Author Topic: Harry's Tutorials  (Read 864 times)

Offline tgervais

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Harry's Tutorials
« on: 2016 December 21 13:37:10 »
I have been following Harry's Tutorials and find them very useful.  But I am left with some confusion here and wondering if someone might be able to help.

It concerns the linear stage and keeping it there when using 'screenTransferFunction'. When I bring up an image saved in linear mode,  I get a blank/black image.  That is correct.  At this point Harry runs the 'ScreenTransferFunction' and he gets a really nice image.  But he is still in Linear mode.

I am trying to do the color calibration process and Harry says to keep the image at the linear stage.  That means that up in the left corner,  the GREEN vertical  line that would normally be along side the file name should be  gone.  I apparently need to keep it gone but as soon as I run the ScreenTransferFunction'  that green line appears. And I am out of Linear mode again.

Any thoughts would be appreciated.

 :(


Online Geoff

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Re: Harry's Tutorials
« Reply #1 on: 2016 December 21 14:10:30 »
The image is still in linear mode when you apply the STF. It is only the display that is altered, but the actual data remains unaltered.
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Offline Alejandro Tombolini

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Re: Harry's Tutorials
« Reply #2 on: 2016 December 21 14:14:04 »
Yes, when you run ScreenTransferFunction your image is still in linear mode. You have to maintain STF applied to see changes during ColorCalibration procedure, and you will visualize the green line.

From tool documentation:

A screen transfer function (STF) in PixInsight is used to improve the visibility of an image as it is represented on the screen, but without altering its pixel data in any way. This is useful to work with processes applied to linear, unstretched data. Such processes may include color calibration, deconvolution, wavelet transforms, and some noise reduction procedures, among many others. Unstretched raw images are often poorly represented on the screen. This is because most of their data are concentrated in small portions of the available dynamic range. This can be easily identified as a narrow peak near the left side of the histogram.

A STF is a histogram transformation that PixInsight applies to the bitmap generated to render an image on the screen. STF parameters include per-channel midtones balance, shadows and highlights clipping values, just as any histogram transformation does. STF includes also dynamic range expansion parameters, but these are only available through the command-line and scripting interfaces of the ScreenTransferFunction process, not on its graphical interface.

In PixInsight, each view can have its own STF, which can be adjusted independently for the red/gray, green and blue nominal channels, plus an additional luminance STF that applies only when the CIE L* and CIE Y* components are being rendered on the screen, in the corresponding display modes. This allows for very precise and fine-tuned adaptations of screen renditions to raw data. ScreenTransferFunction allows you to work with linear images, just as if they were nonlinear, in an easy and completely transparent way.


Hope this helps.
Saludos, Alejandro.