Re: Auto levels

"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio
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il barbi wrote:
"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio
news:83sgm.136645$FP2.110534@xxxxxxxxxxxxxxx
il barbi wrote:
"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio
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il barbi wrote:
(I think
I'm puzzled with such operation: I apply auto levels to some jpgs
from my digital camera (1600x1200 pixel), with file size about 600K,
and I get file size about 300K!
Someone can explain?
il barbi


Your Photo Editing Program COMPRESSED your image more than it was
originally.
In Photoshop, and probably others, you can choose the degree of
compression you want in the SAVED image.
Tell us what photo Editor you are using and maybe someone can tell you
how to control the compression.
I only know Photoshop, but many in this NG have used every imaginable
photo editor. They can surely help you.
Bob Williams

Of course I'm using Photoshop, this is a NG about Photoshop, isn't it?
Now I get a file from my digital camera, say org.jpg, size 599K,
1600x1200
pixels = about 2Mpixels, I enter Photoshop, open org.jpg and choose
autolevels, then file - close, it asks me "Save the
modifications?", I choose yes and I get org.jpg with size 375K
I thought it must have expanded the file as a bitmap, applied auto
level by scaling the 3 RGB bytes representing each pixel, then
re-compressed the file at the same degreee as at beginning (it did not
ask me the jpg compression level)
but why is the size about halved?
il barbi



After you do your Auto Levels operation.
Try the FILE >>> SAVE AS option and then select the highest quality.
Check the number of pixels then.
Bob

saved the file with quality 12 - the pixels are still 1600x1200 and file
size 1069K (!)
il barbi
Yes! I misspoke.
The number of pixels does NOT change.
The number of kilobytes needed to describe the pixels does.
That is what Compression is all about.
High Compression = Fewer kilobytes....Low Compression = More kilobytes
If you want to get a feel for how compression affects the file size in
kilobytes, take any picture from your camera, look at the file size. Do an
Auto Levels or anything else and SAVE AS (give it a new name e.g. pic2.)
and select the highest compression (Image quality = 0)
Now just SAVE AS again. Call it pic3 and select the Lowest compression
(Image Quality = 12)
Now compare the file size for each image...The differences in file size
are huge. The number of pixels remains the same.

I found where is the problem - I was using PS rel.6.0... You can verify, if
you want, that after applying autolevels to org.jpg and closing the file, PS
6.0 does not ask what level of compression is to be chosen, in the contrary
PS 7.0 does. Moreover PS 6.0 assumes the current level of compression, which
may vary between 1 and 12 depending on the last operation involving the
request of this parameter. So by doing autolevels+close the result may vary
very much. I don't know if this is a bug of PS 6.0 or a problem of my PS
installation, anyway it is better to pass away, I'll use PS 7.0.
Now my question arose from this problem: I want to apply autolevels to the
jpg files from my Fuji digital camera (this is a compact one, no raw
output). I don't know what jpg algorithm Fuji applies to the 2Mpixel of each
photo, I only know the photos occupy about 600K. Now since autolevel
function only applies some scaling to each group of 3 bytes representing
each pixel, I expect to get more or less the same memory size but I don't
want to add a further loss to the loss that Fuji has already caused. On the
other side if I save the autoleveled file with maximum (12) degree I get a
much greater filesize (1069K for the 599K file I quoted) and this seems
nonsense
il barbi


.

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