Luminance Combine

Step: Finish

Merges two open images into one. A stack with several filters, for example CL and DB, gives one result window per filter, all aligned onto the same reference, so they lie exactly on top of each other. This module takes the active image and a second open image and combines them: either the brightness of the second image with the color of the active one (Luminance + color), or a plain blend of both (Mix). The result is a new step in the active image's history; the second image stays as it is, and exporting the single filter results works as before.

The module is only offered while more than one image is open. Both images must have the same size and the same number of channels, otherwise the module refuses with a message; it does not resize anything.

Parameters

Parameter	Range	Default	Effect
Second image	open images	the first other open image	The image to combine the active one with. The active image itself is not offered.
Method	Luminance + color, Mix	Luminance + color	Luminance + color: the second image supplies the brightness, the active image keeps its color. Mix: a blend of the two images.
Amount	0…1	1	How much of the second image goes into the result. 0 changes nothing. With Luminance + color, 1 takes the brightness completely from the second image; with Mix, 1 is the second image alone and 0.5 an even blend.

When to use it

Use it in the Finish step, after both images are stretched in the same way: the luminance is taken from the values as they are, so a stretched color image and a stretched luminance image fit together, but a linear one and a stretched one do not. A typical case is a one-shot-color camera with a dual-band filter (DB) for the color and a clear or luminance filter (CL) for the fine detail: stack both, stretch both, then open the DB result, choose the CL result as Second image and apply Luminance + color. Do Saturation, Noise Reduction and sharpening afterwards, on the combined image.

Lower Amount if the combined image looks washed out or too bright in the nebula: it moves the brightness only part of the way towards the second image.

How it works

Luminance + color. The brightness of a pixel is the Rec. 709 luminance of its three channels:

L = 0.2126 R + 0.7152 G + 0.0722 B

With L_c of the active image and L_s of the second image, the target brightness is

L_t = L_c + amount × (L_s − L_c)

and the three channels of the pixel are multiplied by one common factor, L_t / L_c. The luminance of the pixel becomes L_t, while the ratios between red, green and blue, and with them the color, stay as they were. Where the active image is practically black (L_c is 0.0001 or less) there is no color to keep; the pixel becomes the gray L_t instead.

Mix. Every value is blended on its own:

result = (1 − amount) × active + amount × second

Mix also works on single-channel images, as long as both have the same number of channels.

The result is not clamped; the display and the export clamp it. A pixel with no value in one of its channels, in either image, is left as it is in the active image. The file information (exposure time, filter and so on) of the active image is kept.

Edit projects and sessions

An edit project saves the second image by its name and file, not its pixels. When you open the project, Siderea looks for an open image with that file first, then with that name. If it is not open, the steps before this one are applied, the replay stops with a message naming the missing image, and you can open the image and the project again. If more than one open image fits (two windows on the same file, or several images with the same name), Siderea does not guess: the replay stops with a message that names the image as ambiguous, and you close the others and open the project again. When you open a saved session, Siderea restores the image a window combines with before that window, whatever order the windows were saved in.

Sources

• International Telecommunication Union, Recommendation ITU-R BT.709 (the luminance weights).

Errors

• Choose the open image to combine this one with. No second image is chosen, or the chosen one was closed.
• The images differ in size. The two images must have the same number of pixels in width and height, for example because they come from the same stack.
• One image is a color image and the other is not. Both need the same number of channels.
• Luminance + color needs two color images. Use Mix for single-channel images, or run Debayer on a mosaic first.