How do you stretch?

[ollypenrice]

I've always used Curves to stretch open my linear images in Ps. I begin with an aggressive stretch, rising steeply and then flattening to make a straight line to the top. I back off the aggressiveness in subsequent iterations and I then make special stretches for overly bright parts of the first stretch and layer these in. I cut back the black point (conservatiely!) in Levels between stretches.

However, the normal method in PI, and one I see several Photoshop tutorial-makers advise, is to stretch by moving the mid point to the left, giving a pure log stretch.

I've no axe to grind but I'm curious to know what others do...

Olly

[IanL]

Olly,

I have not used PhotoShop since about 1996 (i.e. never for Astro-imaging), but I have used various cheaper alternatives prior to switching to PI. As I see it, the different processes you describe would achieve exactly the same result for the intial non-linear stretch:

- Adjusting the mid point slider in a 'levels' type tool does exactly the same thing as using a histogram tool. It's just a different (simpler) interface. By way of example, in PI the STF tool (a non-permanent 'levels' tool) and the histogram tool can be used interchangably:

I've taken a raw image, done an auto-STF on it to generate the STF instance at the bottom left. As you can see the mid-point slider is way over to the left. Then I've just dragged the blue triangle from the STF to a histogram instance (bottom right chart). This transfers the parameters directly from one to the other, and would allow me to make the temporary STF permanent if I apply the histogram to the image.

Now in PS and the like, the levels tool is not a temporary change, it is permanent, but it shows that the two tools are just different visualisations of the same process.

Now oddly enough, the curve you describe sounds like the histogram in my example - rapidly rising and then flattening out. If you used a curves tool and drew the same curve as shown in my histogram, you'd get exactly the same non-linear stretch. The curves tool and the histogram tool are (again) the same thing, except the histogram is constrained in terms of the shape of the curve you can create - you only have the end points and the mid point and no control over how the curve blends through the three. With the curves tool, you can make the up/around/flatten out bit of the curve more or less aggressive by adding more control points and making the curve more (or less) 'round shouldered'.

I know you already realise this and your question is whether it is better to go for a hard mathematical transformation (histogram/levels) or something more controllable and variable (curves), but I thought it helpful to illustrate the relationship between the tools for those who may not appreciate this.

I don't really think there is a 'right' or 'wrong' way to do it, and usually I experiment to see what gives me the best/most natural contrast between the various elements of the image.

For me, sometimes an aggressive histogram stretch feels better to start with, especially if what I have captured is fairly dark compared to the stars. On other brighter higher dynamic range targets, a histogram stretch compresses things too much and then I waste time trying to restore subtle contrast that I should have preserved. In that case I would tend to use a less aggressive curve for the first non-linear stretch.

As a PI 'convert' I tend to use curves a lot less than those who learned their trade in PhotoShop, simply because the various HDR/Wavelet Transformation tools make it less necessary to do so; they expand/compress brightness ranges in a less intuitive way (at first), but a much more repeatable one. It is still important to get the right choice of initial non-linear stretch though, so I can see what is going on.

Usually I will try both because PI makes it so easy to do so. I find it far easier to back and forward track through a processing workflow by having parametrised process icons that I can apply, undo and re-apply at will, rather than relying on trying to re-draw the same curve numerous times as I realise I wanted to do something different three steps earlier in the process which makes experimentation in photo-editing tools more of a chore.

I will sometimes use a bit of the classic 's' curve during the process, but it is not always necessary to do so.

[ollypenrice]

I agree with all of that, Ian. While I do like my 'non pure log' curve on some targets, notably Ha which tends to have a very black background sky, I think the repeatability would be highly desirable in preparing images for mosaic making, for instance. At the moment I stretch images for a mosaic open side by side, repeating the curve by eye on each one, but I think using the mid point slider would be more logical.

Olly

[IanL]

Well PI gives you best of all worlds in terms of repeatability. You can create a curve of any shape using the curves tool (or a simple log curve with the histogram), drag the blue 'new process instance' triangle to the PI desktop and save it. (It's a nice tool as well since you can create a standard RGB/K curve, or one for each colour channel, one for alpha, one for luminance, one for each of the CIE channels, hue and finally saturation. You can have them all active in one curves session if you want, though that would be insane!)

Double-click the process icon and up pops the curves tool with exactly the parameters you previously set. You can just apply that curve to the next image, and the next. If you apply several curves iteratively, just save each one as a process icon and give it a meaningful name, and you can apply them in sequence to each of the panes in your mosaic, or make duplicates of some or all of them and adjust slightly to get the best match, and of course go back to previous images and re-run the processes if you can't get a later one to match and need to do something to the first one differently.

You can organise the whole think into an arbitrary number of workspaces to keep it tidy, and save the whole project to a single meta-file so you can pick up the next day exactly where you left off. Also processes can be saved individually so you can re-use them between projects. PI also has a dedicated GradientsMergeMosaic process to help blend mosaics without seams, but I haven't had cause to use it yet.

The documentation is a bit patchy, and it can be a baffling learning curve at times, but there is a helpful user community and personally I am glad I have committed to this type of processing early on, as I can see it would be harder to unlearn many years of the PS way of doing things and make the switch. About the only thing it doesn't really do is layering (not that this should be needed with the HDR composition tools in PI) and cosmetic retouching (has a basic cloning tool and that's about it), which is definitely not in the author's philosophy of processing!

[Tim]

In Pixinsight I generally do two or three stretches using the histogram tool.

In the first instance, I bring the mid point to the end part rhs of the data spike, then nudge up the black point (never cutting into the start of the spike)

The next stretch is a lot more conservative, moving the mid point to the front of the new position of the spike, then bring up the black point again.

If the data is up to it, one more gentle moving of the mid point to the left.

I dont really bother with curves anymore.

Horses for courses though, so many ways to achieve the same thing

[ollypenrice]

Now Ian, if the author were half as articulate as you, or employed someone half so articulate, PI would be a different animal...

I can see from your comments that if I could understand PI I'd use it more thoroughly. I'd gladly go on a course. A three year BSC, maybe???

Olly

Edit; Tim came in as I was posting. His images speak for themselves.

[IanL]

Haha Olly, I'm not an expert in PI, just a dabbler (as in most things).

Due to my career I started with personality #1 as a perfectionist and a detail obsessed nerd so I can talk to the techies on their own terms, then flip to alternate personality #2 and talk to normal human beings who have to use the technology and be understood/understand them, then flip to personality #3 which is seeing the bosses' big picture that the business has to make money and that you can't always make everyone happy when you are doing that.

Trouble is that most really smart software developers (better ones than I ever was) have personality #1 and that's it. I worked hard to get #2 and then #3 because whilst I like nerdiness, I also like people and didn't want to spend my whole working life behind a screen.

To be fair, the PI interface is pretty logical and self consistent, but because it is Windows/Mac/Linux cross platform, it is a bit odd compared to a standard application. Once you have mastered the basics of how to open an image and apply a process the rest is easy to pick up as you go along. Take a look at Harry's Astro Shed, he has some easy to follow videos that give you all the basics of processing an image in PI:

http://www.harrysastroshed.com/pixinsighthome.html

Not even the most avid fan would call the documentation half complete (but then lots of people use IRIS all the time and half of that doesn't even have an interface to speak of, never mind documentation). On the other hand if you post a question to the PI developers saying 'what does obscure box <x> do and why in this widget?' you will get an answer same day, and it will be clear and precise. I'd really urge anyone to try it out and see if they can get over the initial hurdle because it has so much to offer that other packages don't, or don't do half as well.

[andrewluck]

My feeling is that relying on the mid-point histogram slider has some adverse effects on stars, tending to both inflate & desaturate them. Since moving to Pixinsight I've started to use the masked stretch script for the initial stretch. The procedure I use is as follows:

1. Perform a test stretch using the histogram tool. Once the overall brightness looks OK, use the statistics tool to determine the image median value.
   
2. Revert to the original image and use the masked stretch script with a target value about 50% of what I determined in step 1
   
3. Perform a histogram stretch to finish the job.
   

Regards

Andrew

[ollypenrice]

My feeling is that relying on the mid-point histogram slider has some adverse effects on stars, tending to both inflate & desaturate them. Since moving to Pixinsight I've started to use the masked stretch script for the initial stretch. The procedure I use is as follows:

1. Perform a test stretch using the histogram tool. Once the overall brightness looks OK, use the statistics tool to determine the image median value.
   
2. Revert to the original image and use the masked stretch script with a target value about 50% of what I determined in step 1
   
3. Perform a histogram stretch to finish the job.
   

Regards

Andrew

To be investigated... Thanks.

Olly

[Mark72]

Olly,

You can also check Warren Keller courses on PI. Very helpful

Mark

[melsky]

I tend to use the curves to adjust my images in Photoshop in a single adjustment layer to start with but using blending options to subtlety control highlight and shadow tones. I also use Hue/Sat and selective colour adjustment layers to tweak the colours, if you are putting together a mosaic I would recommend grouping your adjustment layers then you can simply drag them from image to image for consistent results.

Mel

[opticalpath]

Until recently, I used several gentle stretches using Curves in PS. I saved as an action (with a shortcut key so it's just one key press) a gentle Curves adjustment with just two points: 64 going to 128 and 128 to 192. I would apply that several times, readjusting the black point with Levels as needed in between. Then more tailored curves adjustments to bring up specific tonal ranges to finish off.

If you want to save and repeat a Curves adjustment in PS, the simplest way is to define the curve with 2 or 3 actual points - typing in the input and ouput values of each point. Typing in the same values on subsequent adjustments will create the identical curve. If you find you use the same shape curve frequently, save it as an action with a shortcut key.

I found that several gentle curves in PS is good for control, but tends to brighten the outer area of stars, so my preferred method now is to do a much more aggressive DDP stretch in Maxim, taking the 'auto' midpoint value but dropping the background auto setting a little to ensure I avoid clipping. This gives me a good starting point for subsequent tuning in PS. It also has the benefit of doing the first big stretch more smoothly in floating point FIT before moving to 15 bit integer format in PS.

BTW I tend to set the black point in Levels adjustments fairly aggressively, but only after examining the image carefully to determine where the 'real' black level in the image data is, careful not to clip image information but discounting spurious dark pixels/ frame edges etc.

Adrian