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Introduction: When combining a
luminance layer with RGB in Photoshop, the color is oftentimes subdued
Gendler describes several approaches to minimize
this problem, and I describe another approach here. The
developing this technique comes from Paul Kanevsky's
website, which describes a related approach, although I've modified
it in important ways as described below.
Rationale and Method:
In the standard LRGB technique, desaturation occurs because the
luminance layer is too bright, relative to the RGB signal. Simply
decreasing the opacity setting of the luminance layer in Photoshop to
achieve a luminance "blend" results in greater color saturation, but at
the expense of luminance detail (since the luminance contribution that
we work so hard to obtain is diluted out by this method).
to this problem is to maintain the luminance image at 100%, but modify
the luminance image by 1) decreasing the overall intensity of luminance
signal (which will improve color saturation), and by 2) selectively
luminance signal in
areas of high color content (which will preserve luminance
new method adaptively changes the luminance image to better match
RGB layer, thereby improving color saturation. This is
why I refer to it as "Luminance
Corrected LRGB". The
technique is surprisingly simple and is described as follows:
following directions apply to users of Photoshop:
1. Obtain your best RGB image in
the usual manner. Although not necessary, it is preferable to use
HaR blend for the red channel when processing emission nebulae.
In some cases, it may also be helpful to apply a mild Guassian blur to
the RGB image, prior to proceeding with the next steps (this depends
upon how much noise is present in the RGB image).
2. Duplicate the RGB image into another layer. Desaturate
this layer only (go to Image, Adjustment, Desaturate). Name this
layer "Lum correction".
3. Now construct the color image as follows: Luminance
image (e.g., Ha or clear filter Luminance) in the BOTTOM layer; "Lum
correction" image in the MIDDLE layer; and RGB image in the TOP layer.
4. Assign "normal" blend mode to the luminance image layer
assign "multiply" blend mode to the Lum correction layer (MIDDLE);
"color" blend mode to the RGB layer (TOP).
5. Adjust the opacity of the "Lum correction" layer until you
achieve the proper balance between color saturation and image
detail. An opacity of about 30-40% is a good starting
point, but you should experiment with this.
6. Adjust the RGB layer
(TOP) using the Selective Color and/or
Hue/Saturation tools (under the Layers menu in Photoshop) as needed to
NOTE: The technique as described above will permit the "multiply"
layer to affect all parts of the underlying luminance layer
equally. This may be fine for some images, but not others.
For instance, an opacity setting of 40% may be optimal for one part of
the image, and yet an opacity of 20% might be best for another part of
the image. To achieve greater control over the opacity level, try
1. Choose a reasonably high opacity setting for the
multiply layer (like 30-50% for instance).
a layer mask for the multiply layer as follows: Click
on multiply layer so that it is active, then go to "Layer, Add Layer
Mask, Reveal All".
4. Choose the Brush tool, Hardness level of about
25-30%, and assign the color to black.
5. Set the brush tool opacity (i.e., setting on the
top menu bar) to about 50% to start with. This will be changed
depending upon your needs as noted below.
6. Click on the newly formed layer mask (i.e., the
white box created in step 2) to make it active.
7. Go to the image and use the paint brush to mask
out areas as needed, in order to reduce the multiply effect in those
locations. Experiment with adjusting brush size, brush hardness,
brush opacity to reveal the best detail, depending upon the region.
8. Repeat in other areas of the image as needed.
You will find that this technique permits a great deal of
flexibility in constructing the final LRGB image.
try this "Luminance Corrected LRGB" method and e-mail me with