Curves vs Histogram Transformations

[mcintyre_sj]

I am struggling with trying to integrate an HDR shot of M42. I have manages now to combine the 3 exposures into one image. However, i can't manage to stretch the lower end to bring out that detail without burning out the core.

I tried the MaskedStrectch script, but it left dark areas (negative halows?) around the mid to bright stars. It did a reasonabl job otherwise but still didn't manage to get at the dat in the lower third of the histrogram.

So...

If i stretch with HistogramTransformation, the core just burns out. I tried masking bits of it, but the areas don't blend very well. Perhaps the mask wasn't very good.

I am trying to do the stretching with CurvesTransformation and having some success.

But i am wondering it curves can mimick what the histrogram is doing. Histrogram transfomation is probably the  the right process. So i am wondering is there is a fundamental difference between the two approaches.

Is curves just a finer grain method of doing a histogram transformation.

p.s. when i enter a long post, the window for typing stops scrolloing? If i manually scroll, it jerks around when typing, but leaves me at the top so i can't see what i type. Any one else having this probelm?

[Niall Saunders]

p.s. when i enter a long post, the window for typing stops scrolloing? If i manually scroll, it jerks around when typing, but leaves me at the top so i can't see what i type. Any one else having this probelm?

Yes - you are using the latest incarnation of Windows Internet Explorer - and it is CRAP

Time to change to Firefox

If you really are a glutton for punishment, or have an IT department that controls your PC (and hates you as well ), then there is a workaround - go to the Pixinsight Forum 'Home' page, and then click on the little 'broken page' icon ("Compatibility View"), next to the address bar in IE. This tells Internet Explorer to stop being so clever (and/or stupid) and to treat the page with respect

You should find everything works fine after that - and IE will (should) 'memorise' the fact that you need to be in 'compatibility' mode thereafter, every time you visit the PixInsight Forum.

However, as I said, time to change to Firefox

Cheers,

[mcintyre_sj]

Thanks. For anyone else with the same troublg, I had to first enable the "compatibility view button" by right clicking on the tools area before the "broken page" icon showed up. Now typeing in the post windows works.

Now does anyone have a comment on curves vs histograms for doing a stretch that doesn't burn out the high areas?   

[Silvercup]

Hello:

Depending on the image, you can use either tool. If the overall image is dark I would use histograms, if you just want to highlight a background area, midtones or highlights zones, would use Curves.

To avoid burning high areas you can use masks to protect these areas or make a HDRWavelets in low layers (2,3,4) and then stretch.

Greetings. Silvercup.

[Carlos Milovic]

In other cases, like M42, you may apply a histogram transform, let the core burn (do not clip data), and then recover it with HDRWT.

[mcintyre_sj]

Ok - now there are two threads going here.

To avoid burning high areas you can use masks to protect these areas or make a HDRWavelets in low layers (2,3,4) and then stretch.

Creating masks: My process to date is to extract the luminance channel, mess with it to the areas the i want to protect and then apply it to the image. When you say use HDRWavelets, is this the process for adjusting the mask to suite? Can you recommend a tutorial for this?

In other cases, like M42, you may apply a histogram transform, let the core burn (do not clip data), and then recover it with HDRWT.

I was under the impression that once i burned out a section that that was it. I presume that in order to be recoverable, while it might look burned out, it should not be saturuated.

As above, is there a tutorial recommended for how to recover an area using HDRWT.

[Nocturnal]

As long as you don't move the white point the 32 bit floating point resolution will preserve detail in the bright areas when you stretch. I'm not sure if Harry's videos cover it but I've discussed this in my articles in Astro Photo Insight.

[Carlos Milovic]

Sander already ansered. As long as you don't clip your data, with 32bits there is no problem at all to keep the information alive, even when you see it almost white. Just play with HDRWT to see witch are the settings that gives the best results. Try first changing the number of layers, and then with the checkboxes.

[mcintyre_sj]

Thanks.

I had reasonable success with my M42 data, including recovering the detail in the bright areas using HDRWT. The defaults were fine. I will experiment later with more options.