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