A-level Computing/AQA/Paper 2/Fundamentals of data representation/Bitmaps

Overview

This example shows an Bitmap image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. Try looking closely at your monitor or mobile phone screen to see if you can spot the pixels

Bitmaps are very good for storing things such as photographs

Resolution

The higher the resolution, the more pixels are available. Therefore the crisper the picture

There are many different video display formats out there, with different widths and heights, and total numbers of pixels

Example

Template:ExampleRobox Using the diagram above we are going to work out how many pixels are required to display a single frame on a VGA screen.

Checking the resolution:

Height = 480
Width = 640
Area = Width * Height = Total Pixels
Area = 640 * 480 = 307200

Questions

Template:CPTExercise Template:CPTQuestion Template:CPTAnswerTabThe smallest possible addressable area defined by a solid colour, represented as binary, in an image. Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTabThe amount of pixels an image contains per inch/cm Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTabthe number of pixels per row by the number of pixels per column Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab100 * 70 = 7000 pixels Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab30 * 40 = 1200 pixels Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab1920 x 1080 = 2073600 pixels Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab700 / 35 = 20 pixels Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab higher resolution images are able to display more detail, providing crisper images Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTabIt will require a lot of space to store it. Meaning you'll quickly run out of memory, or it'll take a long time to transmit images across the internet or other data route.Template:CPTAnswerTabEnd Template:Robox/Close

Colour Depth

Colour depth	1 bit	2 bit	4 bit
Example
Description	Mono-chrome, only stores black and white	stores 4 colours: RGB(70,61,55), RGB(79,146,85) RGB(129,111,134), RGB(149,146,166)	Stores limited colours
Number of colours per pixel	$2^1=2$	$2^2=4$	$2^4=16$
Colour depth	8 bit	24 bit
Example
Description	close to reality	hard to see any difference between reality
Number of colours per pixel	$2^8=256$	$2^{24}=16777216$

It seems pretty obvious that the higher the colour depth, the closer the picture will look to reality. Why then don't we just ramp up the colour depth on every image that we make? The answer should be obvious, for a fixed resolution, the higher the colour depth, the larger the file size.

Example

Template:ExampleRobox All the images above are of the same resolution:

300*225 = 67500 pixels

If the first image uses 1 bit to store the colour for each pixel, then the image size would be:

Number of Pixels * Colour Depth = Image Size
      67500      *     1 bit    = 67500 bits

For the second image uses 2 bits to store the colour for each pixel, then the image size would be:

Number of Pixels * Colour Depth = Image Size
      67500      *    2 bit    = 135000 bits

See the rest of the calculations:
For the third image using 4 bits to store the colour for each pixel, then the image size would be: Number of Pixels * Colour Depth = Image Size 67500 * 4 bit = 270 000 bits For the third image using 8 bits to store the colour for each pixel, then the image size would be: Number of Pixels * Colour Depth = Image Size 67500 * 8 bit = 540 000 bits For the third image using 24 bits to store the colour for each pixel, then the image size would be: Number of Pixels * Colour Depth = Image Size 67500 * 24 bit = 1 620 000 bits

Questions

Template:CPTExercise Template:CPTQuestion Template:CPTAnswerTab The number of bits used to represent the colour of a single pixel Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab $2^8=256$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab10 as $2^{10}=1024$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab $30*40*4=4800bits$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab $30*40*6=7200bits$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab $30*40*2=2400bits$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab $2^4=16colours$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab 6 as: $2^6=64colours$ Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab 8 colours is 3 bits per pixel as: $2^3=8colours$
h * w * b = 20 * 30 * 3 = 1800 bits Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab When you want to save file space or when you only need a specific palate of colours such a mono-chrome Template:CPTAnswerTabEnd Template:Robox/Close Template:BookCat