How To Stack Images To Remove Noise Photoshop

Between a recent post here on PetaPixel about the Beauty of Infinite Photography, and my own experiments on blending series of images using averaging techniques, I noticed some rather interesting alignments in technique.

In image averaging, I had previously blended images together past averaging the value of each pixel in an image to produce something entirely new.  The way that this process relates to astrophotography is that the general method is unremarkably used as a means of noise reduction.  It becomes interesting when y'all realize that y'all don't have to be blending images or taking photos of the creation to benefit from the same methods of dissonance reduction.  Information technology works incredibly well for any images that are relatively static.

Have a look at this image (click it for a larger version):

Although a relatively boring prototype, the interesting thing most it is that it was shot at ISO 25,600. To be off-white, there is a little scrap of cheating going on. The result above is derived from combining x images of the same scene. This is the power of using median epitome stacking to increase the betoken-to-noise ratio in images.

Here are a few 100% crops to demonstrate what is happening (single ISO 25,600 left, median stack right):

The last epitome really shows the strength in this technique.  The words in the banner "Fast, Focused, Fearless" are not even distinguishable in the single ISO 25,600 shot.  Stacking 10 images cleans upward the noise enough to clearly read the text.

What Median Stacking Does

Median stacking will consider all the values of a pixel at a single location across each image in the stack, then cull the last value for that pixel based on the median effect.

Assuming you took 5 images for a stack, then for a given pixel location, at that place volition exist at most 5 different pixel values:

Ruby  ( 115, 120, 130, 190, 191 )
Green( 150, 166, 169, 200, 210 )
Blue ( 200, 205, 209, 211, 220 )

A Median stack volition give a final pixel value as RGB( 130, 169, 209 ). Basically, median will pick the value as the number that falls in the middle of the ordered list. If there are an even number of values, it will average the two values that fall on either side of the middle. This means that values that are spurious or  outside of the range will be overlooked.

This works wonderfully, as noise is random, while the signal should remain constant at a location. It is this property of median stacking that is so useful for astrophotographers who often need to shoot at college proceeds to capture their subjects. By combining multiple images they tin can significantly increase their Signal-to-Racket ratio.

Pushing the Results

An interesting effect of this method is that it tin can be useful even when shooting in platonic weather condition.  Granted, you may not shoot a still subject at ISO 25,600 – but even shooting at a cameras base of operations ISO, y'all can subtract that noise even farther than what a photographic camera is capable of in a unmarried shot .

Below is a 200% ingather from an paradigm shot at my cameras base of operations ISO 200:

Hither is the aforementioned portion from a median stack of vi images:

You tin can see that even at my cameras base of operations ISO 200, there is nevertheless some dissonance when pixel-peeping (200%).  The extra effort to create the median stack issue was picayune overall.  In fact, about modern enthusiast cameras can be setup for high-speed capture – so firing off half dozen frames is effortless once everything is gear up already.

This technique can exist exploited by others as well – not just for astrophotography.  It should be immediately apparent that this can exist quite useful for whatsoever still image being captured.  Some groovy uses for this technique tin can exist applied in other areas also:

• When shooting stock images you will sometimes shoot static scenes, and if it requires a very dissonance-gratuitous image, so this is a perfect technique to consider.
• I've seen some tutorials recently about digitizing negatives/slides using a dslr – this would once more exist a perfect technique to minimize the noise coming from the digital sensor, and to maximize the quality of the terminal browse.
• If you demand to capture an image of a highly trafficked surface area, and want to easily remove people, cars, etc. from the scene quickly and easily (see below).

Creating a Median Stack

I'1000 primarily a Free/Open Source Software user – so the tools to create these stacks were already in my basic toolkit (Hugin/Imagemagick/GIMP).  Median stacks can also be created using Photoshop (from CS3 Extended on).

Photoshop

Creating the median stack in Ps is relatively straightforward (disclaimer: I'm not a Ps user personally – so I apologize if there is another method of achieving this that I am overlooking).

1. Open each of the images in your stack as layers in Ps.
2. Marshal them (Edit → Motorcar-Marshal Layers…).  Auto should work fine here.
3. Select all of the layers and turn them into a Smart Object (Convert to Smart Object).
4. Now apply the Median Stack mode (Layer→ Smart Objects→ Stack Mode→ Median).

Gratuitous/Open Source Tools (Hugin/Imagemagick)

Hugin is an open source panorama software that gets fairly heavy use on my system for a myriad of things, but in this instance you just demand the command align_image_stackto automatically align your images.  It's fairly straightforward:

/path/to/align_image_stack -m -a OUT_PREFIX file1 file2 ...

Where file1, file2, etc… are the images to be stacked.

One time this control is finished, there will be a serial of images that are at present aligned and named with the out_prefix from the control.  At this point it only takes one more command using Imagemagick to create the median blended output:

convert OUT_PREFIX* -evaluate-sequence median OUTPUT.jpg

This will stack all of the OUT_PREFIX images, evaluate them using a median stack, and save the results to OUTPUT.jpg.

Interesting Side Effects

An interesting side effect of doing median stacks is that objects that movement between the different images in the stack might be removed.  In fact – this is another common apply of median stacking.  A good case is if y'all want to capture an epitome of a location that has people/cars/etc. moving nigh, simply you don't desire those objects in the terminal image.  A great example of this is trying to shoot touristy locations, simply the desire is to remove all of the other tourists from the final upshot.  Manually masking out each object could be fourth dimension consuming, whereas a median stack might remove them quickly and easily.

Hither is a uncomplicated instance to illustrate:

The top 5 images have an object that is moving beyond the frame, and the bottom image shows the upshot of doing a median stack.  As far every bit the median stack is concerned, the pixel information that makes up the moving object is spurious data, and is not considered for the final output.

Overall, median stacking tin can exist an incredibly powerful method of producing very clean and noise-gratis images.  The only caveat is that the subject be static between each of the frames.

Source: https://petapixel.com/2013/05/29/a-look-at-reducing-noise-in-photographs-using-median-blending/

Posted by: pryorperap1975.blogspot.com

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