Help with Histograms and general processing

[atudorica]

Hi,

     I have just started with PixInsight and I'm having quite a few issues with processing images since the beginning. First of all, I'm using very, very large images (~1 gpx, 30k x 30k) but I have a computer system that can deal with that without any problems. However, to speed up things a bit I thought to apply all of the processes on a small preview before doing it on the large images. I hope that's a valid approach.

    The very first thing I'd like to do after creating the color image from three filters is to stretch the histograms (as seen in the first screenshot). I'm then trying to cut the shadows and highlights where there is no or little information stored to increase the contrast (screenshot 2). Unfortunately, there's really not much of a difference and the histograms look very different from what I've seen here, for example: http://www.harrysastroshed.com/pixinsight%20images/histogram%20pic/histDT.jpg
     Only by using the black/midtones/white point readout mode and then playing with the screen transfer function I get something like in the third screenshot on the right, but then the background is to black and it's all about clicking in "the right place".  The automatic screen transfer function (Ctrl+A) gives me the horrible yellow image on the right.

     What on Earth am I doing wrong here? It is rather frustrating and I'd appreciate a lot some advice on this


    Thanks,
Alex

[atudorica]

... and the third screenshot is now attached.

[Geoff]

Hi Alex
The fact that the histogram shows distinct peaks in each of R, G and B shows that you have a strong colour bias. Have you tried DBE and/or Background Neutralization to remove this bias? The workflow should be:

• RGB combine linear images
• Stretch with STF so you can see what's going on.  Reset as necessary after applying steps below
• Use DBE to flatten background and remove colour bias
• Use background neutralization
• Use ColorCalibration to get colour balance
• Apply STF stretch to the histogram process. 
• Histogram stretch

See the first video here http://pixinsight.com/videos/index.html , which goes through the above processes
Geoff

[atudorica]

    Thanks Geoff, that helped a lot! You relieved me from a healthy dose of frustration. PI is so different than other software that have used until now extensively!

    Now I've understood how this works in principle and I processed my first image. It looks quite fine However, it's a bit too large to share it here right now, the jpeg is 240 mb and I couldn't even save it in tiff, as it was larger than 4 gb.

 

[Tom OD]

Just wanted to ask about the bigger than 4 Gigs and saving as a TIFF. I had problems with that in photoshop. It flattens the layers. Is there a 4 gig limit also in PI? I am still trying to make large mosaics with the GMM tool, and channel combine full mosaic R, G and B frames.
Thanks Tom.

[Andres.Pozo]

The TIFF format uses 32bits offsets so TIFF files can not be bigger than 4GB.

There is a proposal named BigTIFF that uses 64bits offsets and allows files bigger than 4GB. This format is not standarized although it is used widely for cartography.

[Tom OD]

Thanks for the reply Andres.