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 dialog.

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:
http://carol.gimp.org/files/bay-bridge1-levels.jpg

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 photograph.

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 different.

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?

carol