On Sat, Aug 26, 2006 at 04:57:26PM -0700, Tom Williams wrote:
> Hi!  I took a photo using a Canon PowerShot A75 while driving at approx 
> 65 mph.  The shot came out ok and I didn't get into an accident.  :)
very fun when there are no injuries or damage to vehicles :)

> Anywho, when I do an "auto" levels adjustment on the photo, the result 
> looks quite a bit different and not as good as the original.  I'm 
> wondering if the result is actually correct or not.

i do not know what the auto portion does for this tool.  i assume it
does something like determining the lightest color and it turns that to
white and similar with the darkest color and black.  i assume this
because that is how the little eye dropper thingies work with the levels

> Here is the original image:
> http://www.bay-online-media.com/tom/gimp/bay-bridge1.jpg
> Here is the image after the "auto" level adjustment is performed:
> http://www.bay-online-media.com/tom/gimp/bay-bridge1b.jpg
i actually enjoy playing with the levels tool and photographs, so i
endulged myself:

> The original image was scaled to 1024x768 resolution at 72 dpi.  The 
> original photo was taken at 72 dpi.
> Is the resultant image correct?  If so, why does it not look as good as 
> the original?
if you look at the different channels in the levels dialog, you can see
what the Auto button did.  it consistently moves the light side to where
the color histogram starts for all three of the color channels. and it
seems that the gray (center) triangle stays in the middle of the other
two triangles.

i have no idea if this works for a majority of photographs or not.

i used the tool just on the "Values" Channel (which is not a real
channel) and i like mine better than your original and also the auto
adjusted image.  to me, the only thing that was wrong with your
photograph was that 'all over gray' haze that scanned photo prints would
really get and digital photographs still seem to get somewhat.

i honestly think that it is almost impossible to make a tool that will
automatically fix the colors of every photograph and every photograph
will look better.  series of the same photograph (with the same subject
and lighting -- as you might find in a GAP frame stack) can use the same
levels settings.  but that is a different situation than a one tool does
all, like the Auto button attempts.

also, there are some people who have the opinion that crisp bright
colors are better.  they might like what the auto levels did to the

i usually don't like the auto button results.  i am also not very good
at working on the levels of the individual color channels.  i think the
best 'hack' in the levels dialog is that fake channel called "Value".
you can, without an understanding of colors or images at all, move those
triangles until you get something that you like.

usually, moving the left side and the right side triangles to where the
color starts (in this particular image, only the right side needs to be
adjusted) and then the center one just alittle bit to manage the
contrast (and the direction it gets moved depends on how much you moved
the other two) and the photograph looks better without looking

if photographs were like children, the auto button on the levels dialog
would be like trying to provide a snack for 30 to 100 children.
depending on the snack, you might get 30 to 100 children that are very
unhappy.  well, i don't know if that is a good analogy for making a
plug-in like this to work successfully all the time, but it is not that
far off.

i didn't answer your question, did i?


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