Overstretching background

[Mike Reid]

I'm still climbing the PixInsight learning curve and need some help.  I have observed on many of the tutorials that the Histogram Transformation seems to be the preferred tool for non-linear stretching.  I have been processing images using this tool instead of my old flow of an initial DDP in ImagesPlus followed by a modest curves transformation in PS.  The problem that I have encountered is that I invariable end up overstretching the shadow region.  This results in greatly exaggerated noise in the background that then must be dealt with later in the flow.  I have had more success with using only a very light Histogram Transformation and then using the Curves Transformation for further non-linear stretching while keeping the shadows low.  But the curves tool lacks the histogram display that HT has so it's quite difficult to use with precision.

I expect that I am simply missing something.  Please share your PixInsight solutions for stretching.

Thanks,
Mike

[Carlos Milovic]

The Histogram's MTF is quite aggressive, in the sense that it increases more the contrast than a gamma curve, or DDP (btw, you may find it under the obsolete processes). So, you are absolutly right that sometimes noise would be "emphasized" from the same contrast enhancement. You may deal with that later, decreasing contrast with curves, applying noise reduction algorithm, etc.
If, you want to avoid the MTF, but at the same time beeing able to "predict" the histogram, this is a nice solution:
- Create a new preview, with the same size as the main image (you may drag and drop the image tab on the same panel).
- Open the HistogramTransformation process, and view the preview's histogram.
- Open the CurvesTransform process (or any other process), modify the parameters, and then launch it on the preview.
- The Histogram should reflect all the changes automatically.

So, this way, you may try the parameters without modifying the main image, and you don't have to worry about undo/redoing the process.

[Juan Conejero]

Hi Mike,

Since PI 1.6.0 you can also use the Real-Time Preview as a virtual view. In this way you can obtain real-time histograms and image statistics while you are previewing the result of CurvesTransformation, also in real time. This is explained in these release notes:

http://pixinsight.com/forum/index.php?topic=1734.0

On the post above, look for "Virtual View Support for Real-Time Previews" (about half way the whole thread height) for an example of virtual views being used for real-time previewing.

[Mike Reid]

I just tried selecting RealTimePreview with HistogramTransformation while adjusting the CurvesTransformation.  It works great!  This is a tremendous tip.  Harry, you should add this to your next video.

Thanks,
Mike

[Harry page]

Hi

I have not done a video for a while and summer no nights is here in the uk, so I must make time and I think a video covering little tricks like this might be a good idea and always open to what people want 

Suggest away people

Harry

[harist]

Suggest away people

"Everything you wanted to know about Deconvolution but were afraid to ask"

Keep up the good work,
Tasos

[Harry page]

Hi

Yup thats on the list  8)

Harry

[RobF2]

The Histogram's MTF is quite aggressive, in the sense that it increases more the contrast than a gamma curve, or DDP (btw, you may find it under the obsolete processes). So, you are absolutly right that sometimes noise would be "emphasized" from the same contrast enhancement. You may deal with that later, decreasing contrast with curves, applying noise reduction algorithm, etc.
If, you want to avoid the MTF, but at the same time beeing able to "predict" the histogram, this is a nice solution:
- Create a new preview, with the same size as the main image (you may drag and drop the image tab on the same panel).
- Open the HistogramTransformation process, and view the preview's histogram.
- Open the CurvesTransform process (or any other process), modify the parameters, and then launch it on the preview.
- The Histogram should reflect all the changes automatically.

So, this way, you may try the parameters without modifying the main image, and you don't have to worry about undo/redoing the process.

Please pardon me being a bit thick guys....

Are you basically explaining here that Curves/Histograms and other processes can be simultaneously open and working together in Preview mode, allowing you to then apply once you have things right?  So you could do a hard stretch in Histogram, then tweak it a bit with Curves, before dragging both onto the image?

Ta,
R

[Mike Reid]

Hi Rob,

What we were talking about is that for any operation that supports a preview window you can also open the HistogramTransformation window and select RealTimePreview under the view.  In this way you can see the effect of operations other than the HistogramTransformation in real time on the histogram. IMHO, this blows every other tool out of the water for monitoring the histogram while processing.

Mike

[Carlos Milovic]

You apply the processes sequencially, but you may open them, any number of them, without restrictions (well, don't include dynamic processes in my affirmation). You may even open processes without any image opened at all. This is because of the object oriented paradigm that PixInsight implements. Processes are live objects, that interact with images, but don't need them.

Having said that, what I intended to say is that you may be reading the histogram, and at the same time tweak the curves. Then, you launch the result over a preview to update the histogram. Or, as Juan explained, you may read the RealTimePreview's histogram, own it with curves, and then any changes to the curves interface will be reflected live on the histogram window.

[RobF2]

Cool.  Thanks guys.  I believe I have a better grip on the possibilities now + better understanding of "object orientated" model.
(or at least know enough to be dangerous  )