The resolution of an image is determined by the number of individually addressable points that make up the image, whether it is the number of pixels that make up a screen image, or the number of dots that make up a printed image.

The more dots that are used to create an image, then the more detail the image can resolve and the sharper it appears when viewed.

When using bitmap graphics, whatever resolution is chosen, information needs to be stored for each pixel or printer dot. This means the higher the resolution, the more information needs to be stored for an image of any given size.

This does not apply when using vector graphics, as the decision about resolution, only needs to be made when the image is printed, or exported as a bitmap.

Colour Depth

Colour depth is used to describe the maximum number of colours that are used in the image. The higher the number of colours then the more realistic the image will appear. Once again, with bitmap images, the chosen colour depth will affect the final file size.

Let's look at how these colour models work, although these examples refer to pixels, the same applies to dots in a printed image.

Monochrome (e.g. Black & White)
This has the lowest storage requirement, because we only need to know whether the pixel should be on or off, the information for each pixel an be stored in a Bit, or put another way, we can store the information for eight pixels in one Byte of storage space.

Greyscale (256 shades of Grey)
Because a Byte can be one of 256 values, then each pixel requires one Byte of storage space.

256 Colours (Palleted)
This format allows each pixel to be one of 256 colours stored in a Colour Look up Table (CLUT), more commonly referred to as the palette. The palette is also saved with the file so this adds slightly to the file size, but the information for each pixel is stored in one Byte of storage space.

Note. The GIF format, is a variation of the above, in that it allows you to decide upon the size of the palette stored with the image, this allows you to reduce the overall file size, if your image does not use many colours. Generally 256 colour formats store the whole 256 colour palette, whether the image uses them or not.

16 Bit Colour (High Colour)
Though not quite as common now as a few years ago, this format uses two Bytes to store the information, one Byte for the colour and one Byte for the shade of the colour. This gave a total of 65,536 colours (256x256). So each pixel requires two Bytes of storage space.

24 Bit Colour
This format stores the Red, Green and Blue value for each pixel. Each of these can be one of 256 values, giving a total of 16,777,216 colours (256x256x256). Using 16 million colours allows for very photorealistic images, but increases the storage space requirements to three Bytes for each pixel.

32 Bit Colour (True Colour)
This format uses the same format as above for the Red, Green and Blue colours but also stores transparency information for each pixel. This allows each pixel to be one of 256 values from fully opaque to fully transparent. Because of the extra transparency information, the storage space for each pixel now requires four Bytes.

Combining the Two

Using simple calculations, it is possible to estimate the approximate storage space or memory required for a particular image. Here are some examples.

Creating bitmap images for a slide show
First of all calculate the number of pixels, let's assume a screen resolution of 800x600. This gives 480,000 pixels. We will use a 24 Bit colour depth. This means we will need three Bytes for each pixel, or 1,440,000 to display the image.

Scanning a photo for retouching.
Let's imagine that you have been asked to repair damage to an old 5" x 3" black and white photograph.

If we want to print the final result at 600 dpi, then we will need to scan at the same resolution. So our 5" x 3" photograph will use 3000 x 1800 dots or 5,400,000 dots in total. Because it is a black and white photo, we can use greyscale (one byte per dot), which will require 5.4 Mb for the initial scan.

If it were a more recent photo, let's say 6" x 4" colour. Then using the same resolution with a 24 Bit colour depth would mean 25,920,000 dots. That's 25 Mb for the scan.

I'll let you do the calculations for a 10" x 8" colour photo!

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