PixInsight 1.5

The Officially Unofficial Reference Guide

Rev.0.1 – 3/29/2010

Section 5: Convolutions

LarsonSekanina

LarsonSekanina is an efficient implementation of the rotational gradient filter algorithm of Larson and Sekanina (Sekanina Z., Larson S. M., Astronomical Journal, 1984), commonly used for the morphological study of comets. It allows you to apply LS either as a true rotational gradient filter, or as a high-pass filter in polar coordinates. This implementation also includes a deringing algorithm, the possibility to modulate the filtering effect by combining the filtered and original images by arbitrary amounts, and a dynamic range extension feature to fix over-saturated areas.

Filter Parameters

Radial Increment: Radial increment in pixels to calculate the gradient. The slider on top defines large scale structures (above 10 pixels). When that slider reaches its minimum value (10), then the slider on the bottom takes over, to define values between zero and 10. This is done with the purpose of being able to obtain more accuracy in the low scales while using the graphical interface. You can always enter the exact desired values manually.

Angular Increment: Angular increment in degrees to calculate the Laplacian. The double slider interface works the same way as for the radial increment parameter.

X-Center / Y-Center: Center of polar coordinates.

Interpolation: Here you define the method to be used to perform the interpolation. Bi-cubic interpolations tend to be smoother than bi-linear interpolations.

Filter Application

Amount: Filtering strength.

Threshold: Threshold value to protect low-contrast image features

Deringing: Threshold value to fix dark rings around bright image features.

Use Luminance: If enabled, LarsonSekanina will only be applied in color images to the luminance.

High-Pass Mode: Use LarsonSekanina as a high-pass filter.

Dynamic Range Extension

The dynamic range extension works by increasing the range of values that are kept and rescaled to the [0,1] standard range in the processed result. Use the following two parameters to define different dynamic range limits. You can control both the low and high range extension values independently.

Low Range: Shadows dynamic range extension

High Range: Highlights dynamic range extension

Disable: Only allowed for floating point images. Be aware that if disabled, out of range values may arise.

UnsharpMask

This is PixInsight's implementation of a fundamental classic. It however includes a number of improvements over more conventional Unsharp Mask filters, such as an efficient deringing algorithm – so you can apply an unsharp mask without generating dark halos or rings around bright image features for example – an accurate threshold parameter to protect dark, low SNR regions from adverse unsharp masking effects and dynamic range extension parameters.

USM Filter

StdDev: Standard Deviation of the Gaussian low-pass filter, in pixels. By increasing this parameter, the unsharp mask filter will act at higher dimensional scales. Likewise, a lover value will have the unsharp mask filter to act at lower dimensional scales.

Amount: Unsharp mask filtering strength.

Luminance: Apply the unsharp mask filter only to the luminance of the target image.

Linear: Use the CIE Y component instead of the CIE L* as the luminance of the target image. Enable this option to apply USM to the luminance of a linear RGB color image (no separate luminance). Examples: DSLR and OSC CCD images. Disable this option to apply USM to the original luminance of a LRGB linear image. In all cases, a linear RGBWS must be used (gamma=1.0).

Deringing

Dark: Deringing regularization strength for dark ringing artifacts.

Bright: Deringing regularization strength for bright ringing artifacts.

Output deringing maps: Generate an image window for each deringing map image. New image windows will be created for the dark and bright deringing maps, if the corresponding amount parameters are nonzero.

Dynamic Range Extension

The dynamic range extension works by increasing the range of values that are kept and rescaled to the [0,1] standard range in the processed result. Use the following two parameters to define different dynamic range limits. You can control both the low and high range extension values independently.

Low Range: Shadows dynamic range extension

High Range: Highlights dynamic range extension



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