Bit-Mapped Graphic Data:Input (Capture) Hardware

Multimedia – Section 2

CCD & ADC

• CCD (charged coupled device):• Uses sensors to capture light and then

convert into analogue electrical signals.• Currently only see light levels not colour

• ADC (Analogue/Digital Converter)• Converts the analogue signals into digital

signals so that the computer can process them.

Digital Camera

• In a digital camera a CCD array captures light coming in through a lens

• The Resolution is the number of pixels in the image produced

• Capturing colour– A colour filter array is placed over the CCD array– Each sensor records light of one colour (red, green or blue)

Scanner

• Requires 3 images for colour– Red, Green & Blue

• Capture images from a flat object

• Linear CCD– Narrow strip passes over image.

Multimedia - Section 2

Bit-Mapped Graphic Data:Storage of Graphic Data

File Formats• Bitmap (BMP)• Graphic Interchange Format (GIFF)• Joint Pictures Expert Group (JPEG)• Portable Network Graphics (PNG)

Bitmap File FormatUncompressed:

– A file which holds a binary number for each pixel in an image

Limitation:– large file size

Compression: – RLE (Run Length Encoding)

• uses the fact that large areas have same colour of pixel. (LossLess)

Extension: – .bmp

Bitmap File Format

Colour Depth

• The number of colours that can be represented

• Calculation:

Number of colours = 2 bit depth

Example

Colour (bit) Depth = 8Number of colours = 28

= 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2

= 256

GIF File Format• Lossless compression

– LZW

• Limited number of colours: limited to 28

– 256 colours

• Features: – Animation– Transparency

• Extension: .gif

GIF File Format

Good For:• Flat areas of colour• Few colours• Animated images• Transparency

Bad For:• Photographic images• High colour graphics• High quality videos

Interlacing• Allows you to view an image before it is

downloaded• Image is gradually revealed

JPEG File Format• Lossy compression

– DCT

• Greyscale or full-colour image compression • Exploits human eye limitations• Related to MPEG• Extension: .jpg

JPEG File Format

• Advantages:– Greater compression than lossless– Good for photographic images (real-world)– Compression ratio can be controlled by user– Best for large files

• Disadvantages– Doesn’t support interlacing or transparency

PNG File Format• Has all advantages of GIF without the

restrictions

• Allows 24 bit colour

• Allows for partial transparency

• Lossless compression– Variation of LZW

• Extension: .png