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Bit Mapped Graphics and Compression. This graphic is 16 by 16 pixels. The designer has chosen to limit it to 2 colours. Each pixel can only be black or white. 1 could represent black and 0 could represent white. So each pixel takes up 1 bit of memory (a 0 or 1).
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This graphic is 16 by 16 pixels. The designer has chosen to limit it to 2 colours. Each pixel can only be black or white. 1 could represent black and 0 could represent white. So each pixel takes up 1 bit of memory (a 0 or 1). There are 256 pixels (16 by 16), so it needs 256 bits. There are 8 bits in a byte, so that’s 32 bytes for the graphic. We say this bit mapped image has a 1 bit colour depth.
This graphic is 16 by 16 pixels. The designer has chosen to limit it to 4 colours. Each pixel can be black, white, red or yellow. 3 could represent yellow, 2 red, 1 black and 0 white. 3 is 11 in binary, 2 is 10, 1 is 01 and zero is 00 So each pixel takes up 2 bits of memory. There are 256 pixels (16 by 16), so it needs 512 bits. There are 8 bits in a byte, so that’s 64 bytes for the graphic. We say this bit mapped image has a 2 bit colour depth.
This graphic is 16 by 16 pixels. The designer has chosen to limit it to 16 colours. Each pixel can be colour number 0 to 15 15 is 1111 in binary, so each pixel takes up 4 bits of memory. There are 256 pixels (16 by 16), so it needs 1024 bits. There are 8 bits in a byte, so that’s 128 bytes for the graphic. We say this bit mapped image has a 4 bit colour depth.
256 colours require 8 bits 0 to 11111111 (255) 65536 colours require 16 bits 0 to 1111111111111111 (65535) Most modern computers have 24 bit colour which gives 16,777,216 colours, usually referred to as True Colour or 24 bit colour depth. If our house picture used true colour, it would take up 768 bytes ie: 256 pixels, each using 24 bits (3 bytes) And this is still a tiny picture, shown greatly enlarged. Here you can see it about the right size.
How much RAM does the monitor display use? A modern computer screen may be 1280 pixels horizontally by 1024 vertically and has true colour – 24 bit colour depth.
How much RAM does a camera picture use? Like any bit mapped graphic, the answer depends on the resolution of the camera and its colour depth. If a camera has 6 megapixels and 24 bit colour depth – fairly standard values – then a vast amount of memory would be used. Hard discs would become full very quickly, transferring pictures over networks would be too slow. Even displaying pictures on the monitor would be slow. Because of this, large bit mapped images are usually compressed.
Compression - GIF format (1) • GIF (Graphics Interchange Format) compresses images by limiting them to 256 (28) colours. • If you’ve got fewer than 256 actual colours in your image, there is loss of detail. It’s best for cartoons, charts etc. • GIF is poor for photographs. • GIF allows for transparency and even simple animation when they are strung together.
JPEG format (1) • JPEG is a “lossy” format – it ignores detail which the human eye can’t see – loss of detailed shading etc. • Good for photographs – used by most digital cameras.
