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:Output of Graphic Data

Multimedia - Section 2

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