Auto stretch

[Gfischer]

Hi Robin,
I was thinking about how auto stretch works. I’m not sure how sc determines the black and mid tone levels. When I use auto stretch the black is always to left of histogram peak and the midtone to far right. This gives a flatter view than I get manually.
Is it possible to have auto stretch place black on histogram peak and have midtone a user set midtone pixel value to the right of peak?
I use a sequence to live stack and apply color calibration then auto stretch periodically. This keeps view reasonable for entire stack but flat looking. I tried the histogram auto tracking feature and it worked great for color but I still need to move black and midtone levels periodically.


Thanks for all your help

[admin]

Hi,

have you tried adjusting the auto stretch strength in the main SharpCap settings? That should apply to all auto stretch buttons in the software, and by turning it up you will get a stronger stretch (less gap between the black and mid points), although I'm not sure if it will help regarding the positioning relative to the histogram peak.

I'd worry about putting the black point at the histogram peak - that tends to be near the median brightness of the background, which would make a large fraction of the background pixels come out pure black. Personally I don't like that sort of heavy pure black background, preferring just a tiny hint of very dark grey fading towards black instead.

cheers,

Robin

[Gfischer]

Robin,
Thanks for the reply. I have tried the main setting stretch level and it does make the overall image darker or lighter.
Normally when livestack on a cluster or small galaxy the standard stretch works fine. I think because there is not a lot of brightness variation.
A couple of nights ago I was live stacking the pac man nebula. When auto stretch the background is the normal dark gray. The nebula was very dim. So I moved the midtone slider to the left to bring up nebula brightness. Of course this also brings the background up to an over bright level. I then bring the black level to peak of histogram or even just to the right of peak and the background darkens and nebula remains bright.
Now if I leave sliders alone and start live stacking on a small galaxy I can pull midtone to the right to get correct background. This looks good to me.
In both of these cases the black level is near the histogram peak.
This might be a function of my particular setup and exposure times. Any suggestions?

[admin]

Hi,

if you are using the 'Arcsinh' stretch option in live stacking then increase the 'Soften' parameter to about 2 to 3 or so - that doesn't change where the auto stretch puts the black level, but it does reduce the slope of the stretch line up to the point where it reaches about 20% of maximum output brightness - that will help darken the background of the auto-stretched image, but without pushing it to hard black, which might give a pleasing result.

cheers,

Robin