Marc Lehmann wrote:
> 
> On Sun, Jun 03, 2001 at 10:51:32PM +1000, Peter <[EMAIL PROTECTED]> wrote:
> > just need a couple of things so I can replace a collection of Windows
> > tools with Gimp. Is this the right place to request changes?
> 
> Yes. Please note that me picking out a specific point does not mean your
> other requests won't be answered by somebody ;)
> 
> > I found how to change the compression level in a JPEG when I "save as"
> > but could not find out how to view or change the compression level prior
> 
> How would you get the compression level? Before you save it there is no
> such thing as a "compression level". For png, it would be between 0 and 9
> (on gimp), for jpeg it's 0..1 (in gimp) etc... for other formats there
> is no compression level at all.

In PaintShop Pro, the compression settings are from 0% to 100%, where
100% seems to retain the image unaltered. The percentage figure displays
roughly equates to image size, at the higher settings, and I rarely use
the lower settings except to demonstrate picture quality loss. On
cameras, the picture quality is often designated as the number of images
they store on a device (barely big enough to store one quality image). I
do not mind how it is represented numerically, as I prefer to view the
result on each image rather than rely on one level for every image.

> 
> > displaying something like the colour range (I do not remember the JPEG
> > term, it is the part that indicates the 24,000,000 colours are squashed
> > down to an actual range of n colours.
> 
> that's not what happens with jpeg. you can't, in general find out the save
> quality as this is a number not really defined in the jpeg "standard".

My mistake. I thought there was just one compression technique and at
some stage it started reducing the colour range. I have one image that
compresses to 90% before losing colour accuracy yet one of my customer's
major home page images starts turning an unacceptable colour if I
increase compression from 25% to 30%.

> 
> please also note that it's disagreeable wether saving the image with the
> same compression level does have any sensible semantics at all.
> 
> > from .75 to 1 and only discovered the setting when I found a few files
> > were damaged after minor adjustments.
> 
> a pity that this indeed cannot be changed (but also note that a level of 1
> does not mean the image won't be "damaged").
> 
> > Which leads to the next thing. If I start with a JPEG that has zero
> > compression
> 
> Such a thing doesn't exist. Do you mean compression level 1? That's not
> the same as no compression.

I did not know there was a difference until now. Some cameras just refer
to a setting as zero compression. I notice in some of the JPEG
literature that there are references to varying techniques but I read it
as one software compression routine that varied the techniques applied
as the compression level increased.

I have 90 Mb original images stored as TIFFs and some proprietary
formats in case I need to print but 90 Mb is overkill for the web and
few cameras have TIFF as an option (I think on Canon has TIFF and then
only on their most expensive models).

> 
> > I see the change image from editing but not after compression. Is it
> > possible to view the file as saved to disk, without having to close the
> > file then open the saved file?
> 
> I think the jpeg save plug-in does indeed do previews, do you mean
> something else?
> 
The aim of viewing the image "as saved", is to tune the image for the
web. I was adjusting an image, and saving the adjusted image as I went,
and could see my adjustments in the image window. Because the image
window did not show pixelation errors from compression, I assumed the
damage from compression was zero or so slight as to not impact on
visible quality. When I reviewed the images before uploading, I found
they were unusable because of compression. What I would like to do is
turn up the compression, a bit like the volume control on the radio, and
watch the image degrade. When the image becomes noticeably bad, then
turn it back a notch (undo) then save.

I teach web developers to always check compression by subject. Images of
the familiar can stand far less compression than the unfamiliar and some
types of detail are lost before others, so one image can take 15% loss
while another can stand 50% loss. Hence the desire to see the effects of
compression live on screen and undo back to to most useful level.

I know I face a long battle on quality with major camera makers selling
3.3 Mp cameras but still suggesting, in their advertising, that they can
store 950 images on a floppy disk. Some software makers also contribute
to the problem with advertising that suggests interpolation (and other
techniques) can restore the missing quality.
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