PixInsight 1.5

The Officially Unofficial Reference Guide

Rev.0.1 – 3/29/2010

Section 2: Color Calibration

BackgroundNeutralization

The BackgroundNeutralization tool adjusts the colors in an image so as to produce a background color as neutral as possible. The (linear) color adjustment is applied to the entire image, not just the background.

BackgroundNeutralization requires a good background reference. A good background reference includes mainly data that actually represents the sky background in the image.



Reference image: BackgroundNeutralization will use pixels read from this image to compute an initial mean background level for each color channel. If you leave this field blank (or with its default <target image> value), the target image will be also the background reference image during the neutralization process.

You should specify a view that represents the true background of the image. In most cases this means that you must select a view whose pixels are strongly dominated by the sky background, as it is being represented on the target image. A typical example involves defining a small preview over a free sky area of the target image, and selecting it here as the background reference image.

Lower limit: Lower bound of the set of background pixels. Background reference pixels with values less than or equal to this value will be rejected for calculation of mean background levels. Note that since the minimum allowed value for this parameter is zero, black pixels are never taken into account.

Upper limit: Upper bound of the set of background pixels. Background reference pixels above this value will be rejected for calculation of mean background levels.

Working mode: Use this option to select a background neutralization mode.

In target background mode, BackgroundNeutralization will force the target image to have the specified mean background value (see the target background parameter below) for the three RGB channels. In this mode, any resulting out-of-range values after neutralization will be truncated. There can be some (usually negligible) data clipping, but only additive transformations are applied to the data.

In rescale mode, the target image will always be rescaled after neutralization – rescaled here means all pixel values are recalculated so they all stay within the available dynamic range of the image, that is, no clipping can occur. In this mode, besides no data clipping, the neutralized image maximizes dynamic range usage. However, in this mode you have no control over the resulting mean background value, and the rescaling operation is a multiplicative transformation that redistributes all pixel values throughout the available dynamic range.

The rescale as needed mode is similar to rescale, but the target image is only rescaled if there are out-of-range values after neutralization. This is the default mode.

In truncate mode, all resulting out-of-range pixels after neutralization will be truncated, which usually results in severely clipped data. This mode is useful to perform a background subtraction to a working image used for an intermediate analysis or processing step.

Target background: In the target background working mode, this is the final mean background level that will be imposed to the three RGB channels of the target image.

In the other working modes (rescale, rescale as needed and truncate) this parameter is not used.

ColorCalibration

The ColorCalibration tool is used to calibrate the image by sampling a high number of stars in the image.

ColorCalibration can also work with a view as its white reference image. This is particularly useful to calibrate an image using a nearby galaxy, for example. The integrated light of a nearby galaxy is a plausible white reference, since it contains large samples for all star populations and its redshift is negligible.

White Reference

Reference image: White reference image. ColorCalibration will use pixels read from this image to compute a set of three color calibration factors, one for each channel. If you leave this field blank (or with its default <target image> value), the target image will be also the white reference image during the calibration process.

Lower limit: Lower bound of the set of white reference pixels. White reference pixels with values less than or equal to this value will be rejected for calculation of color correction levels. Note that since the minimum allowed value for this parameter is zero, black pixels are always rejected.

Upper limit: Upper bound of the set of white reference pixels. White reference pixels greater than or equal to this value will be rejected for calculation of color correction factors. This parameter allows you to reject saturated pixels, or pixels with very high values pertaining to the nonlinear regions of most CCD response curves. Note that since the maximum allowed value for this parameter is 1, white pixels are always rejected.

Structure detection: Detects significant structures at small dimensional scales prior to evaluation of color calibration factors.

When this option is selected, ColorCalibration uses a multiscale, wavelet-based structure detection routine to isolate bright image structures within a specified range of dimensional scales (see the next two parameters). This feature can be used to perform a color calibration based on the stars recorded in the white reference image.

Structure layers: Number of small-scale wavelet layers used for structure detection. More layers will use larger image structures for calculation of color calibration factors.

Noise layers: Number of wavelet layers used for noise reduction. Noise reduction prevents detection of bright noise structures, including hot pixels and cosmic rays. This parameter can also be used to control the sizes of the smallest detected stars (increase to exclude more stars).

Manual white balance: Perform a manual white balance by specifying the three color correction factors, literally. If you select this option, no automatic color calibration routine will be applied, and you'll be allowed to enter the correction factors for red, green and blue, as the next three parameters.

Output white reference mask: If this option is selected, ColorCalibration will create a newer image window with a white reference mask. A white reference mask is white for pixels in the white reference image that have been used to calculate color correction factors, black anywhere else. You can use this mask to check whether the Lower limit and Upper limit parameters are doing a good job selecting the pixels that you intend to use as a white reference.

Background Reference

Reference image: Background reference image. ColorCalibration will use pixels read from this image to compute an initial mean background level for each color channel. If you leave this field blank (or with its default <target image> value), the target image will be also the background reference image during the calibration process.

You should specify a view that represents the true background of the image. In most cases this means that you must select a view whose pixels are strongly dominated by the sky background, as it is represented on the target image. A typical example involves defining a small preview over a free sky area of the target image, and selecting it here as the background reference image.

Lower limit: Lower bound of the set of background reference pixels. Background reference pixels below this value will be rejected for calculation of the mean background levels.

Upper limit: Upper bound of the set of background reference pixels. Background reference pixels above this value will be rejected for calculation of mean background levels.

Output background reference mask: If this option is selected, ColorCalibration will create a newer image window with a background reference mask. A background reference mask is white for pixels in the background reference image that have been used to calculate mean background levels, black anywhere else. You can use this mask to check whether the Lower limit and Upper limit parameters define a suitable range of values to select the pixels that you intend to use as background reference.

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