Life: Global Light Diffraction Remodelling of Large Scale Structures

A before and after image showing the Life module's 'Isolate' preset in action.
The Life module's Isolate preset at work, 'pushing back' a busy star field and refocusing attention on the nebulosity.

The Life module brings back 'life' into an image by remodelling uniform light diffraction, helping larger scale structures such as nebulae and galaxies stand out and (re)take center stage.

Throughout the various processing stages, light diffraction (a subtle 'glow' of very bright objects due to lens or mirror diffraction) tends to be distorted and suppressed through the various ways dynamic range is manipulated. This can sometimes leave an image 'flat' and 'lifeless'. The Life module attempts to restore the effects of uniform light diffraction by an optical system, throughout a processed image. It does so by means of modelling an Airy disk pattern and re-calculating what the image would look like if it were diffracted by this pattern. The resulting model is then used to modulate or enhance the source image in various ways. The resulting output image tends to have a re-established natural sense of depth and ambiance, with better visible super structures.

For example, the Life module's Isolate preset, when applied to the whole image, is particularly adept at pushing back busy star fields and noisy backgrounds, refocusing the viewer's attention to the larger scale structures. As such it is a very powerful, yet easy to use tool to radically change the feel of an image.

The Life module may additionally be used locally by means of a mask. In this case the Life module can be used to isolate objects in an image and lift them from an otherwise noisy background. By having the Life module augment an object's super-structure, faint objects that were otherwise unsalvageable can be made to stand out from the background. Please note that, depending on the nature of the used selective mask, the super structures introduced by using the Life module in this particular way with a selective mask, should be regarded as an educated guess rather than documentary detail.


A 4-panel image showing StarTools' Life module examples
3 examples of the Life module presets manipulating the visibility of large scale structures. Top left; original. Top right; 'Moderate' preset. Bottom left; 'Heavy' preset. Bottom right; 'Shroud' preset.

As with most modules in StarTools, the HDR module comes with a number of presets;

  • Moderate - applies a moderate application of the 'life' algorithm.
  • Heavy - a more aggressive application of the 'life' algorithm.
  • Less=More - pushes back anything that is not a super structure, imparting depth to the image by manipulating brightness
  • Shroud - Helps brighten an image without emphasising background noise or star fields.
  • Isolate - pushes back anything that is not a super structure (similar to Less = More) while enhancing energy allocated to super structures.

Going beyond the presets, very detailed adjustments can be made, starting with the 'Glow Threshold' parameter. This parameter is determines how bright a pixel needs to be before it is considered for diffraction by the Airy disk diffraction model.

A 3 panel left-to-right image comparing the subtle qualities of the 'Less is more' and 'Isolate' presets in StarTools' Life module.
A comparison of the subtle qualities of the 'Less is more' and 'Isolate' presets. Left; original. Middle; 'Less is more' preset. Right; 'Isolate preset'.

To view just the model that Life is using to enhance the image, the 'Output Glow Only' parameter can be set to 'Yes'. Optionally this output can be used to manipulate the image later using the Layer module, or in a separate application.

The 'Strength' parameter governs the overall strength of the effect.

The 'Inherit Brightness, Color' parameter determines whether brightness or color information is inherited (and thus unchanged) from the source image.

The 'Saturation' parameter controls the colour saturation of the output model (viewable by setting 'Output Glow Only' to 'Yes'), before it is applied to the source image to generate the final output. This parameter can be quite effective for enhancing the color of nebulosity.

A computer-generated image of an Airy disk
A computer-generated image of an Airy disk. The grayscale intensities have been adjusted to enhance the brightness of the outer rings of the Airy pattern. Source: Wikipedia.

The 'Detail Preservation' parameter selects the detail preservation algorithm the Life module should use to merge the model with the source image to produce the output image;

  • Off - does not attempt to preserve any detail.
  • Min Distance to 1/2 Unity - uses the pixel that is closest to half unity (e.g. perfect gray).
  • Max Contrast - uses whatever pixel maximises contrast with its neighbouring pixels.
  • Linear Brightness Mask - uses a brightness mask that progressively masks-out brighter values until it uses the original values instead.
  • Linear Brightness Mask Darken - uses a brightness mask that progressively masks out brighter values. Only pixels that are darker than the original image are kept.

The 'Detail Preservation Radius' sets a filter radius that is used for smoothly blending processed and non-processed pixels, according to the algorithm specified by the 'Detail Preservation' parameter.

The 'Compositing Algorithm' parameter defines how the calculated diffraction model is to be generally combined with the original image:

  • Screen - works like projecting two images on the same screen.
  • Power of Inverse - Power of Inversed Pixels (PIP) function.
  • Multiply, Gamma Correct - multiplies foreground and background and then takes the square root.
  • Multiply, 2x Gamma Correct - similar to 'Multiply, Gamma Correct' but doubles the Gamma Correction.

The 'Airy Disk Sampling' parameter controls the accuracy of the point spread function (PSF) that describes the diffraction model (an Airy disk).

  • Default is 128 x 128 pixels. Range is 128 x 128, 256 x 256, 512 x 512 pixels.
  • Increasing this value will give a more accurate simulation but will take longer.

The 'Airy Disk Radius' parameter sets the radius of the Airy disk point spread function (PSF) that is used to diffract the light. Just like in nature, you may spot some (very) subtle rings around the stars after processing. The way this looks can be adjusted using this setting.

Finally, as with most modules in StarTools that employ masks, a 'Mask Fuzz' parameter is available to smoothly blend the transition between masked and non-masked pixels.