Correcting Entire Image For Uneven Lighting

IA
Posted By
i_am_jim
Sep 3, 2006
Views
1004
Replies
9
Status
Closed
I hope I can make this question clear.

I have a situation that happens off and on where I have an image that’s unevenly lit. Unfortunately it’s not a simple gradient. Instead the uneven light is splotchy. It may help to think of it as irregular eccentric vignetting combined with a gradient.

Throughout the image I have a consistent background with patterns overlaid on it. Imagine the uniform wall of a building with posters, graffiti and advertising on it.

So, I have a consistent reference throughout the image. It seems like I should be able to use this reference to automatically adjust for the lighting irregularities, but I don’t know how.

I imagine something like marking 50 points on the "wall" throughout the image, then telling Photoshop, "Make them all the same density and, by interpolation, apply the correction to the area between points." This is a hypothetical command to illustrate what I want to do.

Anybody know how to do this?

What I do now is use dodge and burn to try to even things up, but this is crude and doesn’t do a very good job.
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C
chrisjbirchall
Sep 4, 2006
Jim. Post and example for us to see. That way you’ll get more useful advice. <http://www.pixentral.com>
CN
Cybernetic Nomad
Sep 4, 2006
Adjustment layer(s) and juducious use of layer mask(s).
IA
i_am_jim
Sep 4, 2006
On Mon, 4 Sep 2006 00:35:26 -0700,
wrote:

Jim. Post and example for us to see. That way you’ll get more useful advice. <http://www.pixentral.com>

Here’s a crude hypothetical
http://img73.imageshack.us/img73/8743/illustrationkc6.jpg

The dots are on the background and the area I would like to make a uniform density.
C
chrisjbirchall
Sep 4, 2006
Hi Jim

Here is a (very) quick job which I hope is what you had in mind. ..
..

..
..
First I used the eyedropper to sample the mustard colour.

Then clicked on the background with the Magic Wand tool set to a tolerance of 32 "contiguous".

This created a selection of all the background tone which I then filled by going Alt+Del

Job done – in less time that it took to write this 😉

Hope this is of some help. There will be other ways (there always is in Photoshop), and I’m sure someone else will look in here shortly with a different solution.

Chris.
IA
i_am_jim
Sep 5, 2006
On Mon, 4 Sep 2006 15:33:57 -0700,
wrote:
Here is a (very) quick job which I hope is what you had in mind. .
First I used the eyedropper to sample the mustard colour.
Then clicked on the background with the Magic Wand tool set to a tolerance of 32 "contiguous".

This created a selection of all the background tone which I then filled by going Alt+Del
Job done – in less time that it took to write this 😉

Hope this is of some help. There will be other ways (there always is in Photoshop), and I’m sure someone else will look in here shortly with a different solution.

Thanks very much for your try. As with everything a hypothetical illustration is incomplete. I failed to explain there is a faint pattern in the background. So my referring to it as consistent was ambiguous and thus misleading. I can’t post the real image because for now it is of a pre-release product still under wraps.

Here’s another try at a hypothetical
http://img68.imageshack.us/img68/6360/illustrationtb7.jpg

The faint patten is not as bold as in the illustration (width of the "lines" that make it up are smaller) and the density of these lines is not consistent (they fade in and out, but they are not darker/more distinct than in the illustration (maybe less distinct).
C
chrisjbirchall
Sep 5, 2006
Jim. In this example, I take it you would want to keep the paisley pattern whilst evening up the mottled effect. As it stands, the pattern is effectively a watermark using very similar tones to the darker patches in the BG.

How would I tackle this one? I’d do what I did in the first example then add the paisley pattern from another (its original) source. To attempt to even up the blotches without affecting the pattern would be a mamouth task. (cue someone else to take up the challenge and prove me wrong 🙂 )

It is sometimes quicker to go "back to the drawing board" than patch up an unsatisfactory graphic. If the image (background) is a photograph, can it be reshot?

I suspect this (#2) Hyperthetical is way different to the real image in question. I’m going back to your wall of a building reference in the OP. We’d really need to see the actual problem (or a small section of it perhaps? One that wouldn’t give any trade secrets away?) to know the best approach.

Chris.
IA
i_am_jim
Sep 5, 2006
On Tue, 5 Sep 2006 01:49:06 -0700,
wrote:

Jim. In this example, I take it you would want to keep the paisley pattern whilst evening up the mottled effect. As it stands, the pattern is effectively a watermark using very similar tones to the darker patches in the BG.

How would I tackle this one? I’d do what I did in the first example then add the paisley pattern from another (its original) source. To attempt to even up the blotches without affecting the pattern would be a mamouth task. (cue someone else to take up the challenge and prove me wrong 🙂 )

It is sometimes quicker to go "back to the drawing board" than patch up an unsatisfactory graphic. If the image (background) is a photograph, can it be reshot?

It’s not a graphic, it’s a photograph. The the illustration is a graphic to illustrate the problem. The pattern in the actual photograph is irregular and intrinsic to the subject.

I have no practical way to get uniform lighting on the subject, otherwise I wouldn’t faced with this problem.

I haven’t been able to come up with a way to do this manually either That’s why I posted. I think a plugin or program feature would have to exist. It could average the density at all the sampled points, and adjust the density at each point to this average, then adjust the areas between points by interpolating between them.

I only imagine this method.
C
chrisjbirchall
Sep 5, 2006
I think a plugin or program feature would have to exist.

In Photoshop there are usually several different ways of dealing with a particular proble.

If only you were able to show us (just a small portion) we might be able to offer more specific advice.
CN
Cybernetic Nomad
Sep 5, 2006
Adjustment layer(s) and judicious use of layer mask(s).

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