[robski]

Let us first consider how much memory an uncompressed bitmap raster image uses.

Using a small 1280 x 1024 pixel desktop image as an example.

Each pixel requires 3 memory locations to store the RGB colours.

So 3 Blocks ( arrays-grids) of 1280 x 1024 memory are used = 3932160 memory locations

Therefore any 1280 x 1024 image will always use 3932160 memory locations

The Windows Paint bmp files stores data in this uncompressed format plus some header info.
With larger images the huge file sizes create a problem for data storage and transmission over networks.

To overcome these problems data compression is used to reduce the file size.

The first data compression methods devised were loss-less. That is after compression and decompression you get back the original data.

I will give you a basic idea of some of the loss-less methods used but they relied on the data being inefficiently coded in the first place to get good compression ratios. With a few exceptions as a general rule of thumb if the compression ratio is better than 10:1 then it is not a loss-less method used.

To improve compression ratios further lossy methods were developed. That is after compression and decompression you get back something that looks like the original data.