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Re: Image Workshop

  • Subject: Re: [iris-photos] Image Workshop
  • From: John I Jones <jijones@usjoneses.com>
  • Date: Mon, 28 Jun 2004 17:14:41 -0700

On Jun 28, 2004, at 9:48 AM, oneofcultivars@aol.com wrote:

> In a message dated 6/28/2004 10:25:45 AM Central Daylight Time, 
> kittencampi@earthlink.net writes:
>
>
>
> JPEG compression formulas don't produce linear results. The scale you 
> see as you save an image to JPG is a quality index scale.
>
>
>  I do not see a scale identified as such when using PhotoStudio to 
> resize photos.
>
>  On the other hand, when scannin' images to a specific size I am given 
> percentage reduction information for the scan being sized. Is this the 
> same, or similar to, the scale to which you refer?

There is some confusion about terms (understandably) based on the same 
word being used by different programs to mean different things.

To specifically address Bill's quession:

When you scan something, the scanner scans with its native resolution 
(what ever it happens to be) and hands you a data file sized according 
to the resolution you specified.
For instance: suppose you are scanning with a scanner that has a 
mechanical limit of 300 dpi. You scan a 2 X 4 section  of a picture and 
ask the scanner to scan at 150 dpi. That would result in the same image 
as if you had asked the scanner to scan at 300dpi but scale the image 
to 50% size. In each case half the dots would be thrown away. (They 
don't just throw every other dot away but do some fancy interpolation 
and "dithering" which takes into account adjoining colors intensities 
and so forth). Pretty straight forward. If you displayed both those 
images would appear to be the same size on your monitor because they 
each have the same number of pixels.

Now, suppose you want that 2 X 4 section blown up to 4 X 8 at 300 dpi. 
Well now you are asking the scanner for more dots than it is capable of 
producing (mechanically). Once again it does some fancy interpolation 
and "dithering" which takes into account adjoining colors intensities 
and so forth, but this time it is making up dots instead of throwing 
them away.

I have to admit I don't know what algorithms the scanners use to do 
thesse operations, but since mathematics is not their core strength, I 
prefer to use Photoshop to do that kind of manipulation. I always scan 
(well things that are important anyway) at the highest native (not 
interpolated) resolution of the scanner and at 100% size and then 
manipulate the image in photoshop. If it is a quickie scan of something 
that is not color critical, using the scanner features to produce the 
final file is ok and saves time.

Jpeg compression is a different process. Basically the compression 
routine surveys the color map and tries to blend similar colors in to a 
single entry in its list of colors and and then assigns and index to 
each color. It is more complex than that but that is the basic idea. 
When the image is being decompressed, the program changes the index 
numbers back into real colors, but because some of the original colors 
were blended to reduce the list of colors, the reconstructed image is 
not quite the same as the original. It is a "lossy" process indicating 
that some of the original information is lost.

The compression routine generally gives you a range of numbers to 
choose from. I am purposefully not using the word "scale" here because 
it has nothing to to with the size of the image only the amount of 
compression and thus the size of the FILE. At one end of the range you 
get less compression and better a quality in the reconstructed image. 
At the other end you get a smaller file and lower quality image. 
Generally speaking, for an image that is going to be shown on a 
computer monitor, you don't notice that much difference until you get 
into really high compression.

Beside which, you have no idea what you picture is going to look like 
on someone else's monitor anyway since every monitor shows colors a 
little (some times a lot) differently.

Probably more than you wanted to know.

John                | "There be dragons here"
                          |  Annotation used by ancient cartographers
                          |  to indicate the edge of the known world.

List owner iris@hort.net and iris-photos@yahoogroups.com
________________________________________________
USDA zone 8/9 (coastal, bay)
Fremont, California, USA
Director, American Iris Society
Chairman, AIS Committee for Electronic Member Services

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