what does compression mean

iota

can anyone out there explain the term "compression", how do I know if my pics are over compressed? I am a total tec idiot so need to have things explained in very simple term.

Paulw

Compression.

If your image is jpg, then it's compressed. Many other image formats are compressed too.

CabanSail

Compression is a technique used to make data files (in this case Photographs) smaller.

You can have Lossless compression. This is where no data is lost but the file is is made somewhat smaller. This is used in some TIFF formats & also some RAW files.

There is also Lossy compression. This can make files a lot smaller but data is sacrificed. This is what happens in JPEG files. As you compress the file you get artifact in the image. if you compress too much the artifact becomes very noticable.

CabanSail

These links may be of interest. Probably more information there than most require.

Wiki about Jpeg

Comparison of Graphic File Formats

iota

thanks for the link, I'm sure it will help

sheesh

iota wrote: »

can anyone out there explain the term "compression", how do I know if my pics are over compressed? I am a total tec idiot so need to have things explained in very simple term.

you don't need to know much compression only that it happens

one thing to know is what happens when you use jpeg the program compressing the image goes through the image and finds areas that have areas of similar colour it sets that area to the same colour there by saving a certain amount of memory space. so the if there was a image with alot of different colours it might look a bit dull when you compress it

DeVore

I know a great deal about compression. If you have more questions, put them here and give me a PM and I'll swing by. No sense in repeating whats in those links but it would be helpful I'm sure to have an interactive answerer

the TL;DR answer:

Typically you know you have over compressed when you get little itty bitty squares of the same colour in the picture.

Compression takes small squares of the picture which APPEARS to be the same colour, and MAKES it the same colour. The more aggressive the compression, the bigger the square and the wider the range of shades that will be considered the same colour. At low settings the human eye cant tell but the amount of information saved is large. Pushing it further can reveal these squares to the naked eye.
You sometimes see these in a dodgy cable-tv signal, particularly on big HD tvs which expose the problem to the eye even more because of their high definition.

Theres a lot more to it then that but basically compression is a necessary evil but should be the last thing you do to the pic before uploading it or copying it to CD etc.

DeV.

iota

thanks to everyone who answered my question about compression, what a usefull sight this is:)

DeVore

No problem. You'll find some part of it better then others!

One thing I meant to say is that not ALL compression is bad. There are two main types:

"Lossy" which means that information from the picture was destroyed/discarded during the process of compression

and

"Lossless" which means the information was just kind of squished and it is NOT lost. This means the picture, remains just as detailed but smaller in file size.

Lossless compression is far less space-saving then lossy generally, which is the catch!

DeV.

iota

hi, thanks for all the answers to my compression question, I'm sure you wont be suprised by my next question. How in do I avoid said compression, in very simple words, as I really am a digital twit.

CabanSail

You avoid compression by not doing it, but you have to then work with large files.

I think what you mean is "How do I avoid artifact?" This will be a compromise between quality and size. You need to decide on the use of image. You should keep your original image as taken & only compress as a copy. If the image is display on the web then you would normally resize & compress the file to make it managable. If you are printing the file you probably are best only using lossless compression or keeping the jpeg as large as possible.