I Power Multimedia Technology Video Data

Video data Video is a sequence of individual pictures or frames, taken one after another. These are played back at speed to trick the brain into seeing something moving. The standard rate that the frames are played is 25 frames every second. This frame rate is measured in frames per second (fps).

Video data (Cont.) Remember! The bit depth give us the number of colours that each pixel can be. The resolution is the number of pixels in a given area, measured in dpi. A digital camera uses a CCD, an array of light- sensitive sensors to capture the image. Digital video can also be captured using a web cam(using a CCD array). Better Quality cameras have three CCD’s(one for red, green and blue), this give a much better quality image.

Video Capture Card Once the film is captured, it must be transferred to the computer. If the video was taken using an analogue camera this would be via a video capture card. This card takes the incoming data stream and translates it into a format that the computer can use. It will perform any encoding or compression necessary.

Video Capture Card (Cont.) The video capture card may process the video slower than the transfer rate of the camera so we need to reduce the amount of data to be transmitted. The data rate is measured in kilobits per second or megabits per second (Mbps). Note that is bits, not bytes!

Calculating video file size The size of the file depends on: The physical size of the image. The resolution. The bit (colour) depth. The frame rate. The length of the video.

Calculating video file size (Cont.) The file size of a video clip can be calculated as follows: 1.No. of pixels in a single frame = (Height in pixels) x (width in pixels) 2.Storage for a single frame = (No. of pixels) x (bit depth) 3.Storage for 1 sec = (size of single frame) x (frame rate) 4.Total size = (Size for 1 second) x (duration of clip)

Example What is the uncompressed file size of a 30 second video clip, taken at 25 frames per second(fps). The resolution is 640 x 480 and the video was shot in 24-bit colour. Note the resolution given is in pixels not dpi!

Solution Frame size = 640 x 480 x 24 = bytes 1 second clip = x 25 = bytes = 22500kB 30 second clip = x 30 = kB = 659.2MB This is a very large file and only just fits on a CD-RW. This is why most video is compressed.

Techniques to reduce the file size The file size of a video depends on many factors. If we adjust the numbers, without reducing the quality noticeably, we can reduce the file size.

Techniques to reduce the file size (Cont.) Using our previous example. Reduce the frame rate. If we cut the frame rate from 25 to 15 fps we reduce our file to 395MB. Too far and the clip becomes ‘jumpy’. Reduce the bit depth. Cutting the bit depth from the standard 24 bit full colour(16 million colours) to 16 bit we reduce our file to 439MB. Too far and the colour looks flat.

Techniques to reduce the file size (Cont.) Reduce the picture size. By reducing the size of the picture, less pixels are needed to fill the window, reducing the size of the file. Reduce the resolution. By reducing the resolution we reduce the amount of pixels that need to be stored. If we reduce the resolution too much then the picture will become ‘blocky’.

Techniques to reduce the file size (Cont.) Apply compression to the file. This means that the data is encoded to reduce the amount of space that it takes up. Unlike the other methods this does not have to mean a reduction in quality.

Video file formats AVI. Audio Video Interleave format is used in Windows applications like Media Player. This format does not have built in compression. The file size is limited to 2 GB. This is being superseded by the Windows Media Video(WMV) which does support compression.

Video file formats (Cont.) MPG. The Moving Picture Experts Group format is one of the most common. MPEG-2 is the standard for DVDs and can compress a 2 hour Video into a few GBs. MP3 is the audio format spin-off as it is the soundtrack layer from this format. MPEG-4 was launched in 1997 and allows variable data rates.

File Compression There are two types of file compression: lossy compression and lossless compression. Both involve restructuring the data to make it fit into a smaller space. Lossy compression removes some of the data to make the file smaller. JPEG uses lossy compression.

File Compression (Cont.) Lossless compression is when no data is lost during compression. GIF uses lossless compression.

Video editing Once your video clips have been captured you will want to start making them into a single piece of video. To do this you should first decide on a timeline,a sequence of events. Once you have got the clips trimmed and placed in order (sequenced) you will have to decide on how to join them up again.

Video transitions There are several types of transition available. Wipe: A line wipes across the old image, bringing in the new picture. Fade out: The old image gradually fades to black. Fade in: Starting with a black screen, the new image gradually appears. Fade in/out: a fade out followed immediately by a fade in.

Video transitions (Cont.) Dissolve: The old image images fade out while the new image fades in. The screen never goes black. Hard cut: The clip suddenly jumps from one image to another. Peel/page turn: The old image is peeled off the screen to reveal the new image.