Photo compression

[Polly]

I think that my query has something to do with the title!

I was wondering what causes the slight aberration that I, and others, sometimes get on their photos when they are downloaded to a website.

The distortion I mean is the "crinkling" or spotting (don't know how to really describe it) above the butterfly. For example you can see it on Gary, Pete and Lisa's photos in the thread "Interesting Comparison" along the top edge of the wing and up the antennae but it isn't on Rogers' photo (nothing personal guys, I just want to learn ). I seem to get it a lot when I download photos

Is it to do with how the photo is re-sized for the web? How do you avoid it please?

Thanks

Polly

[Dave McCormick]

What I know is that when using JPEG its probably to do with compression. I found this that might help what your asking: http://photo.net/learn/jpeg/ although I am trying to understand a few of the terms used.

[Polly]

Thanks for the link Dave. The article is rather too technical for me though there is a good example of what I was trying to describe in the right-hand photo under the heading "Visual Perception".

Polly

[Polly]

Can anyone please help with my query about JPEG compression?

[Padfield]

Can anyone please help with my query about JPEG compression?

I was leaving it to the experts, but if I write some nonsense here you'll get an answer as they all queue up to correct me!!

A raw image has each pixel separately coded for and is huge. It represents the light that reaches the sensor. A compressed image condenses the information in many ways, including effecively 'describing' the image rather than representing it. For example, instead of recording 30 consecutive pixels with the same colour it could indicate simply that 30 identical pixels are coming up. But to describe wholly accurately is still heavy, so approximations and shortcuts are made, with the result that information is lost. There are algorithms that minimise the effect of loss of information by recognising lines, borders &c. and slightly modifying the result to reflect them, accentuating (sharpening) borders for example. The greater the compression, the more 'after work' is needed to keep the picture looking good.

All this introduces artefacts of one sort and another. A common artefact is a kind of heat haze over edges. I imagine this is preferable to the blurred edge that would result from compression without the post-compression adjustments.

Different compression algorithms will result in different effects too, I imagine.

That should get someone to explain it to us properly, Polly!!

Guy

[Markulous]

I think that my query has something to do with the title!

I was wondering what causes the slight aberration that I, and others, sometimes get on their photos when they are downloaded to a website.

The distortion I mean is the "crinkling" or spotting (don't know how to really describe it) above the butterfly. For example you can see it on Gary, Pete and Lisa's photos in the thread "Interesting Comparison" along the top edge of the wing and up the antennae but it isn't on Rogers' photo (nothing personal guys, I just want to learn ). I seem to get it a lot when I download photos

Is it to do with how the photo is re-sized for the web? How do you avoid it please?

Sounds like pixellation - when pixels take on the attributes of the neighbouring pixels and so form blocks. Can be introduced/accentuated in Post Production but it's an artefact usually associated with compression - can only be avoided by compressing the JPG less

A second possibility is haloisation (a dark edge against a bright background - usually a skyline, tree branches, etc) which is a sharpening artefact introduced in PP, reduced by lowering the degree of sharpening or using alternative methods of sharpening

[Polly]

A second possibility is haloisation (a dark edge against a bright background - usually a skyline, tree branches, etc) which is a sharpening artefact introduced in PP, reduced by lowering the degree of sharpening or using alternative methods of sharpening

From the description it sounds as though the problem may be from over sharpening rather than JPEG compression. I think that I'm going to have to do some work on it.

Thanks Guy and Markulous

[Zonda]

I agree, this edge pixelation is sometimes caused by over-sharpening as well. Also, when you save an image in jpeg format, always go for the least compression setting. I never use RAW,,life's too short.

[Vixpix]

Hi Polly,
Something else to consider would be the file type, jpegs are known as "lossy" files which means that each time you save the same jpeg file there is a loss of information resulting in reduced image quality. Saving a jpeg file is re-compressing an already compressed file and if done too many times will lead to "compression artifacts".
ALWAYS save a jpeg image as "save as" which will keep your original image untouched.
NEVER save your original jpeg image as "save" because the information lost in compression is unrecoverable unless you have backed-up the files in their original form.
A combination of compression artifacts and over sharpening of an image can result in the sort of edge definition problems that you refer to.

Hope this is of some help

Regards Vic.

[Polly]

Yes, very helpful - thanks