Digital Video and Multimedia If images can portray a powerful message then video (as a series of related images) is a serious consideration for any multimedia

Digital Video and Multimedia

If images can portray a powerful message then video (as a series of related images) is a serious consideration for any multimedia application.

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To produce videos that are worthy of inclusion in a web-based multimedia application you need to understand:

How video is captured?How video is stored?How video is transferred?How video is displayed?How to shoot a good video?


How video is captured?

Light passes through the camera lens and is converted into an electronic signal by a charge-coupled device (CCD). Also known as a sensor.



In a consumer level camera there is one sensor capturing the red, green and blue data required to record color. In a professional level camera there are 3 sensors, one each for red, green and blue.



Video is captured as a series of still images. Each still image is known as a frame. The rate at which the frames are captures is measured in fps (frames per second).

So that a video can be viewed without the individual images being evident, more than 24 images need to be captured per second.

Common capture rates are 24, 25, 30 and 60 fps. These different capture rates are required to suit the hardware on which the video will be displayed. We will look more into how video is displayed in the next lecture.



Video can captured in different aspect rations. For example 4 x 3 and 16 x 9.

Analog TV

Widescreen TV

Cinema



Digital video can be captured at different resolutions. The resolution can often be set on the camera.

Standard Definition – 720 x 480 or 576 pixelsHigh Definition – 1280 x 720 or 1920 x 1080 pixels

The resolution a video is captured at will depend on the capabilities of the camera and the requirements of the output hardware.


How video is stored?

The question of “How video is stored” is important because of the size of video files.

For example: What is the file size of a 1 hour digital video, frame size 640 x 480, 24 bit color, 25fps? Answer in the lecture



So digital video often needs to be compressed before it is stored or transferred over the Internet.

To do this software called a codec (compression/decompression) is used.



Processes that involve codecs include ...

•recording/filming to the storage in a camcorder•storage on the hard disc of a computer•burning a DVD•transmission over the internet•broadcast via satellite or cable•transfer to a portable device (eg iPod or mobile phone)



To compress a video to a smaller size the software needs to store the pixels more efficiently (loseless compression) or throw away some pixels (lossy compression).

Fortunately digital video contains a lot of duplicated pixel information, which makes it very suitable for loseless compression.

With loseless compression a digital video can be returned to its original size.



To compress a digital video from the High Definition recorded on a camera to a size suitable for YouTube, for instance, some lossy compression will be required.

Lossy compression can be Intraframe or Interframe.

Interframe compression exploits the similarities between successive frames, known as temporal redundancy, to reduce the volume of data required to describe the sequence.

There are several interframe compression techniques, of various degrees of complexity.



Sub-sampling

Sub-sampling can also be applied to video as an interframe compression technique, by transmitting only some of the frames….

Either the viewer’s brain or the decoder would be required to interpolate the missing frames at the receiving end.

Difference Coding

Difference coding, or conditional replenishment, is a very simple interframe compression process during which each frame of a sequence is compared with its predecessor and only pixels that have changed are updated.

These images are successive frames from a table tennis sequence and illustrates how, frequently, very little changes from one frame to the next.



Intraframe compression techniques can be applied to individual frames of a video sequence….

2 examples: Sub Sampling and Coarse Quantization



Only a fraction of the pixels from the original are used. The same procedure is illustrated below.

Sub-sampling

Sub-sampling reduces the amount of data by throwing some of it away….

The quality of the sub-sampled image is lower than the quality of the original.



Only a fraction of the pixels from the original are used. The same procedure is illustrated below.

Sub-sampling

Sub-sampling reduces the amount of data by throwing some of it away….

The quality of the sub-sampled image is lower than the quality of the original.



Coarse Quantization

Compression is accomplished by restricting the number of colors.…

These three images have different numbers of colors. The first has thousands of different colored pixels. The middle has 64 different colors and the rightmost uses only 8 different colors.



http://video.answers.com/web-video-compression-and-codecs-98762724

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Digital Video and Multimedia If images can portray a powerful message then video (as a series of related images) is a serious consideration for any multimedia