Stretching in PI

[frasax]

Hello Everyone,

this is my first Post here, so i´m very new to PI and its functions.

One of the first and most interesting things about PI is the automatic stretching function. I was told that PI is able to compute for instance, the same stretching for a Set of LRGB Images.
So this is described in one of the video tutorials on the PI Webpage.

I´m now trying to stretch some narrowband Data. I used the Auto Button in STF and drop and dragged this tho the Histogram function window. But the Auto Stretch seems to be very aggressiv and my resulting picture shows a lot of noise.

I´m a bit confused about the "right" way to stretch data in PI now. Maybe someone could bring some light into my confusion.

Thanks
CS Frank

[Herbert_W]

Hi Frank!

Press CTRL + A (Autostretch Button) in STF to set the aggressivness of STF.




Best regards.

Herbert, Austria

[marekc]

Hi Frank and Herbert,

Herbert is right, the Ctrl+A (or Cmd+A on OS X, I think), is the way to control the parameters of the `auto-stretch' (also called the Auto STF).

I happened to stumble into this trick myself, recently, and I think it's a little-appreciated way of controlling the stretch, but a powerful one.

- Marek

[frasax]

Hello Herbert,

frohes Neues Jahr!:-)

OK, i thought the aggressivness is calculated by PI automatically . So if not what is the benefit of using STF instead of a standard histogram curve?

Maybe i have a total wrong idea of stretching at all:-)

CS Frank

[Herbert_W]

Servus Frank!

Yes, STF calculates automatically based on 3 Sliders and 2 parameters - only click Button A - 1. advantage.

3 sliders (shadow, midtone, highlight): equates to histogramtransformation.

2 Parameters (Shadows Clipping, Target background): here is th 2. advantage!

Set Target background (for example 0.1) and apply STF to several images - all of them will have a median background of 0.1.
For example, to get the same target background for a RGB and Luminance image.

Its easier with numbers then to draw a curve in HistrogramTransformation.


Have a look on my homepage > PixInsight > Tutorials > ScreenTransferFunction STF > Video (sorry, austrian language)

Best regards.

Herbert, Austria

[frasax]

Hello,

my question was maybe misleading. I thought PI computes the stretching factors (including the background) depending of the S/N ratio of the picture. But maybe this is to complex or it doesn´t make sense at all.

CS Frank

[kwiechen]

I´m now trying to stretch some narrowband Data. I used the Auto Button in STF and drop and dragged this tho the Histogram function window. But the Auto Stretch seems to be very aggressiv and my resulting picture shows a lot of noise.

I´m a bit confused about the "right" way to stretch data in PI now. Maybe someone could bring some light into my confusion.

OK, i thought the aggressivness is calculated by PI automatically . So if not what is the benefit of using STF instead of a standard histogram curve?

Maybe i have a total wrong idea of stretching at all:-)

CS Frank

Hi Frank,

STF and AutoSTF do not touch the image data - these tools are used to show linear data on the monitor. Non-linear data stretching is done for example by the histogram or curve tool.

Kai

[frasax]

Hello Kai,

of course i know. I´m talking about the possibility of transfering the stf stretch to the histogram.

CS Frank

[Gary]

Hi
Once you have the stf stretch as you like it open up the histogram transfer window and drag and drop the little blue triangle bottom left of the stf window onto the lower bar of the histogram transfer window. This will transfer settings from stf to the histogram tranformation window.
Good luck

Gary

[topboxman]

I never transfer STF to histogram. I only use STF just for viewing purpose to see what the linear image would look like when stretched. After I am done with processing linear data, then I use Histogram Transformation. I run HT iteratively until I am satisfied.

I find it somewhat difficult to control STF to transfer from STF to HT. It there an advantage of transferring STF to HT?

Peter

[pfile]

in vicent's tutorial he uses STF to stretch the L and the RGB before combination. this ensures that the stretches are 'compatible' with one another. but there's no reason you can't do it by hand, you just have to take care that the two images are not wildly different.

[topboxman]

What if L and RGB were exposed differently that if you stretch the same way, it may end up looking bad? I simply use HT for L and RGB until both look great regardless of whether they were stretched equally or not. I was not aware that you need to stretch both L and RGB near equally before combination. I thought it depends on the original content of L and RGB.

Peter

[pfile]

autoSTF is going to compute some statistics on the image and stretch accordingly. so by "the same" i don't mean stretching with the same parameters/amount but stretching such that the brightness of the results is similar. you just want the L and RGB to have similar histograms before combination. that's what autoSTF does...

[topboxman]

I understand and thanks for the explanation.

I guess this is a personal preference. I still prefer to manually stretch using Histogram Transformation. I use it iteratively by adjusting and applying HT to the image until it looks good.

Peter

[frasax]

autoSTF is going to compute some statistics on the image and stretch accordingly. so by "the same" i don't mean stretching with the same parameters/amount but stretching such that the brightness of the results is similar. you just want the L and RGB to have similar histograms before combination. that's what autoSTF does...

Hi,

this is my Question. What is happening here exactly? I can´t find anything according this "computations" neither in the description of STF nor in any Tutorials.

CS Frank