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Old Mar 10, 2012, 6:54 PM   #1
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Default Recent experiments with JPG with unexpected results

G'day all

This post covers some recent experiments I have done with JPG saving of images where I would like some feedback from others who have also done experiments.
I am not after comment from arm-chair experts quoting magazines, other internet sites or third-hand pronouncements from other sources

For starters...
I guess that I, like you, have read & been told "don't use JPG, you'll lose too much image quality" ...
I have also read photo magazine articles "claiming & showing test images" that purport to show that JPG images go fuzzy from pixellation if you save a JPG to another JPG - so I decided to try a number of things and see where it got me

Firstly I took a 12mpx [4000 x 3000px] ex-camera jpg image as my 'base-line' image

experiment-1
a) I opened the image [original]
b) then cropped the image to a smaller size
c) then chose 'undo' to return it back to its former size
d) then saved the ex-camera image as JPG > JPG with PS Quality=12/12, & renamed the file as 'image-1'
ie- no real & lasting change to the image occurred, but it was saved JPG > JPG

This was repeated 4 times with image-1 being opened, altered, undone, then saved as image-2; then repeated image-2 to image-3; image-3 to image-4 etc

Results-
The ex-camera JPG image is 6,2Mb in size, the image-1 -2 -3 -4 & -5 saved images are all 6,1Mb in size and all still 4000 x 3000px in size
result- no loss of image size either via pixel dimensions or file size

experiment-2
I repeated experiment-1 with a 3mpx [2048 x 1536px] image and using a Win-95 version of Photo Express software [rather than PSE]. The ex-camera image was 1.04Mb in size, ALL the saved images were all 660kb in size when using the default JPG > JPG saving regime
result- again no loss of image size either via pixel dimensions or file size beyond the initial JPG > JPG save

experiment-3
Involved printing the image(s) - I wanted to see what visible changes occurred via JPG > JPG saving of the image
I then saved the 12mpx image [from experiment-1] with JPG > JPG format and set PSE Image Quality as 12/12, then 9/12, then 6/12, then 3/12 and finally 1/12. To ensure no mixups occurred during printing, a small text comment was inserted into each image via a text layer then flattened before saving & printing

I then took the above 12mpx ex-camera jpg image & all the saved copies down to my local print shop. Each was printed as 12" x 8" prints
So with a series of 6x 12" x 8" prints on the table, I sought the comment from the local photo-professional as to each image

He was unable to see any visible difference between any of the 12" x 8" prints - and expressed amazement that the details in the PSE 1/12 quality image was just as sharp as the 12/12 quality image. [the image subject was my missus, and we were looking at the weave of her dress fabric along with fine-ness of her hair] He did locate a very slight colour-shift in one small portion of the image, but stated that "without the original to compare it with, he would not have noticed anything"

Overall he was astonished and could not answer the basic Question - why is there so little IQ difference between PSE quality 12/12 versus 1/12

Attached here is a 1024px image showing a portion of the image from experiment-3, where a new canvas of 4000px wide was prepared and partial images from the experiment-1 images were pasted and some text added. This image has been reduced to <250kb for posting here and you can see the resulting "loss" of sharpness from repeated JPG saves



So - over to you ... what's going on? ... why do we keep getting told that "JPG > JPG saves will cripple our images?" when quite obviously from the above it does nothing of the sort

Regards, Phil
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Old Mar 10, 2012, 8:19 PM   #2
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Crop and undo means nothing has happened to the image file. PS may even have restored the original file.

I assume that PS Quality=12/12 is the minimum JPEG compression. Have you tried the experiment using a higher compression such as 6?
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Old Mar 10, 2012, 10:43 PM   #3
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First, there are lots of edits that can be performed on an image that can avoid the image being re-compressed.

Second, you didn't actually chage the image, so it may very well never have been recompressed.

Third, the procedure you used doesn't explicitly say anything about it, but if you just opened the orginal file, and then saved it four times, I wouldn't expect to see much difference in them. But if you opened the original file, saved a copy, opened the copy, saved a copy of that, opened that copy and save a copy of that, etc., etc., etc., I'd expect to see some difference there.

So if you were always working with the same image in memory, and never actually did anything that required the image to be recompressed, then I have every expectation that they'd all look about about the same.
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Old Mar 10, 2012, 10:55 PM   #4
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As Bob says, in experiment 1, by using the undo, you didn't do anything to the original. I don't think that matters much, anyway, as using the minimum compression level, it takes more than 20 iterations to even begin to see changes in the picture. (I got bored after twenty, and gave up) Try it again at a '4' setting, and you should see some changes.
As far as printing the images goes - the resolution of your camera is high enough that when downsized to fit the print, the pictures are going to look pretty much the same. Part of this is due to the way printer drivers deal with jpeg artifacts, and part because you are starting with more resolution than the printer can use. From my own tests, I have found that the size of the photo is more important when printing, than the amount of compression.
Jpeg is a 'lossy' compression method, in that some data is thrown away when the compression is done. Some people just can't stand the thought of losing any part of their precious photos, and dislike using jpeg for that reason. At minimal compression, I have never found any significant loss of quality or resolution. Maybe if I were printing banners for the side of buses, there might be enough difference to make it worthwhile to use lossless compression.

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Old Mar 11, 2012, 7:03 AM   #5
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That's an interesting test Phil.

Quote:
no loss of image size either via pixel dimensions or file size
That suggests that the image was either completely unchanged or had
little or no compression applied.

Here are a few shots of a JPEG compressed image using various
levels of compression in Gimp. The original shot can be found here:
http://pix.ie/corkpix/2764989/size/1600/in/album/371360
Click on the image for details.

The image was scaled to 1024x768 so that it would fit within forum limits
then converted to Gimp's native uncompressed format (.xcf)
and then saved at several different levels of JPEG compression. To avoid
repeated re-compression of the same image, each JPEG was saved from
the uncompressed version. The number in the JPEG filenames represents
the compression level on the Gimp's 0-100 quality scale. File sizes are:

2.4M gull-orig.xcf
116K gull-90.jpg
48K gull-70.jpg
36K gull-50.jpg
28K gull-20.jpg

At quality=90, there is a substantial reduction in file size from 2.4MB to 116kB
but there is little or no loss of perceived image quality. At lower quality
(higher compression) the degradation is easily seen.
Attached Images
    

Last edited by corkpix; Mar 11, 2012 at 7:05 AM.
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Old Mar 11, 2012, 3:50 PM   #6
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G'day all ~ thx for the replies so far
I may not have made myself completely clear here - so forgive me if I repeat myself a bit ...

Quote:
Originally Posted by Bob Nichol View Post
Crop and undo means nothing has happened to the image file. ... I assume that PS Quality=12/12 is the minimum JPEG compression. Have you tried the experiment using a higher compression such as 6?
@Bob > in experiment-3, I saved the file 5 times, each time closing & reopening the original file between saves and each save altering the PSE quality figure from 12/12 [maximum quality] thru 6/12 [medium quality] down to 1/12 [minimum quality] ... and with no discernable difference in the printed result

Quote:
Originally Posted by TCav View Post
.... Third, the procedure you used doesn't explicitly say anything about it, but if you just opened the orginal file, and then saved it four times, I wouldn't expect to see much difference in them. But if you opened the original file, saved a copy, opened the copy, saved a copy of that, opened that copy and save a copy of that, etc., etc., etc., I'd expect to see some difference there. .....
@TC > I thought my text was clear, but yes, the image was saved then closed & then reopened and resaved etc to force a series of recompressions to occur

Quote:
Originally Posted by VTphotog View Post
As Bob says, in experiment 1, by using the undo, you didn't do anything to the original. I don't think that matters much, anyway, as using the minimum compression level, it takes more than 20 iterations to even begin to see changes in the picture. (I got bored after twenty, and gave up) Try it again at a '4' setting, and you should see some changes.brian
@Brian > Yes you're right in that I didn't want to do anything to the original but I did want to force it to save & resave & resave again via a series of open & close of consecutive files

Quote:
Originally Posted by VTphotog View Post
As far as printing the images goes - the resolution of your camera is high enough that when downsized to fit the print, the pictures are going to look pretty much the same. Part of this is due to the way printer drivers deal with jpeg artifacts, and part because you are starting with more resolution than the printer can use. From my own tests, I have found that the size of the photo is more important when printing, than the amount of compression.brian
Quote:
Originally Posted by VTphotog View Post
Jpeg is a 'lossy' compression method, in that some data is thrown away when the compression is done. Some people just can't stand the thought of losing any part of their precious photos, and dislike using jpeg for that reason. At minimal compression, I have never found any significant loss of quality or resolution. ...
@Brian - yes again ... I am well aware of the JPG operations - I just wanted to confirm for myself how far I could go with it and little or nothing happened which is where my surprise came in

and @Corkpix > thx for your results too ... I would expect that by downsizing to 1k in size, any reductions would become immediately visible and they have done so, so good one there

I am still a bit mystified & will continue experimenting

Regards, Phil
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Old Mar 11, 2012, 5:21 PM   #7
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Quote:
Originally Posted by Ozzie_Traveller View Post
@TC > I thought my text was clear, but yes, the image was saved then closed & then reopened and resaved etc to force a series of recompressions to occur
A better test would have been to open the most recently saved copy instead of the original, and resaved that.

Quote:
Originally Posted by Ozzie_Traveller View Post
@Brian > Yes you're right in that I didn't want to do anything to the original but I did want to force it to save & resave & resave again via a series of open & close of consecutive files
A better option to force a resave would have been to rotate the image 180 degrees. That would have done nothing to the actual image, but would have forced theJPEG to be recompressed.
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