Merge LRGB Images

The Merge LRGB Images command combines 4 separate images that contain red, green, blue channels and a luminance channel. The result is an RGB color image that can have, in comparison to a normally merged RGB image, high signal to noise ratio, sharpened details, and less color channel noise. These improvements result form the addition of the luminance channel image. Each input image is a luminance image rather than an RGB image.


This command uses 4 input images to produce 1 output image, which is the merged RGB image. The luminance channel is spread over the R, G, and B channels so that 4 images end up producing a 3 channel RGB image. Using the Image Tree boxes in the dialog, the 4 input images may be selected from the same Image Set in one window or they may be selected from different windows. Each input image must not be an RGB image itself.

In an LRGB image, the R, G, and B channels contribute the color information while the Luminance image (usually an unfiltered, high signal to noise ratio image) contributes signal. The nature of human color perception makes it so the luminance channel image can be sharpened to show sharper detail in the output image without incurring the grainy effect of sharpening the color channel images. Conversely, the color channel images are of lower signal to noise ratio and they may be de-speckled to remove color channel noise without making the merged image appear more blurry. The Luminance image is usually merged in at a percentage level so that it does not overwhelm the color channel data. Often a Luminance % setting in the range of 15 to 40% is appropriate.

When doing trial merging to approach the ideal result, you are usually spending effort getting the brightness and color balance correct. These attributes are not affected by the sharpening and de-speckling operations but these 2 operations do occupy most if the processing time spent in the merging process. To disable them during the trial phase, without actually unchecking the options, check the Quick option. The name "Quick" tells you that the merging is ignoring the Sharpen and De-speckle parameters to obtain a quick result. When you are ready for the final merging, uncheck Quick to use the Sharpen and De-speckle options a you have them set.

The Auto check box enables automatic merging after a parameter change so that you don't have to keep clicking the Merge button. If you want Mira to automatically update the merged image after you change a parameter, Check this item. if the Auto box is not checked, Mira does an update only when you click the [Merge] button.

Oftentimes you will choose new setting that give a poorer result. The [Revert] button recalls the set of values used in the most recent, prior merging and updates the preferences with them. This is an "undo" for the preferences but not for the merged image.

The critical determinant of the brightness, contrast, and color balance is the choice of the Min Value and Max Value levels in the 4 input images. As described below, Mira gives you 3 ways to set these values.

Using Min Value and Max Value

The Min Value and Max Value parameters control the placement of the "black point" and the "white point" in the luminance values of the input images. When merged into the output RGB image, all pixel values lower than the Min Value are assigned "black" and all pixels above Max Value are assigned "white". These values do not need to be the same for each of the images. Typically they may be different if the images have different background signal or inherently different contrast. The particular values you use affect the overall brightness of the RGB image as well as its contrast and color balance.

Beyond wild guessing, there are 3 ways to choose the Min Value and Max Value parameters:

Note that this is an interactive procedure and you can tune the preferences after each trial to home in on the best possible result. Simply adjust the parameters and re-Merge the images to get a new result.

Using the Lock Option

When you adjust the Min Value or Max Value, there are times when you want to set the values independently, and there are other times when you want them to change together. To make the entire column of values change when you change one of them, check the Lock check box in the appropriate column.

After merging, you can refine the color using simpler methods than Min Value and Max Value. You can finalize the brightness, contrast, and color balance using the palette stretching technique with the mouse or by making adjustments in the Palette Properties dialog.

Related Topics

RGB Image Commands, Merge RGB Images