© GCSE Computing

Candidates should be able to: explain the representation of an image as a series of pixels

represented in binary explain the need for metadata to be included in the file such

as height, width and colour depth discuss the effect of colour depth and resolution on the size

of an image file.

Slide 1

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Bitmap images are made up of individual pixels (picture elements).

The colour of each pixel is represented as a binary number.

The image is therefore stored as a series of binary numbers.

The program loading the image needs to know things like the image resolution and colour depth before it can convert the binary back into an image and display it.

Slide 2

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The colour depth describes how many memory bits are used to store the colour of the pixel.

If only 1 bit is used (1 bit colour depth) then 2 colours can be stored (0 = the 1st colour and 1 = the 2nd colour).

This would result in a monochromeimage such as that used by earlycomputer monitors.

Little memory was needed to store thescreen because: Screens were low resolution

(used few pixels). Each pixel only used 1 bit of memory.

Slide 3

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If only 1 bit is used per pixel then 2 colourscan be stored.

If 2 bits are used per pixel then 4 colours can be stored. 00 = colour 1 01 = colour 2 10 = colour 3 11 = colour 4

This means the amount of memory needed to store an image with 2 bit colour depth is twice that needed to store the same 1 bit image.

Slide 4

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Up to 8 bit colour depth it is possible to have a code for each colour. This is called direct colour.

Beyond 8 bits it is easier to define a colour by how much RED, GREEN and BLUE (RGB) it is made up of.

This is called true colour. Modern computer displays use 24 bit colour depth. 8 bits are used for the amount of Red, Green and Blue. Pure yellow would be stored as 111111111111111100000000.

Slide 5

Colour depth

Number of colours

Notes

1 2 Monochrome

2 4 00, 01, 10, 11 etc.

3 8 000, 001, 010, 011 etc.

4 16 0000, 0001, 0010, 0011 etc.

8 256 Direct colour - 00000000, 00000001 etc.

24 16,777,216 True colour - 8 bits for Red/Green/Blue

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Resolution is a measure of how much detail there is in an image.

A high resolution image can be magnified and still stay sharp. A low resolution image will appear pixelated (the individual

pixels will be clearly visible) if it is magnified too much. The resolution depends on the pixel density, the number of

pixels per unit of distance (not the total number of pixels in the image).

It is usually measured in pixels per inch (PPI).

SUMMARY: A bitmap image with a high resolution will have a greater the file size than the equivalent low resolution depth image because more memory is used to store the colour data of the extra pixels.

Slide 6

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Metadata is needed in a bitmap image file because the software that displays an image needs to know:

The height and width of the image so each line of the image

starts in the correct place. The resolution

so the image displays at the correct size.

The colour depth so the correct number of

bits are used to represent the colour of each pixel.

Slide 7

The correct width

metadata.

The correct width

metadata.

The wrong width

metadata.

The wrong width

metadata.