JPG - a 'lossy' image format.

[werdan]

Here's a good video showing why jpg is called a lossy compression format.

Generation Loss on Vimeo

The bloke went through a load & save cycle on an image 600 times.

[dullbird]

good way of seeing...not a lot of people realise that, the more they open and save a jpeg file the more it loses it quality

[vnx205]

That demonstration, while useful and dramatic, does not mean that we should all abandon JPEG files.

When I ran the video I found it hard to see any change in the first 5 seconds. If the 20sec video represents 600 saves, then that means that for the first 150 saves there was a negligible difference.

There is an added complication, because most programs allow you to set the level of compression. If you set the quality high (or the compression low), you lose less each time.

The loss of quality only occurs if you open, modify and save the file. Opening and closing the file is not a problem. In fact some programs are able to do some modifications without the file being recompressed on saving.

Here is just one article of many that shows that for most of us, the loss of quality is not something to get paranoid about.

Muddyboots Photography: How bad is Jpg's lossy compression? (http://muddybootsphotography.blogspot.com/2008/10/how-bad-is-jpgs-lossy-compression.html)

A sample test is shown here for those who don't want to read the whole article. One version of the picture has been recompressed 10 times at 85%. If the quality was set higher, the deterioration would be less.

http://www.fileden.com/files/2008/7/16/2005444/2944888497_8956e93f01.jpg

[werdan]

I wasn't suggesting we throw away jpg, far from it.

I've always been told get the best quality out of an image, scan & edit in TIFF or PSD and only then save in jpg when putting on the web or printing it at a photo place. However I've never noticed any degradation of the image saving as jpg a few times, unless the compression setting was wound right up.

I guess I just liked this vid because it shows that the effect is exponential; slowly adding artifacts and then quickly falling to bits.

[slug_burner]

Surely you only get that repetetive compression loss if you decompress and recompress the file. Most people compress once and that is it. Every time you view the file you don't resave/recompress. It is only if you open the jpg into an editing program, (change something, why else would you resave) and then recompress it that you will suffer the losses shown.

[vnx205]

I wasn't suggesting we throw away jpg, far from it.

I've always been told get the best quality out of an image, scan & edit in TIFF or PSD and only then save in jpg when putting on the web or printing it at a photo place. However I've never noticed any degradation of the image saving as jpg a few times, unless the compression setting was wound right up.

I guess I just liked this vid because it shows that the effect is exponential; slowly adding artifacts and then quickly falling to bits.

I didn't think you were suggesting that people shouldn't use JPEG. It was not my intention to criticise the video or your dcision to post it. I think my reaction was based more on a fear that some people tend to see things in simplistic terms and overreact when given some information critical of a product or service. I was just doing my bit to discourage people from overreacting.

I was struck by the same thing that you have commented on in your last paragraph. It was not so much the deterioration that astonished me; it was the rate at which the deterioration accelerated once it started.

[vnx205]

Surely you only get that repetetive compression loss if you decompress and recompress the file. Most people compress once and that is it. Every time you view the file you don't resave/recompress. It is only if you open the jpg into an editing program, (change something, why else would you resave) and then recompress it that you will suffer the losses shown.
That's right.
If you only look at the picture, it isn't compressed again as you close it.
You can look at it as many times as you want and it won't change at all until you make a modification and then save it.

[mfc]

the losses that concern me most are from the discarded information in the alogrithem{?} in the first place, shoot 4 or 5 scenes in both raw and jpg then compare the loss of information in the jpg as compared to the raw file!!! Its extensive and quite frankly no acceptable especial as d sensors are so bad at recording high contrast...buy a few more cards and batch process
my 96 cents worth
cheers mark