Bit-Mapped Graphic Data: Input (Capture) Hardware

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 (GIF)• 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

Extension: – .bmp

Bitmap File FormatColour Depth

• The number of colours that can be represented

• Calculation:Number of colours = 2 bit depth

ExampleColour (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• Limited number of colours: limited to 28

– 256 colours• Features:

– Animation– Transparency

• Extension: .gif

GIF File FormatGood For:• Flat areas of colour• Few colours• Animated images• Transparency

Not Good For:• Photographic images• High colour graphics• High quality videos

JPEG File Format• Lossy compression• Greyscale or full-colour image compression • Exploits human eye limitations• 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

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).

Graphics card

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