Color Mosaics

The basic steps for creating a color mosaic are the same as for a black and white mosaic.  However, adjusting the individual frames can be more difficult when color is added to the image.  When creating the RGB (or LRGB) images, try to be as consistent as possible in the adjustments you make.  In fact, making as little adjustment as possible to the individual frames is probably the best idea;  hold off on image enhancement until after the mosaic is completed.

Combine Individual Frames

Below are two images for a mosaic of the California Nebula (NGC1499).  The images are each composed of 3x10 minute exposures in H-alpha and 3x3 minute exposures in each color RGB (for a total of 57 minutes exposure for each frame).  For tips on creating such a picture, see the page on Using an H-Alpha Image as a Luminance Channel.

Above:  Top and bottom frames of the mosaic.

Above:  The two frames overlaid.  This definitely needs some work to properly blend these frames!

To match these two images, there are three main steps:  Equalizing brightness and contrast, matching color, and blending the images.  These steps are best done in that order as will be shown below.

First, note what needs to be changed about the image to determine the best action to take to correct it.  The problems with the above image are:

• Top image background is too light

• Top background is too green

• Bottom nebula is too magenta

• There is a pretty severe seam between the two frames

The first problem will be corrected by equalizing the brightness and contrast.  The second two will be fixed in the color matching step.  The seam should mostly disappear during the first two steps, but any remnants will be eliminated by blending the images with the layer mask technique.

Equalizing Brightness & Contrast

There are a number of ways to correct brightness and contrast mismatches in a mosaic.  The best tools to use in Photoshop for this are:

• Levels

• Curves

• Selective Color

Any of these methods will work okay, but there are advantages and disadvantages to each.

Levels

For the above example, adjusting the middle slider in Levels (Ctrl-L) to 0.85 matches the top image background to that of the bottom image.  However it also darkens the nebula, meaning more work matching the bottom half of the nebula to the top.

Curves

Curves (Ctrl-M) gives more control of the light and dark parts of an image.  The settings below match the background very well in the two halves.  The nebula is left brighter than when using Levels since the Curves settings below concentrate more on the darker parts of the image (the background).  However, the contrast is increased, again requiring more work on the bottom half of the image.

Selective Color

The Selective Color command (Image > Adjustments > Selective Color) allows you to adjust the attributes of only a single color at a time.  For this image, select Black from the Color menu at the top of the Selective Color window.

Increasing the black value will darken the black portions of the image, in this case the background.  It does this with no effect to the nebula itself, making this method ideal to this particular image.  Keep the Levels and Curves methods in mind for other images where they may work best.

Above:  Mosaic after Selective Color adjustment.  Color is still mismatched but background brightness matches.

Matching Color

Matching color includes not only making the background and subject color coincide, but also includes adjusting color saturation and contrast.  Color matching methods include:

• Color Balance

• Selective Color

• Levels

• Curves

• Hue/Saturation

Again there are good and bad aspects to each method.  Some methods work better for different parts of an image.  Remember in our example that we have two separate color issues.  One is that the background color does not match.  The other is that the nebula color is mismatches.  These may be best adjusted using different methods.

Color Balance

This method works very well for setting the background color.  Access the Color Balance window by pressing Ctrl-B.

Note that there are three Tone Balance settings in the Color Balance window.  You can isolate the adjustments you make to the color balance between the Shadows, Midtones, or Highlights.  For matching the background color, select the Shadows setting.  This is very sensitive.  When adjusting midtones, you often make larger changes, but subtle changes have a noticeable effect in the Shadow regions.  Above, some green was subtracted and some blue added to make the top frame background match the bottom frame.  Notice how small the necessary adjustments were.

Selective Color

Just like for the background adjustment, Selective Color can be used to alter the color cast of just a single channel.  In this case, we want to take out some of the magenta color in the nebula in the bottom frame.  This is easily done by selecting the Red color and decreasing the cyan slider (which adds red, the complimentary color to cyan).  Also, selecting the Magenta color and eliminating magenta and cyan and increasing yellow removes any leftover magenta cast.

Since there was little magenta in the image to begin with, large adjustments have a subtle effect.  But there was a ton of red in the image, so small adjustments have a big effect in that channel.

Levels

It turned out not to be necessary to use Levels for this image, but it is a good technique to know.  To adjust color in Levels, you can select one of three channels (red, green, and blue) to adjust independently.

Red, green, and blue colors can be adjusted separately in Levels.  Adjustments work as follows:

• Moving a slider to the right decreases that color in the image

• Moving a slider to the left increases that color in the image

For example, moving the middle slider in the Red channel to the right would make the image redder.  Moving it left would make the image less red (or more cyan, depending on how you want to think of it).

Curves

Curves can be adjusted in the same way as Levels, in red, green, or blue channels.  Adjustments work as follows:

• Bringing the curve up increases that color

• Bringing the curve down decreases that color

Hue/Saturation

This is an excellent tool if used properly.  Hue/Saturation (Ctrl-U) allows adjustments of individual color channels without necessarily affecting brightness and contrast within the image.

The Hue slider alters the each color in an image.  The image below will help to understand how it works.

When the Hue slider is moved, the bottom spectrum of colors at the bottom of the window is shifted relative to the top spectrum.  Note, for example, that in the above window, red in the top spectrum now corresponds to a yellow-orange color in the bottom spectrum.  This means that anything in the original image that was red (like the nebula) is now yellow-orange.  Using this method you could make the entire California Nebula purple. 

But what good does that do? Well, none really.  But by making subtle adjustments, you can remove an unwanted color cast.  Notice also that in the above window magenta now corresponds to red.  So by moving the Hue slider to the right, a magenta cast could become more red.  An adjustment more like +5 might be better suited than +30, but the extreme example shows how the function operates.

Color can also be adjusted by using the Saturation slider in one of the specific channels available.  For example, decreasing the saturation in the Magenta channel would eliminate that color from the image without affecting other colors.  Similarly, red saturation could be increased without changing the other colors.  Also, when imaging with a camera that is deficient in one color (say, a CCD that is not very blue sensitive), you could increase that color saturation alone to bring out that color without oversaturating the remaining colors.

Blending the Images

The best method for blending the images is the Layer Mask technique outlined in the last section.

Above:  A layer mask with gradient has been added to the top layer

Another technique is to adjust the layer mask itself using Levels or Curves to alter the brightness of the gradient.  This can further smooth out the transition between frames.  To adjust the layer mask rather than the image itself, click on the layer mask in the Layers window.  Then open the Curves window.

Above:  Curves adjustment applied to layer mask

Final Image

Above:  Final mosaic of the California Nebula.  Click for larger image.

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