© 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

© GCSE Computing  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

© GCSE Computing  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. Slide 3

© GCSE Computing  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. Slide 4

© GCSE Computing  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 Slide 5 Colour depthNumber of coloursNotes 12Monochrome 2400, 01, 10, 11 etc , 001, 010, 011 etc , 0001, 0010, 0011 etc. 8256Direct colour , etc. 2416,777,216True colour - 8 bits for Red/Green/Blue

© GCSE Computing  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

© GCSE Computing  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 wrong width metadata.