Develop: Stretching with Photographic Film Emulation

Top: linear image, Bottom: image developed by photographic film curve using Develop module by 'homing in'. Notice the lack of star core bloat, courtesy of the automatic black and white point detection.

The Develop module was created from the ground up as a robust equivalent to the classic Digital Development algorithm that attempts to emulate classic film response when first developing a raw stacked image.

The Develop module effectively functions as a classic digital dark room where your prized raw signal is developed and readied for further processing.

The module can also be used as swiss pocket knife for gamma correction, normalisation and channel luminance contribution remixing.


First off, please note that this module emulates many aspects of photographic film, including its shortcomings. These shortcomings include photographic film's tendency to "bloat" stellar profiles. If your goal is to achieve a non-linear stretch that shows as much detail as possible, the far more advanced AutoDev will always do an objectively better job for that purpose.

First off, please note that this module emulates many aspects of photographic film, including its shortcomings.

Enhancements over the classic Digital Development algorithm (Okano, 1997), are the introduction of an additional gamma correction component, the removal of the edge enhancement component, and the introduction of automated black and white point detection. The latter ensures your signal never clips, while making histogram checking a thing of the past.

A semi-automated 'homing in feature' attempts to find the optimal settings that bring out as much detail as possible, while still adhering to the Digital Development curve.

Finally, a luminance mixer allows for re-mixing of the contribution of each color channel to brightness.

Color retention

Non-linearly stretching an image's RGB components causes its hue and saturation to be similarly stretched and squashed. This is often observable as "washing out" of coloring in the highlights.

Traditionally, image processing software for astrohptography has struggled with this, resorting to kludges like "special" stretching functions (e.g. ArcSinH) or Color enhancement extensions to the DDP algorithm (Okano, 1997) that only attempt to minimize the problem, while still introducing color shifts

While other software continues to struggle with color retention, StarTools Tracking feature allows the Color module to go back in time and completely reconstruct the RGB ratios as recorded, regardless of how the image was stretched.

This is one of the major reasons why running the Color module is preferably run as one of the last steps in your processing flow; it is able to completely negate the effect of any stretching - whether global or local - may have had on the hue and saturation of the image.

Because of this, the digital development color treatment extensions as proposed by Okana (1997) has not been incorporated in the Develop module. The two aspects - color and luminance - of your image are neatly separated thanks to StarTools' signal evolution Tracking engine.