LOCAL CONTRAST ENHANCEMENT Technique : explanations please

[Marie]

Hi,

I read the "LOCAL CONTRAST ENHANCEMENT" yesterday and I tried and tried and tried. I need more technical informations to get it. I am used to do the "Smart sharpening", as explained here. But in the Local contrast enhancement, I don't understand what you do with the Blur.

I started by making a duplicate image and adjust the curves.
Then I tried to make a duplicate image of that one, then blur it to max, then copy that blur image as a layer in the adjusted image and set the mode to subtract (as it looks like a subtract sign in the photos). But then I am supposed to make the edges from that, but how could I? This is a two-layers image. So I flatten the image. Then I decompose to HSV, I select the Value layer and applied the Edge filter.

For the next steps I figured I would make another duplicate image (the 3th) of the adjusted image, make a duplicate layer, add layer mask (or channels? I am in GIMP, just look for the general steps, it is really similar). So I copied the edges image (made out of the adjusted image minus the blur image) and made that layer in overlay mode. Then I added a new layer and copied there the original image (non-adjusted).

Is that what I am supposed to do? And yes, it did change the colors a little. But as I read in the same article the lightness technique change even more the colors, is that right?

Thank you for your help!

[David]

Hi Marie - Welcome to the forums. I think that you may be making things too complicated for yourself. If you go to the bottom of McQ's tutorial you will see a simple way of implementing Local Contrast Enhancement using the Unsharp Mask tool. That saves the bother of trying to do the blurring etc yourself.

I note that if you use smart sharpening then you may be used to the Gimp. If so, LCE works well with the Unsharp Mask tool in that software.

Hope this helps.

David

[Marie]

Thank you David for answering me! You are right, I use Gimp. But I can adapt the techniques.

I thought that technique was more steps than just using the unsharp mask. I think the unsharp mask work quite well too, but I thought the LCE used a blur layer and combine light and sharpness... it is what it looks like in the article. I found that idea interesting.

So is that simply to make a layer of blur and a layer of sharpening? I do that to some pics. But in the article it looked like the sharpening mask was made out of the image minus a blur layer...? Do you know more about it? Or do you have another suggestion?

Thank you!

[Marie]

Thank you David for answering me! You are right, I use Gimp. But I can adapt the techniques.

I thought that technique was more steps than just using the unsharp mask. I think the unsharp mask work quite well too, but I thought the LCE used a blur layer and combine light and sharpness... it is what it looks like in the article. I found that idea interesting.

So is that simply to make a layer of blur and a layer of sharpening? I do that to some pics. But in the article it looked like the sharpening mask was made out of the image minus a blur layer...? Do you know more about it? Or do you have another suggestion?


Thank you!

[David]

Hi Marie - I've checked out the article again, and I'm sure that the USM can be used as described by McQ. I've also checked what I do using the Gimp. Take an image and duplicate the original layer. Do this so that you can check the difference, in due course, by switching the modified layer on and off. On the new layer go to the USM tool (Filters>Enhace>USM). Use a radius of 30 to 100 and an Amount 0.3 - 0.5. Threshold should be 0. Apply to the new layer. You could set up several layers and use different settings for, say radius, on each. See what happens. I find that settings of Radius = 30 and Amount = 0.3 often give the effect I want.

Regarding, blurring etc., you could also try the following technique: duplicate the layer you are working with; desaturate the new layer using Luminosity in the desaturation tool (other image editors don't give you the choice); blur with Gaussian Blur set at about a radius of 10; blend the upper to the lower layer with Overlay. This achieves a similar effect. In fact, there are lots of effects like this, a well known one being the Orton effect, where you do not desaturate the upper layer.

On a slightly related point, have you downloaded and used the "wavelet" denoise, sharpen, and decompose tools for the Gimp - see the plugin registry? These are very powerful. I prefer to use the wavelet sharpen tool now rather than USM.

Cheers

David