PixInsight 1.5

The Officially Unofficial Reference Guide

Rev.0.1 – 3/29/2010

Section 4: Color Spaces

ChannelCombination

ChannelCombination is used to combine images defined different separate channels of an images into a new image containing all the channels, as defined by the individual single-channel images. This is useful for example to combine the previously calibrated and aligned RGB channels of an image into one single color image. For combinations that include luminance and RGB channels, see the LRGBCombination tool below.

Color Space: Select the color space to be used for the combination. Depending on the color space used, the channel/source image descriptors will change accordingly.

Channels / Source Images: Once you have selected the color space, enter on each of the boxes the images corresponding to each of the channels you wish to combine. For example, if you selected the RGB color space, you need to enter an image for the red channel, one for the green channel and another one for the blue channel.

You can check/uncheck each channel as needed.

If a text box is left with the default <blank>, PixInsight will try to use an image with the same name as the target image plus the suffix _X where X corresponds to the abbreviation for that particular channel (_R for red, etc).

Target: You can select a target image where the resulting combination will be placed, although normally the apply/new instance icons are used instead.

ChannelExtraction

The purpose of the ChannelExtraction tool is to create individual images from a source image, where each of the newly created images contains information about only one of the channels that form the source image. For obvious reasons, ChannelExtraction cannot work on grayscale images.

Color Space: Select the color space that contains the channels we want to extract.

Channels / Target Images: Once you have selected the color space, use the check-boxes here to indicate which channels you want to extract. You can also enter the image identifiers if you wish, or leave it as <Auto> in which case PixInsight will name these identifiers with the same name as the source image plus the suffix _X where X corresponds to the abbreviation for that particular channel (_R for red, etc).

Sample Format: You can select Same as source to produce individual images with the same format (bit depth) as the source image, or specify a different format.

ConvertToGrayscale

This process does not contain a dialog box – that is, it's immediate and it's applied to the last active view. It will convert the last active view to grayscale. It can only be applied to color images.

ConvertToRGBColor

Like ConvertToGrayscale, this process does not contain a dialog box and it's immediately applied to the last active view, converting it to RGB color. It can only be applied to grayscale images.

Debayer

The Debayer tool is intended to be used with images captured with One-Shot color (OSC) devices. It will debayer an OSC RGB image (all pattern types supported) applying bilinear interpolation or super-pixel demosaicing methods.

Bayer/Mosaic Pattern: Different OSC cameras have a different pattern. Here you indicate the pattern that corresponds to the camera used to capture the image to be debayered.

Debayer method: The bi-linear method interpolates green and red/blue pixels and generates a high quality full-sized image. The SuperPixel method creates half-sized images of good quality and works faster.

Save as Default: Saves the selected pattern and method options so future uses of the Debayer tool will have the selected options as default.

Restore from Default: Click this button if you change the pattern and method options but would like to restore them back to the default options.

LRGBCombination

This process is used to create a new image from a combination of separate images each defining luminance and RGB data. The procedure is performed within colorimetrically defined, rigorously implemented CIE Lab and LCH color spaces. The entire process is carried out with perfect isolation between chrominance and luminance, so there are no hue changes and no noise transference between chrominance and luminance.

Channels / Source Images: Enter on each of the text boxes the images corresponding to each of the LRGB channels you wish to combine.

If you have a luminance image that you wish to apply to a RGB image – or viceversa – you do not need to extract each of the RGB channels from the color image before using LRGBCombination. Simply uncheck the R, G and B slots, include the luminance file name in the L text box, then apply the process to the image containing the RGB data.

If a text box is left with the default <blank>, PixInsight will try to use an image with the same name as the target image plus the suffix _X where X corresponds to the abbreviation for that particular channel (_R for red, etc).

Target: You can select a target image where the resulting combination will be placed, or use the apply/new instance icons instead.

Transfer Functions

Luminance: The luminance transfer function allows for a precise adaptation of the luminance to the available RGB data, which is essential to achieve the maximum possible color saturation, minimizing chrominance noise. A value of 0.5 does not change existing values. This parameter can be adjusted, if necessary, to adapt the luminance to the existing brightness and contrast of the RGB data. This is very important, since the luminance and chrominance components in a LRGB combination should be well matched in brightness to maximize the possibilities of increasing color saturation with minimal noise boost.

Saturation: The saturation transfer function works cooperatively with a specific noise reduction algorithm (see below), allowing dramatic color saturation improvements with virtually zero chrominance noise. Try decreasing the saturation balance to improve color saturation in your LRGB combined image or – hopefully not - increasing it to desaturate the result.

Chrominance Noise Reduction

A chrominance-specific, multiscale (wavelet-based) noise reduction algorithm can be applied at the final stages of the LRGB combination procedure, as part of the saturation transfer function (see the preceding point) by enabling this option.

Smoothed wavelet layers: Defines the structure scale to where a smooth noise reduction is to be applied.

Protected wavelet layers: Defines the structure scale that will be protected from the noise reduction algorithm.

RGBWorkingSpace

Separating a color image into its luminance and chrominance components is a fundamental task of image processing in astrophotography. To calculate luminance and chrominance values, PixInsight uses a colorimetrically defined RGB Working Space (RGBWS).

Each image window can use its own, local RGBWS. For images that don't have their own RGBWS (as is by default), there exists a global RGBWS.

Note that the concept of RGBWS has nothing to do in PixInsight with color management or ICC profiles. Color management is used to achieve consistent color through different imaging devices. A RGBWS is used strictly for pure image processing tasks. It is defined by the following parameters:

Luminance coefficients. These are the relative weights of red, green and blue used to calculate the luminance of a pixel.

Chromaticity coordinates. The x and y coordinates of the red, green and blue primaries. These colors are the fundamental colorants of the RGB color space.

Gamma. A function used to linearize RGB components when converting them into a linear transformation space, such as CIE XYZ or CIE L*a*b*. Usually, the gamma function is just the raise function and the value of gamma is the exponent used.



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