Author Topic: ?'s for Vicent regarding Wavelets article  (Read 2640 times)

Offline sleshin

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?'s for Vicent regarding Wavelets article
« on: 2011 February 12 14:34:55 »
Hi Vicent,

Your Wavelet article in PI Magazine is very well done and provides an excellent general overview of the topic. I look forward to more specific "how to" details in forth coming articles.

While I use wavelet based tools(ATrous and HDR) in my processing, I am certain that I don't begin to get optimal results for a lack of a clear understanding of how best to apply these tools. It sounds as though your series of articles will provide details but in the mean time I wonder if I might ask you a few questions about the processing details for NGC 6914 as shown on pages 22 to 25.

When applying the multiscale processing and creating the three images of varying scales is the original image used linear or non-linear? Or does it matter?

How did you create the 3 images of increasing scale size? I know you can create the large scale image with ATrousWavelets and then subtract it from the original to get a small scale image. You are showing 2 smaller scale images. How were those created? What are their scales? Perhaps you could show a screen save of the process module with the settings you used.

What process/processes do you like to use to sharpen the small scale images?

Finally, how do you recombine the processed scaled images with the original? Addition with Pixel Math? Perhaps you could show the formula/formulas you would have used for recombining these images.

Thanks in advance,

Steve

Steve Leshin

Stargazer Observatory
Sedona, Arizona

Offline marekc

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Re: ?'s for Vicent regarding Wavelets article
« Reply #1 on: 2011 February 12 23:29:41 »
I'll second Steve's request! I, too, really enjoyed reading about multi-scale processing in Pixinsight Magazine, and I'd love to read some `how-to' details. Steve's questions are great examples of the sorts of things I'd like to know more about.

- Marek Cichanski

Offline vicent_peris

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Re: ?'s for Vicent regarding Wavelets article
« Reply #2 on: 2011 February 13 03:01:43 »
Hi!


I'm glad you liked the article. It is short in text, but usually the introduction is the harder part to write on.

Quote
When applying the multiscale processing and creating the three images of varying scales is the original image used linear or non-linear? Or does it matter?

All this proccessing methodology is with non-linear data. The only point where I work with linear data is at the moment of building the HDR image. But, after this step, data is completely delinearized with an histogram transformation.

Quote
How did you create the 3 images of increasing scale size? I know you can create the large scale image with ATrousWavelets and then subtract it from the original to get a small scale image. You are showing 2 smaller scale images. How were those created? What are their scales?

The philosophy is the same as in the Masked Unsharp Mask technique. Take two images:

Image1 --> We remove the first three layers (we have only structures starting from 8 pixel wide).
Image2 --> We remove the first seven layers (we have only structures starting from 128 pixel wide).

If we subtract Image2 from Image1, we will get an image with structures ranging from 8 to 128 pixel wide.

Of course, this is a classical band-pass filtering. With this technique, you will filter structures according to a size interval. BUT those structures will be affected from the local illumination levels: the structures will loss contrast in the more illuminated areas.

The solution to this problem is the Local Contrast Normalization Function, which I will describe in further articles. It's a simple equation, applied through PixelMath.

Quote
What process/processes do you like to use to sharpen the small scale images?

Once you make the structures independent from the illumination levels, one of the best options to make the sharpening is CurvesTransformation. Yes, sound bizarre, but it is. You will see.  >:D

Quote
Finally, how do you recombine the processed scaled images with the original? Addition with Pixel Math? Perhaps you could show the formula/formulas you would have used for recombining these images.

I usually make a simple addition in PixelMath. You control the addition putting a weight to each image. So, if we have a small-scale (SS), a mid-scale (MS) and a large-scale (LS) images, the formula could be this one:

original + SS*j + MS*k + LS*l

Being j, k and l three different weights for each of the processed images.

In the case of NGC 6914, the difference between the before and after this multiway processing methodology is not great at first glance. But in some cases this can be very powerful. See this figure, extracted from my processing example on NGC7331:



The top-left image is the original one. The top-right is with HDRWT, showing the galaxy cores. But it doesn't show the faint IFNs. To make it visible you must further stretch the image (bottom-left); with this stretching, you lose again the galaxy cores. But if we make a multiway wavelet processing starting from the top-right image, we can have it all (at the bottom-right): we can show perfectly the galaxy cores and the IFNs at the same time.

An easier way would be to directly stretch the image until you see the IFN and then apply a HDRWT. But this will result in a much more "flatter" galaxy body.

With my methodology you are not getting results quickly. The goal of my methodology is to have everything under control. Starting from this basis, you can have the final result you really desire. To me, this way makes me feel the image in my hands, as I was making a sculpture.

Anyway, we will start generating large-scale images. I recommend you to start acquiring data of Barnard nebulae, these techniques are very funny with these kind of objects.  ;)


Best regards,
Vicent.

Offline sleshin

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Re: ?'s for Vicent regarding Wavelets article
« Reply #3 on: 2011 February 13 10:31:57 »
Vicent,

A big Thank You for that very helpful reply, that's just the kind of detail I was hoping you would provide.

I was actually going to ask about the Local Contrast Normalization Function, so I'm glad you mentioned it in your reply. I had looked for it in the List of Processes and of course didn't find it. :) Then I thought it was probably the name of the algorithm at work behind the scenes when one invokes a HDR Process. :) So, I look forward to your future tutorial on this subject. Since it's the "solution to this problem" please don't wait too long to explain it. ;)

At the risk of overstaying my welcome ;D, may I impose on you a little more with a few additional questions for the sake of clarification?

Quote
I usually make a simple addition in PixelMath. You control the addition putting a weight to each image. So, if we have a small-scale (SS), a mid-scale (MS) and a large-scale (LS) images, the formula could be this one:

original + SS*j + MS*k + LS*l

Being j, k and l three different weights for each of the processed images.

Do you assign the weights by trial and error? Can you provide any guidelines to determining weights? For example. do you use higher weights for say small scale structure images, etc.?
What were the weights you used when recombining the 3 images in your example of NGC 6914?

After your reference to NGC 7331, I went back and reviewed your wonderful processing tutorial on this image. Again, a very well done general overview of the processing workflow but missing are the specific settings used to create the masks, achieve noise reduction with ACDNR, etc. For me it is very helpful and educational to see the specific settings a very accomplished astrophotographer and PI expert such as yourself uses to achieve the results shown. Trial and error is certainly helpful and I use it all the time as I try to get the results I'm looking for when using a specific process, but having a good starting point learned from well documented tutorials goes a long way to shortening the learning curve for PI. I hope you will provide more of those kinds of specifics as you continue to help us all with your excellent tutorials.

Again, thanks.

Steve

Steve Leshin

Stargazer Observatory
Sedona, Arizona