Re: Auto levels

il barbi wrote:
"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio news:8BQgm.118883$3m2.9191@xxxxxxxxxxxxxxx
il barbi wrote:
"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio news:83sgm.136645$FP2.110534@xxxxxxxxxxxxxxx
il barbi wrote:
"Bob Williams" <mytbobnospam@xxxxxxx> ha scritto nel messaggio
news:CRPfm.140383$vp.53922@xxxxxxxxxxxxxxx
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


Since your Fuji outputs a 2MP image, and it takes 3 bytes (RGB) to describe a pixel, a totally uncompressed file would be 6 megabytes.
But your camera compresses the image to 600 KB. So Fuji's algorithm uses ABOUT a 10:1 compression. (Actual compression ratio depends on the image content). 10:1 is a pretty high compression ratio. Perhaps you are NOT using the Finest Quality setting on your camera.
Most P/S cameras offer 3 degrees of user-selectable quality.
Something like Super Fine....Fine....Normal or similar.
Choose the Finest Quality your camera offers
You MAY see an improvement in image quality if you make 8x10 enlargements. Probably no visible difference at 4x6.
Bob Williams
.

Relevant Pages

  • Re: dumb question
    ... Then there is the number of pixels in the sensor - the more ... The compression performed by the camera ... cameras with either no compression or lossless compression. ...
    (rec.photo.digital.slr-systems)
  • Re: dumb question
    ... to me so please forgive the dumb question. ... image size one can get out of a camera like the Nikon d700. ... Then there is the number of pixels in the sensor - the more the merrier. ... cameras with either no compression or lossless compression. ...
    (rec.photo.digital.slr-systems)
  • Re: Auto levels
    ... il barbi wrote: ... In Photoshop, and probably others, you can choose the degree of compression you want in the SAVED image. ... After you do your Auto Levels operation. ... Check the number of pixels then. ...
    (comp.graphics.apps.photoshop)
  • Re: Q : which compact digital camera do you recommend
    ... to re-size it to 1920 pixels wide and show us just how sharp it is? ... That shot is straight out of the camera. ... claims to be RICOH DC-4U taken 4th April 1999 and the compression looks ... Post a 1.2 MP image where details are defined by single pixels, ...
    (rec.photo.digital)
  • Re: use and selection of digital cameras
    ... conventional camera and I recommend them to one and all. ... beyond some higher resolution the pixels degrade, ... of detail in the compression, the 'original' still exists if it has ...
    (soc.genealogy.ireland)