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

Multimedia - Section 2

Bit-Mapped Graphic Data:Storage of Graphic Data

- Technical Terms

Bit-mapped Graphic Data

• To calculate the approx file size of an uncompressed bitmap:

Files size (bytes) =

Number of Pixels x colour depth per pixel (bits)8

• If colour depth per pixel is in bytes:

Files size = Number of Pixels x colour depth per pixel

Bit-mapped Graphic Data

• Dithering– Extra colour can be created by placing

different colours next to each other

– Red and blue pixels would appear as purple

Dithering Example

256 Colour 4 Colour Non-Dithered

4 Colour with Dithering

Bit-mapped Graphic Data

• Anti alias– Low resolution images can appear less jaggy

if extra colours are used to reduce staircase effect

Bit-mapped Graphic Data

• Re-sampling– If detail is lost in a

graphic it is not possible to retrieve it

– Re-sampling will make a best guess at what the missing pixels would look like

– Technique used to enhance grainy security camera images

Trading quality for file size

• Resolution: increasing resolution increases the number of pixels, can improve the quality of a graphic but increases the file size.

• Colour depth: increasing colour depth increases the number of colours or shades of grey, can improve the quality of a graphic but increases the file size.

• Lossy compression: reduces file size and, providing the rate of compression is not too high, does not affect the quality of the graphic.

Bit-mapped Graphic Data

Simple bit-map editing and creation software

• Painting programs

Fill tool: a feature for pouring colour into a graphic.

Paintbrush tool: for more precise application of colour.

Bit-mapped Graphic Data

Main features of image editing programs

• Decrease resolution

• Alter colour depth

• Crop

• Alter brightness and contrast

• Insert graphic

• Re-size.

Multimedia - Section 2

Bit-Mapped Graphic Data:Output of Graphic Data

Graphics cards• The graphics card is the device in the

computer responsible for generating images.

• Images are held in the memory of the graphics card (called VRAM - or Video RAM).

Digital to Analogue Converter (DAC)

• Converts the digital computer signal into an analogue one that the monitor needs.

• In future analogue displays will be replaced with ones that accept digital signals

• Many LCD displays currently allow digital signals – Called DVI - Digital Visual Interface

Digital Image Signal DAC on

Graphics Card Analogue Image Signal

Graphics Processing Unit (GPU)

• The GPU is a Digital Signal Processor (DSP)

• Located on the graphics card

• Often more advanced than CPU

• Created mainly for modern gaming

• Generates images faster and with better quality than if generated from the processor

Hardware for Displaying 2D Graphics

• CRT monitor: – Bulky & heavy– Run on mains power– Cost less than equivalent

TFT screens

• LCD and TFT screens: – Flat & light– Need less power– More expensive than

CRT