Video Compression Most video compression is lossy, i.e. it operates on
the premise that much of the data present before compression is not necessary.
For example, DVDs use a video coding standard called MPEG-2 that can compress ~2 hours of video data by 15 to 30 times while still producing a picture quality that is generally considered high quality for standard-definition video
Video Compression is a trade off between video quality, disk space, and hardware needed for decompression
More about compression theory
Video data contains spatial and temporal redundancy.
Spatial Data= Difference within frames Temporal Data = Difference between
framesInterframe vs intraframe compression.
What went wrong?
QuickTime™ and aH.264 decompressor
are needed to see this picture.
What was wrong with that video?
Interlaced Data rate - 300kbs Key frame - every 150f Frame rate - 15fps Quality Settings - High Codec - H.264 Size Ratio 480 x 360 - 4:3 2MB Original-165MB
Interlaced VS Progressive
NTSC = 29.97 interlaced frames of video per second
Interlaced=image is broken into odd and even lines
Progressive scan means every frame is the entire image
Interlaced compression artifacts
QuickTime™ and aH.264 decompressor
are needed to see this picture.
What is Data Rate? Data Rate is the number of bits that are
conveyed or processed per unit of time Data rate is synonymous with bit rate. While often referred to as "speed", Data rate
does not measure distance/time but quantity/time
Data rate includes video and audio combined Video(300kbs) + Audio(128kbs)=
Total Data Rate(428kbs)
Data Rate
Data rate greatly impacts quality of the video as well as size. I usually start with the automatic settings built into most current programs. Rule of thumb 300kbs for med-high bandwidth streaming 100kbs for dial up.
-Some computers cannot handle higher data rates
-Think about how your audience will retrieve your video and let the software do the work
Key framesIn video compression, a key frame is a
frame that contains the entire imageDifference frame or in-betweens contain
only important info to fill in the gaps between Key Frames
Think of an image of a person walking in front of a wall. To show motion only the person needs to be redrawn as the wall stays the same.
Key frames
Key frames
More action needs more key framesRunning video looks better at a setting
of every 60 framesFish still looks good at every 150 framesAlways adjust key frames when
compressing
What is Frame Rate
The frame rate is the number of frames per second displayed during the video.
More frames do not necessarily make the video look better, due to the limitations of the human eyes and brain to process all of the data
Basically, a frame is the same as a still picture
Frame Rate
Most camcorders record at 30fpsProgressive mode is usually 24fpsFilm is 24fpsNTSC is 29.97fps and Pal is 24fpsAdjust by halfs 30-15fps 24-12fps
Quality Settings
The quality factor of encoded video streams is a rough measurement of video encoding quality expressed in bits per pixel.
Most programs let you adjust compressor quality
Start low and work up
Codec A video codec is a device or software that
enables video compression and/or decompression for digital video.
Co = Compressor / Dec = Decompressor Most widley used codec currently for video is
H.264 (MPEG4 Ver10) Other examples, H.261, MPEG-1 thru 4,
H.263, DivX, Xvid, WMV and the list goes on and on
Size
This greatly impacts file size and qualityTry 480x340 for 4:3 SD480x270 for 16:9 HDNTSC is 640x480
Audio Settings
Always set to mono, at this level of compression you gain little from stereo
Encode using AACSet to 22.050khzSet target bit rate to 48kbs
Looking Good!
ProgressiveData rate - AutoKey frame - every 60fFrame rate - 15fpsQuality Settings - MedCodec - H.264Size Ratio 480 x 360 - 4:38.6MB Original 165MB
Looking Good!
QuickTime™ and aH.264 decompressor
are needed to see this picture.
Sample Video Comparison Interlaced Data rate - 300kbs Key frame - every 150f Frame rate - 15fps Quality Settings - High Codec - H.264 Size Ratio 480 x 360 -
4:3 2MB Original-165MB
Progressive Data rate - Auto Key frame - every 60f Frame rate - 15fps Quality Settings - Med Codec - H.264 Size Ratio 480 x 360 -
4:3 8.6MB Original 165MB
What programs can I use?
Sorenson SqueezeFlash Media EncoderQuicktime ProCompression is built into ,most major
editing packages I.E. Final Cut, I-movie, Adobe Premier
File Types
Flash FLVQuicktime MOVRealplayer .ra, .rax, .rv., .rvx, .rm, .rmx,
.rmj, .rmsWindows Media
Formats .asf, .wma, .wmv, .wm