7.Digital Image and Video

Digital Image and Video

Pixelized Image• Image is divided into small dots (pixels) arranged in rows and columns

• The size of the image (S) is given by row-column multiplication

• Each pixel has an associated color• Each color has an associated binary code• The number of bits (N) in a binary code depends on the total number of colors (M):

N = log2 M and M = 2N

Pixelized Images

Bitmap Image• Bitmap image representation is used to store images in files.

• Each pixel in bitmap file is represented by the binary representation of its color

• Total number of bits in the file is N * S

RBG Additive color model

• Any color can be created by combining the 3 basic colors Red Blue Green

• The difference between one color another is the intensity of each of RBG components

• The number of colors will depends on how many levels of each of the RBG colors are available

How does film type camera work?

• Film is made of light sensitive chemicals • Light is reflected from objects• Reflected light is allowed to strike the film for short time by the use of shutter

• Film’s light sensitive chemicals undergoes changes proportional to the light components.

• After that, film is processed.

How does digital camera work?

• Most Digital Cameras have the following Components:– Lens system– Sensor Array– Filter system– Memory– Processor– Software– Viewing Screen

Sensors Array• Sensors array act like the film.

• The smallest unit of Sensor array is known as Charged Coupled Device (CCD)

• Each CCD represent one pixel

• The CCD detect light and produce electricity proportional to the intensity of the light

• example: A 6-megapixel camera has sensors array of

2816 *2112 = 5.95 M pixel

Capturing colored Images• Sensor array do not

capture colors, it capture only light intensity

• To obtain the color, light must pass through a filter

• This filter is known as spinning disk filter and it is made of 3 sections red, blue and green.

• Each section passes its color and block the other 2 colors.

• The intensity of the passed color is detected and sent to the ADC

• Spinning occurs at very high speed, thus it is invisible to human eyes

Image Resolved into basic colors

Color Pallet and corresponding codes

• The number of colors is related to the number of bits assigned to each pixel

• The number of bits assigned to a color is known as bit depth

• Human eye is sensitive to 10 million colors.

• Commercial cameras have bit depth of 24 bit i.e. the number of colors = 16 M.

• 24 bit depth is known as true color

• Each basic color (Red, Green and Blue) is represented by 8 bits

• Codes are assigned to each color in the color palette

• Example: for bit depth of 3, the total number of colors in the color palette is 8

Example• Calculate the number of megabyes in a color image captured by a digital camera that can produce images with 2272*1704 pixels with bit depth of 24? How many images can be stored in the camera's 32 MB flash memory?

# of pixels = 2272 X 1704 = 3,871,488 pixels

# of bits = 3,871,488 X 24 = 92,915,712 bits

# of Bytes = 92,915,712/8 = 11,614,464 byes

# of MB = 11,614,464/220=11.07 MB

Spatial Resolution

• Spatial Resolution is related to the number of pixels per unit length

• When 120*80 pixels viewed from far distance it would look similar to the 900 * 600 pixels

Brightness Resolution

• Brightness Resolution is related to the bit depth and number of colors

• More than 24 bit depth is not needed

Digital Video

• Video is achieved by playing images (frames) in succession

• With a speed of at least 20 frames per second human eyes don’t capture the transition from a frame to another

• 24 fps is the standard used for video

Example• Calculate the number of gigabytes in a 2 hour digital movie if each frame is made of 512*512 pixels, each pixel is assigned 9 bit binary code, and each second of the movie requires 24 frames

# of pixels = 512 X 512 = 262,144 pixels # of bits in every frame = 262,144 X 9 = 2,359,296 bits

# of frames = 2 X 60 X 60 X 24= 172,800 frame # of bits in 2 hours = 172,800 X 2,359,296 = 407,686,348,800 bits

# of bytes in 2 hours= 407,686,348,800/8 = 50,960,793,600 bytes

# of gigabytes in 2 hours = 50,960,793,600/230 = 47.46 GB