GCSE COMPUTING 1 4 IMAGES Candidates should be

GCSE COMPUTING – 1. 4 IMAGES 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. © GCSE Computing Slide 1

HOW CAN A BITMAP IMAGE BE REPRESENTED IN BINARY? 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. © GCSE Computing Slide 2

WHAT IS COLOUR DEPTH? 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 1 st colour and 1 = the 2 nd colour). This would result in a monochrome image such as that used by early computer monitors. Little memory was needed to store the screen because: Screens were low resolution (used few pixels). Each pixel only used 1 bit of memory. © GCSE Computing Slide 3

HOW DOES COLOUR DEPTH AFFECT AN IMAGE? If only 1 bit is used per pixel then 2 colours can 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. © GCSE Computing Slide 4

HOW DOES COLOUR DEPTH AFFECT AN IMAGE? 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 111111110000. 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 - 0000, 00000001 etc. 24 16, 777, 216 True colour - 8 bits for Red/Green/Blue © GCSE Computing Slide 5

WHAT IS RESOLUTION AND HOW DOES IT EFFECT THE SIZE OF AN IMAGE FILE? 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. © GCSE Computing Slide 6

WHY DOES METADATA NEED TO BE INCLUDED IN AN IMAGE FILE? 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 The resolution so each line of the image starts in the correct place. The correct width metadata. The wrong width metadata. 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. © GCSE Computing Slide 7