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





Resolution




Example
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
The smallest possible addressable area defined by a solid colour, represented as binary, in an image.

The amount of pixels an image contains per inch/cm

the number of pixels per row by the number of pixels per column

100 * 70 = 7000 pixels

30 * 40 = 1200 pixels

1920 x 1080 = 2073600 pixels

700 / 35 = 20 pixels

higher resolution images are able to display more detail, providing crisper images

It 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.

Colour Depth
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
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

Questions
The number of bits used to represent the colour of a single pixel

$$2^8 = 256$$

10 as $$2^{10} = 1024$$

$$30 * 40 * 4 = 4800 bits$$

$$30 * 40 * 6 = 7200 bits$$

$$30 * 40 * 2 = 2400 bits$$

$$2^4 = 16 colours$$

6 as: $$2^6 = 64 colours$$

8 colours is 3 bits per pixel as: $$2^3 = 8 colours$$ h * w * b = 20 * 30 * 3 = 1800 bits

When you want to save file space or when you only need a specific palate of colours such a mono-chrome