Equating Images on Luminance

[s...@adobeforums.com]

Hello,

I have a set of 25 images that I need to equate on luminance. That is, I need them to have the same luminance mean and distribution. Is it possible to do this using Photoshop CS4? If so, what are the steps?

Thanks

[Zeno_...@adobeforums.com]

Try copying a merged version of one of them into another image and set the blend mode of the newly created layer to Luminosity. There still remains some deviation but maybe it's good enough for your needs

[Lundb...@adobeforums.com]

Oh yeah, try that.

[christoph_pf...@adobeforums.com]

I suppose adjusting the overall mean according to a target image should be achievable (and probably automatable), but the distribution seems more difficult to adjust.
How exact do the results have to be? What is the intention behind the operation?

[s...@adobeforums.com]

How do I go about adjusting the overall mean?

They don't have to be precise, as close as possible is fine.

[christoph_pf...@adobeforums.com]

I suppose You could place the target image in the file-to-be-adjusted and place a Hue-Saturation Adjustment Layer on top of them set to Zero Saturation; then apply the filter Blur – Average to the target image and apply some random sharp edged mask; then convert the layer containing the image You want to adjust to a Smart Object and apply the Average-Filter as a Smart Filter to that – which should give You a comparison as to their overall brightness.
Then open the Smart Object side to side with the layered file and apply a Curves Adjustment Layer to adjust the Brightness – on saving the Smart Object it should be updated it the original file’s window, then correct the Curves accordingly until both Averages look identical.
After that You can remove the Smart Filter and the rest of the unnecessary layers.
Sorry, but currently no more convenient way comes to my mind.

[Zeno_...@adobeforums.com]

so what's wrong with my suggestion of using the Luminosity blend mode? seems a lot easier

[s...@adobeforums.com]

Thanks for both suggestions. I'll try both methods and see which is best.

[christoph_pf...@adobeforums.com]

Zeno, if I’m not mistaken, when You impose one image set to Luminance on another You get the Hue and Saturation from the underlying one with the Luminance of the top one, so the result might not be what Saj intended.

[Zeno_...@adobeforums.com]

Well, it's hard to say what Saj wants because he didn't say but if he had a base image which he wanted to use to equalize the luminance information from the other images it would have worked like a charm.

If it's only about making the luminance information from the images as close to each other as possible then just use a Curves adjustment layer with the blend mode set to Luminance and muck around in there with the histogram open in extended mode set to Luminosity. That way you get a visual representation of the luminance values of the image and also get a readout for the mean luminance values

[s...@adobeforums.com]

The image set is of different races, and I need to control for the differences in skin colour when I transform the photos to greyscale. It's for a psychophysics exp.

No, I don't have a base image which I can use to equalise luminance.

[christoph_pf...@adobeforums.com]

Have You tested the method You proposed?
Because what I got was a mixture of the two images, the brightness of the one, but the color content of the other quite clearly recognizable.

But I admit I recommended an utterly unnecessary visual representation of what can actually be read out in the Histogram Panel anyway.

[Zeno_...@adobeforums.com]

I know what it looks like, but based on the limited information given in the original post that was imo the easiest way to do it, of course it looks rotten on pictures but it could have been for some very specific images.

Saj, what you want can be best accomplished using the Black&White adjustment layer and selecting the On-image adjustment tool(looks like a hand with a horizontal double pointed arrow below it) with which you can drag in a part of the image to the left or right to make it brighter or darker. If you use Image->Adjustments->Black & White then that tool is automatically selected

[Lundb...@adobeforums.com]

Without a set of the images and a specific requirement, all these ideas are moot.
One of them is idiotic.

[PECour...@adobeforums.com]

Isn't it a task for Image>Adjustments>Match color, maybe with a Edit>Fade on Luminosity?

[Lundb...@adobeforums.com]

Matching 25 two dimensional random distributions is a job for your local university's supercomputer.