Today in the lecture we spoke about moving images and digital videos.
Warning - There is a lot of text here! BUT it is VERY useful so look at it!
Moving Images
We spoke about persistence vision and how it is a theory that states that the human eye will remember and image for one twenty-fifth of a second on the retina and this gives the brain the illusion that the image is moving.
This, however, is and old idea and is now regarded as the myth of persistence of vision.
A more plausible theory to explain motion perception are two distinct perceptual illusions:
- phi phenomenon - http://en.wikipedia.org/wiki/Phi_phenomenon
- beta movement - http://en.wikipedia.org/wiki/Beta_movement
Digital Video
We also spoke about the amount of space need for videos.
Uncompressed HD video files could be large, for example, 3bytes per pixel, 1920x1080 by 60 frames per second. This equals 373.2 megabytes per second.
This is approximately 1 gigabyte every three seconds.
Even as we stand today this is an extreme amount of data.
This is why we have many varieties of compression algorithms and standards to dramatically reduce the amount of data used in video storage, processing, streaming and transmission.
Below is some VERY IMPORTANT terminology:
We then spoke of different video file formats:
MPEG-1 - Development of the MPEG-1 standard began in 1988, Finalised in 1992 and the first MPEG-1 decoder was made available. Compressing video to about 26:1 and audio 6:1, the MPEG-1 format was designed to compress VHS quality raw digital video and CD audio with a minimum of quality loss.
Today, it is the most widely compatible lossy compression format in the world. (ie very blocky compression artifacts) The MPEG-1 standard is part of the same standard that gives us the MP3 audio format. Fortunately, the MPEG-1 video and Layer I/II audio can be now be implimented in applications royalty free and without license fees, since the patents expired in 2003.
MPEG-2 - The MPEG-2 format was an improvement on the MPEG-1 format. The MPEG-1 format had less efficient audio compression, and was restricted when it came to the packet types it accepted. It also did not support interlaced video.
MPEG-2 is the format of choice for digital televison broadcasts.
Work on the MPEG-2 began 1990 — before the first draft of MPEG-1 was ever written. It was intended to extend the MPEG-1 format to provide full broadcast quality video at high bitrates, between 3 and 5 Mbits/s.
MPEG-4 - Is essentially a patented collection of methods to define compression of video and audio, designating a standard for a group of audio and video codecs.(coder/decoder) MPEG-4 encompasses many of the features of MPEG-1 and MPEG-2, while adding support for 3-D rendering, Digital Rights Management (DRM), and other types of interactivity.
QuickTime - Appeared in 1991 under a proprietary license from Apple, beating Microsoft’s Video for Windows to the "market" by nearly a full year.
QuickTime video playback. Possibly the best of the Linux programs that can handle playback of most QuickTime files are VLC and MPlayer, both of which are in the PCLinuxOS repository.
In 1998 the ISO approved the QuickTime file format as the basis of the MPEG-4 file format. The benefit is that MOV and MP4 files (containers) are interchangeable on a QuickTime-only environment (meaning running in an "official" QuickTime player, like QuickTime on the Mac OS X or QuickTime for Windows), since both use the same MPEG-4 codecs.
AVI (Audio Video Interleave) - appeared in1992 by Microsoft as a part of its Video for Windows technology. It is basically a file container the allows synchronized audio and video playback.
Since AVI files do not contain pixel aspect ration information, and many players render AVI files with square pixels, the frame (image) may appear stretched or squeezed horizontally when played back. However, VLC and MPlayer have solved most problems related to the playback of AVI files.
Although being "older" technology, there is a benefit to using AVI files. Because of it being around for so long, coupled with Microsoft’s market penetration, AVI files can frequently be played back on the widest variety of systems and software, second only to MPEG-1. It has gained widespread acceptance and adoption throughout the computer industry, and can be successfully played back, so long as the end user has the proper codec installed to decode the video properly. Additionally, the AVI format is well documented, not only from Microsoft, but also many, many third parties.
WMV(Windows Media Video) - is made with several different proprietary codecs, made by Microsoft. It has gained adoption for use with BluRay discs.
The WMV files are often wrapped in the ASF, or Advanced Systems Format. WMV files, themselves, are not encoded. Rather, the ASF wrapper is often responsible for providing the support for Digital Rights Management, or DRM. Based on Windows Media 9, WMV files can also be placed inside an AVI container. In that case, the WMV file claims the AVI file extension.
WMV files can be played on PCLinuxOS, using VLC, MPlayer, or most any other program that uses the FFmpeg implementation of the WMV codecs.
3GP - is actually two similar formats. The first, 3GPP, is designed as a container format for GSM phones (in the U.S., primary GSM wireless carriers are AT&T and T-Mobile). The second, 3GPP2, is designed as a container format for CDMA phones (in the U.S., primary CDMA wireless carriers are Verizon and Sprint). 3GPP files will often carry a 3GP file extension, while 3GPP2 files will often carry a 3G2 file extension.
(A little complex) 3GP and 3G2 files store video streams using MPEG-4 Part 2, H.263, or AVC/H.264 codecs. Some cell phones will use the MP4 file extension to represent 3GP video. Both formats were designed to decrease storage and bandwidth requirements to accommodate mobile phones.
Software support under PCLinuxOS is, once again, achieved with VLC and MPlayer. Additionally, 3GP files (and most 3G2 files) can be encoded and decoded with FFmpeg.
FLV - Flash Video, are a file container format used primarily to deliver video over the Internet. In fact, it has become the defacto format of choice for such sites as YouTube, Google Video, Yahoo! Video, Metacafe, and many news outlets.—While the FLV format is an open format, the codecs used to produce FLV files are mostly patented. The most common codecs used are the Sorenson Spark (H.263 codec variant) and On2’s VP6. FLV files can also be encoded as H.264 as in the more recent releases of Adobe Flash.
The great the compression on a video means the greater loss of information.
Algorithms that compress video predicatively still have problems with fast unpredictable and detailed motion!
Automatic Video Quality Assessment could be the solution.
LAB
In the lab we produced an edited video using clips and music given to us. Below is a link to the finished video that I made (there was an issue with time at the end so the music goes a bit strange sorry):
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