Other Color Tools

Color Saturation

This tool is used to increase the color saturation of the image. It is possible to choose between a specific hue or the global hue to enhance. The strength of the saturation is adjusted with the slider Amount.

The Background factor slider sets the factor multiplied by the background value. Lower is the value, stronger is the saturation effect. While a high value will preserve the background.


Color Saturation dialog window.

Siril command line

satu amount [background_factor [hue_range_index]]
Enhances the color saturation of the loaded image. Try iteratively to obtain best results.
amount can be a positive number to increase color saturation, negative to decrease it, 0 would do nothing, 1 would increase it by 100%
background_factor is a factor to (median + sigma) used to set a threshold for which only pixels above it would be modified. This allows background noise to not be color saturated, if chosen carefully. Defaults to 1. Setting 0 disables the threshold.
hue_range_index can be [0, 6], meaning: 0 for pink to orange, 1 for orange to yellow, 2 for yellow to cyan, 3 for cyan, 4 for cyan to magenta, 5 for magenta to pink, 6 for all (default)

Remove Green Noise

Because green is not naturally present in deep sky images (except for comets and some planetary nebulae), if the image has already been calibrated, its colors are well balanced and the image is free of any gradient, we can assume that if the image contains green, it belongs to the noise. It is then interesting to find a method to remove this dominant green. This is exactly what the Remove Green Noise tool proposes, which is derived from the Subtractive Color Noise Reduction tool, but for green only.


Remove Green Noise dialog window.


This tool is not intended for direct use on a typical green image from a stack where the background sky level has not been equalized. Its use in such conditions would destroy the image's chrominance.

This tool has 3 settings. The protection method, the amount (called \(a\) in the following section), and a Preserve lightness button. The following methods present the different existing ways to remove the green pixels by replacing them with a mix of Red and Blue. The amount is only available for methods with mask protection. The choice of its value must be done with caution in order to minimize the rise of the magenta cast in the sky background. Do not hesitate to use the Undo and Redo buttons in order to fine-tune the value.

Protection method

Maximum Mask Protection

\[\begin{split}m &= \text{max}(R,B) \\ G'&= G\times (1 — a)\times (1 — m) + m\times G\end{split}\]

Additive Mask Protection

\[\begin{split}m &= \text{min}(1,R+B) \\ G'&= G\times (1 — a)\times (1 — m) + m\times G\end{split}\]

Average Neutral Protection (default method)

\[\begin{split}m &= 0.5\times (R + B) \\ G'&= \text{min}(G, m)\end{split}\]

Maximum Neutral Protection

\[\begin{split}m &= \text{max}(R,B) \\ G'&= \text{min}(G, m)\end{split}\]

Finally, the Preserve lightness button preserves the original CIE L* component in the processed image, in order to only process chromatic component, it is highly recommended to let this option checked.

Siril command line

rmgreen [-nopreserve] [type] [amount]
Applies a chromatic noise reduction filter. It removes green tint in the current image. This filter is based on PixInsight's SCNR and it is also the same filter used by HLVG plugin in Photoshop.
Lightness is preserved by default but this can be disabled with the -nopreserve switch.

Type can take values 0 for average neutral, 1 for maximum neutral, 2 for maximum mask, 3 for additive mask, defaulting to 0. The last two can take an amount argument, a value between 0 and 1, defaulting to 1

Negative Transform

Negative transformation refers to subtracting pixel values from \((L−1)\), where \(L\) is the maximum possible value of the pixel, and replacing it with the result.

The Negative transformation tool is different from the negative view negative-icon in the toolbar. Indeed, the transformation is not only visual, but actually applied to the pixel values. If you save the image, it will be saved as a negative.


Original image with weak signal (Image Cyril Richard).


Negative image where the signal is more visible (Image Cyril Richard).


A common use of the negative transformation tool is to remove the magenta cast from SHO images. In this case one need to apply Negative transformation, then Remove Green Noise, then Negative transformation again.

Siril command line

Changes pixel values of the currently loaded image to a negative view, like 1-value for 32 bits, 65535-value for 16 bits. This does not change the display mode