What does easy compression tell me about a jpeg?

Zak wrote:

As an example, I notice that (with the SAME settings for compression in Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs will compress right down to a surprisingly small 100 KB.
What does the final size in KB compared to the original size tell me about the properties of original jpeg?

Nothing. The file size is related to how much detail there is in the image. More detail<=>larger file.

Or to put it another way, how is the degree of compression (given the same settings on Acdsee) linked to the original jpeg’s properties such as pixel size, print size, etc?

It isn’t.

Andrew

"Zak" wrote in message

Sometimes I compress a jpeg which I have downloaded from the Net.
I always use the same old version 3.1 of ACDSEE for compression mainly because it is simple and convenient although I do have Photoshop and Paint Shop Pro. However I tend not to use them for something which seems so straightforward.

As an example, I notice that (with the SAME settings for compression in Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs will compress right down to a surprisingly small 100 KB.
What does the final size in KB compared to the original size tell me about the properties of original jpeg?

Or to put it another way, how is the degree of compression (given the same settings on Acdsee) linked to the original jpeg’s properties such as pixel size, print size, etc?

Jpeg uses an adaptive algorithm for compression. Lower quality numbers result in smaller files, but the actual size, as you imply, depends on the amount of detail contained in the image. File size, and therefore jpeg file size does not depend on the pixel size or print size of the image, since these are ancillary data that are unrelated to image content.

For example, a 512×512 image containing only white will save to 14KB at jpeg quality 10 in Photoshop, while the same size image containing gaussian noise compresses to 695KB. Blurring the noise image by .5 pixel results in a 424 KB jpeg image.

Normal photographs will vary between these two extremes, with the biggest variable probably being the amount of noise in the image. Certain material, such as scanned cartoons, will save smaller and at a higher quality as gif images, or RLE encoded tiff images, than as compressed jpegs. Each RGB channel is compressed separately, and in theory a somewhat smaller image could be achieved by jpeg compressing an Lab image, with a higher quality for the Lightness channel.
—
Mike Russell
www.curvemeister.com/forum/

"Mike Russell" wrote in message

Each RGB channel is compressed separately, and in theory a somewhat smaller image could be achieved by jpeg compressing an Lab image, with a higher quality for the Lightness channel.

Oops – scratch that. Jpeg images do not compress the RGB channels separately, but treat color data separately from luminance. —
Mike Russell
www.curvemeister.com/forum/

In article ,
Zak wrote:

As an example, I notice that (with the SAME settings for compression in Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs will compress right down to a surprisingly small 100 KB.

A lot depends on how much the JPEG was compressed to begin with.

If it was originally compressed with a high quality setting, and you save it from ACDSee with a medium quality setting, it may shrink a bit. If it was originally saved with an extremely high quality setting, and oyu save it with a medium quality setting, it may shrink quite a lot.

—
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tacit wrote:

In article ,
Zak wrote:

As an example, I notice that (with the SAME settings for compression in Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs will compress right down to a surprisingly small 100 KB.

A lot depends on how much the JPEG was compressed to begin with.

Sort of, no, more like, how well a text file will zip/compress.

🙂

Uni

If it was originally compressed with a high quality setting, and you save it from ACDSee with a medium quality setting, it may shrink a bit. If it was originally saved with an extremely high quality setting, and oyu save it with a medium quality setting, it may shrink quite a lot.

On 15 May 2006, Mike Russell
wrote:

As an example, I notice that (with the SAME settings for compression in Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs will compress right down to a surprisingly small 100 KB.

What does the final size in KB compared to the original size tell me about the properties of original jpeg?

Or to put it another way, how is the degree of compression (given the same settings on Acdsee) linked to the original jpeg’s properties such as pixel size, print size, etc?

Jpeg uses an adaptive algorithm for compression. Lower quality numbers result in smaller files, but the actual size, as you imply, depends on the amount of detail contained in the image. File size, and therefore jpeg file size does not depend on the pixel size or print size of the image, since these are ancillary data that are unrelated to image content.

For example, a 512×512 image containing only white will save to 14KB at jpeg quality 10 in Photoshop, while the same size image containing gaussian noise compresses to 695KB. Blurring the noise image by .5 pixel results in a 424 KB jpeg image.

So blurring will result in a larger file size? That may explain it. I see some very sharp pictures with what seems to be decent detail be quite compressible.

Whereas poorer quality images (perhaps poor scans, images that have been tinkered with, poor photo equipment if photo is very old) do not compress corrtespeondingly better.

I thought the poorer the quality of the jpeg the more it would compress but I guess that often it is the other way around.

Is my understanding broadly correct?

Normal photographs will vary between these two extremes, with the biggest variable probably being the amount of noise in the image. Certain material, such as scanned cartoons, will save smaller and at a higher quality as gif images, or RLE encoded tiff images, than as compressed jpegs. Each RGB channel is compressed separately, and in theory a somewhat smaller image could be achieved by jpeg compressing an Lab image, with a higher quality for the Lightness channel.

From: "Zak"
….

So blurring will result in a larger file size? That may explain it. I see some very sharp pictures with what seems to be decent detail be quite compressible.

No – the other way around. Blurring results in a file that compresses more efficiently.

Whereas poorer quality images (perhaps poor scans, images that have been tinkered with, poor photo equipment if photo is very old) do not compress corrtespeondingly better.

Adding noise, sharpening, increasing contrast, all tend to make the resulting jpeg larger.

I thought the poorer the quality of the jpeg the more it would compress but I guess that often it is the other way around.

Is my understanding broadly correct?

In general, good clean scans will very little noise will compress better than noisy scans. Because most edits result in increased contrast and color, manipulated images, with certain exceptions, will generally be larger than unmanipulated ones.
—
Mike Russell
www.mike.russell-home.net

"Mike Russell" wrote in message

From: "Zak"
…

So blurring will result in a larger file size? That may explain it. I see some very sharp pictures with what seems to be decent detail be quite compressible.

No – the other way around. Blurring results in a file that compresses more efficiently.

Question Mike: Gaussian blurr is, by definition, not an even blur (although it is ‘regular’), so adjacencies calculate to differences that do not compress well. Is that correct?

From: "2"
….

Question Mike: Gaussian blurr is, by definition, not an even blur (although it is ‘regular’), so adjacencies calculate to differences that do not compress well. Is that correct?

Gaussian refers to the "shape" that is used to distribute pixel values when calculating the blur. It tapers off gradually, and would produce a slightly smaller jpeg file than, for example, a box blur which has sharp edges. —
Mike Russell
www.curvemeister.com/forum/