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Fixing Sky with Luminosity Mask

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Going from Here to Here

I have been interested in using luminosity masks for combining different exposures, but only recently found out how to achieve it. One area I found luminosity masking very helpful is controlling the sky. In the pair of images I have a high contrast scene which is exposed reasonably well. The sky needs to be made more blue without affecting the intense yellow of the autumn leaves.

So what is a luminosity mask?

A luminosity mask is continuos tone mask. So it is not dutone, black or white but shifts continuously, between darks and light. We can create luminosity masks using "Channels" or by the
Image->Calculation
dialog. We can also paint on luminosity masks and used gradients.
The way I use luminosity masking in this example is to reduce brightness and contrast is the sky. The benefits of the technique are:
  • It's easy
  • It actually works
  • It will not leave halos around edges
See also a more extensive series of articles on Luminosity Masking Tony Kuyper.

Using virtual copies in Lightroom

Lightroom is based on parametric processing. It does not change any bits in the image but keeps a "recipe" for "cooking" a final image from a "raw-image". One of the benefits with this is that Lightroom can have different recipes associated with each image. For instance, we could have different croppings or different exposures. In this case I'll work with the "original image" and two virtual copies. The original image is used as base image. A virtual image will be prepared for the sky and another for improving the shadows.
  • Create two virtual copies
  • Make the first virtual copy dark with the sky a bit darker than intended
  • Make the second copy light with very good shadow detail

Now, choose "edit in"->Open as Layers in Photoshop

 

Processing in Photoshop

Go to the "Channels" palette and choose different channels, either by clicking on each channel or by hitting "Command-2", "Command-4" and so on. In this case the blue channel will give best separation.

Create a "luminosity mask" using Option-Command-5 as shown below:

Goto the layer palette and make sure that the original image is at the bottom of the layer stack. If not so, drag it to the bottom.

Select the "dark layer", and press Add layer mask (marked with red square below).

 

The result will look like this:

There are some specular highlights in the foreground, possibly light reflected on ice or rime. One issue with this masking is that the luminosity mask is not only transparent in the sky but also to highlight or anything containing blue. The image would be better if those highlight would be preserved. I just painted a path around the area, converted to selection and filled the area with black. Obviously I wanted to fill the mask and not the image. To do that I kept the thumbnail for the mask selected in the working layer dialog.

 

Finally

 

Touching up the shadows

The shadow details were pretty decent in the original image. It would be possible to come up with a decent luminosity mask, but I decided use a color range selection for this. This results in a binary mask (either on or off) and would not be as smooth as a shadow mask based on luminance. The adjustments would be very small, so I went color range. The idea is to sample the image for the darks, using the left dropper initially and than adding more selections using the additive (center) dropper to extend the selection. When selection is done we just click OK, select the light (or overexposed) layer and hit the Add layer Mask button. Finally we adjust the opacity of the layer to fit our taste.

 

Last Updated on Sunday, 07 November 2010 10:21  

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