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 Format Uncompressed : –A file which holds a binary number for each pixel in an image Limitation : –large file size 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 = 8 Number of colours = 2 8 = 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 2 8 –256 colours Features: –Animation –Transparency Extension:.gif

GIF File Format Good 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