RGB compositing

RGB composition dialog

The RGB composition tool allows you to assemble up to 8 monochrome images to form a single color image. The images can be shifted by translation but not by rotation, otherwise it will not be possible to register them. In such a situation, it is necessary to create a mini sequence of the input images and register them with the global registration algorithm.

The operation of this tool is quite simple, just load the images and assign them a color. The first field, optional, is reserved for the luminance layer. Once a luminance layer is loaded you can integrate it or not in the composition thans to the Use Luminance button. Each color can be customized by clicking on it and choosing a new one. When more than 3 images (or 4 if there is luminance) are loaded, it may be necessary to adjust the brightness of each channel. The Adjust layers brightness button performs this operation automatically.


For binning and image dimensions, the first loaded image determines the size of the output image. If you have images of different sizes, you should always load the largest first. If your images are different just because of bining, so with the same field of view, the composition tool will upscale the smaller images when they are loaded to match the size of the first loaded image. It is useful for the common L-RGB taken with the colour filters in bin 2. This also means that if two images have not been taken with the same sensor, it is unlikely they will have the same field of view and pixel sampling after image resampling, and this will not work with this tool.

Three color spaces are available for rendering the composition:

  • HSL (for hue, saturation, lightness)

  • HSV (for hue, saturation, value; also known as HSB, for hue, saturation, brightness)

  • CIE L*a*b*

and are left to the choice of the user.

Once the composition is finished, it is possible to do the color balance by clicking on the button Finalize color balance: this opens the color calibration dialog.

If the images are not aligned with each other, and they are just shifted by translation, then it is possible to align them. Two algorithms are possible:

  • One star registration (deep-sky): you have to draw a selection around a star, making sure that the selection contains the star in all channels.

  • Image pattern alignment (planetary/deepsky): you have to draw a selection around the object you want to align. A high enough contrast is required for the algorithm to work properly.

Командная строка Siril

rgbcomp red green blue [-out=result_filename]
rgbcomp -lum=image { rgb_image | red green blue } [-out=result_filename]
Создаёт композитное RGB изображение, используя три независимых изображения, или LRGB композитное изображение, используя дополнительное яркостное изображение и три монохромных или одно цветное изображение. Итоговое изображение будет иметь название composed_rgb.fit или composed_lrgb.fit, если в необязательном аргументе не указано другое название