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PixInsight Notes

Victor Mategrano
in PixInsight
Adam has invited members to submit their PixInsight notes. Here's mine.

Comments

  • Mike Cranfield
    Hi Victor

    That's a lot of work, thanks for sharing!

    In the GHS section you say you have not been able to get the Colour setting to work. May I offer a few words to help with this?

    The Colour option preserves colour in exactly the same way as the Arcsinh stretch does. It is best used for the initial stretch. In this stretch smaller values are generally stretched proportionately more than higher values so the values in each channel get proportionaly closer - ie saturation is reduced. The colour mode makes sure each channel is stretched by the same ratio and in this way preserves saturation. This is exactly the same as how the arcsinh stretch preserves colour.

    There is an issue with this approach which is that some channels can become clipped. Arcsinh stretch has the "Protect highlights" option to cope with this. In GHS, selecting a Clip type of "Clip" is equivalent to leaving the "Protect highlights" option unticked, while selecting "Rescale global" is equivalent to ticking that option. In GHS, however, we also offer "RGB blend" as a Clip type which we believe is a good way to help control the "wacky" star colours that sometimes result from the arcsinh stretch. 

    As you note when you describe arcsinh stretch the preservation of saturation can be a bit too overpowering in some images - in GHS we provide a Colour blend slider which blends the Colour stretch image with an RGB stretch image to allow this effect to be moderated to taste. 

    You mention the "Use RGB Working space" option when dealing with arcsinh stretch. The same option is available in GHS and works the same way. What I would note is that in both GHS and arcsinh stretch, if you leave this option unchecked the process will equally weight the channels so there is no need to do this as described in your write up.

    So by selecting the Colour mode and arcsinh as your  transformation type in GHS you can reproduce what the arcsinh process provides, the only real difference being that the latter process incorporates the black point setting whereas in GHS we have chosen to separate that out as we believe this gives greater clarity and control.

    I hope this helps.
    CS, Mike

    PS, thanks for the mention re Soft Light blending in the section on ImageBlend, although you may want to correct my surname from Cranston to Cranfield!


  • Victor Mategrano
    That is very helpful. I'm looking forward to working with your suggestions! Thanks much

    I apologize for the error. I corrected it in v.5 and it is the first change I made in v.6.

    Thanks again,

    Vic


  • Victor Mategrano
    Corrected pdf
    pdf
    pdf
    OSC Workflow Notes v5.pdf
    1M
  • Victor Mategrano

    Is this correct? Selecting Clip mimics Arcsinh’s unticked “Protect Highlights Setting”.  Stretching a starless image does not require RGBBlend. RGBBlend is useful if working with an image with stars to control star color. Arcsingh should be turned on.


    • The following was preformed on a linear image with SPCC applied. 
    • Colour, Arsingh, and Clip were selected. 
    • The Stretch factor slider was applied incrementally to the image, (as I do with Arcsinh) stretching the image and bringing out color. 
    • The Linear Blackpoint slider was applied when the background was too bright. 
    • Strange coloration was corrected with the color slider. 
    • Symmetry points were not used. 


    I was very pleased with the result! 


    If I understand your note correctly there is a “Use RGB Working space”, available in GHS, however I cannot find a  “Use RGB Working space” selection. This is probable not relevant since, according to the note it is not needed.


    Thanks much,


    Vic

  • Mike Cranfield
    edited March 2024
    Hi Vic

    I have italicised quotes from your post below and added my comments. I hope these help.

    Selecting Clip mimics Arcsinh's unticked "Protect highlights" setting - That is correct.

    Stretching a starless image does not require RGBBlend - Usually correct but I would just leave RGBBlend selected for starless as well - if there is no clipping it is exactly the same as Clip but if there is any clipping then it is better. I can't think of a situation where Clip is better than RGBBlend.

    Arcsinh should be turned on - Only if that is your preferred transformation. In practice I find Generalised Hyperbolic has so much flexibility with the ability to set different local intensity parameter values that I tend just to use that, particularly for the initial stretch. Some people like to use Histogram Transformation to stretch their stars, GHS offers the Midtone Transfer transformation type which is the same as the one used by HT but you can achieve the same effect with the Generalised Hyperbolic transformation type with local intensity set to 1 (you will need a stretch factor about 0.7 greater than you use for Midtone Transfer to get the same effect - this does not hold for smaller values of the stretch factor but works fine for a strong initial stretch).

    Symmetry points were not used - I think the ability to set a Symmetry point is a hugely powerful feature of GHS. It allows you to really differentiate between the "background" (or, as Adam would say, the stuff you have not earned the right to display in your image) and the useful signal you want to bring out. No other intensity transformation process in PixInsight has this feature - I would recommend using it!

    The "Use RGB working space" parameter is the last one on the GHS Colour options panel. Its effect is generally quite subtle so, as you say, leaving it unchecked is usually fine.

    I hope this is helpful
    CS, Mike

  • Victor Mategrano
    Hi Mike,

    Thank you very much, you has been very helpful. 

    I’m applying some of what we discussed on a Medusa Nebula that I captured last night. Color appears more quickly and is much easier to manage using these GHS techniques. Color really pop’s with the Arcsinh setting but it is more difficult to tame and I will probably use it sparingly. The colour slider does provide some control.

    I very much appreciate the time you took to carefully and clearly describe some of GHS’s advantages that I was totally unaware of that will be very helpful both in color management and stretching.

    BTW I finally found the “Use RGB working space” parameter. “If it had teeth it would have….!”
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