Jpg / Jpeg Downloader

JPEG

What is jpeg? Technically, "jpeg" refers to a picture file encoded to a standard specified by the JPEG Committee.

How do I know a picture file is a jpeg? Typically a jpeg file has the "jpg" or "jpeg" extension. If it is being downloaded by a web-browser, the web-browser software knows that it is a jpeg because it is told it has a mime-type of "image/jpeg".

Just to confuse things; a valid jpeg file does not need to be marked with "jpg" or "jpeg" or "image/jpeg", and a file marked with "jpg" or "jpeg" or "image/jpeg" is not necessarily a valid jpeg file.

How does my computer turn a jpeg into a picture on my screen? The best analogy is to imagine a large grid. In each of the grid's cells is a number which represents a color of that cell's dot. Effectively it is a long list of numbers ... row after row from the grid. This is called a bitmap  and no matter what the original format of the picture (jpeg or gif etc), it is converted into a bitmap before being displayed.

Why then bother with jpegs? Quicker Downloading! Jpegs are so popular because a relatively small list of numbers in jpeg format converts into a much larger bitmap. The jpeg below is 2,104 bytes in size

When converted to a bitmap the size is 40,960 bytes ... around 20 times larger, and therefore would take 20 times longer to download. If you are a frequent jpeg downloader, you end up with 20 times more pictures!

Is there a cost to using jpegs? Yes, you don't get this reduction in size for free. It is referred to as "lossy" ... you lose some information. However, with most photographs, if the picture is compressed up to 20 times smaller, we can't really see any difference. You can go smaller and smaller, but that becomes noticeable:

The above jpeg is the same picture, but it has been compressed to only 503 bytes ... 80 times smaller ... and it looks like a "blocky" version of the original.

Sample jpegs you can download: