Saving files with Siril with good exposure

I am new to Siril (version siril-0.99.4 here) , and I have observed a strange phenomenon that I cannot control. Certainly I am doing something wrong, but I could not figure out which.

After processing the final stacked file with Siril, I like to make some final touches with GIMP. So I save files in 16-bit TIFFs and open in GIMP.

90% of the times, I open something that looks like a totally black image in GIMP. I have to stretch the histogram A LOT to make it look like in Siril. This does not seem to impact the final version a lot (I see no pixelation), but I can see missing frequencies in GIMP histogram. If I save the file in (8-bit) JPG and open in GIMP and I correct the histogram I see a horrible watercolor mess, as expected by the low 8-bit depth.

Yet, sometimes I open the image in GIMP perfectly fine, without any need for histogram correction… Why is this? How can I tell Siril to save the TIFF with the same exposure that I see in its window?

I guess this has to do with the linear vs log image, but I was unable to solve the issue playing with this in both Siril and GIMP.


Hi Luis and welcome.
The behaviour you see is perfectly normal and should be explained in all tutorials. The modes of image display in the Siril graphical user interface are only ways of scaling image data to display them in a human-compatible way. The values of the pixels of the original image are not changed. So when saving, it does not matter which mode you are displaying the image with, it will always be the same, as linear between 0 and the max of the pixel depth, so yes, quite black.

If you want to export what you see in your saved images, you must use the linar mode and change the histogram of the image instead.

The first paragraph is exactly what I expect, and it is actually well explained in Siril manual.

However, sometimes, when I open the image in GIMP, I do not need to fix the histogram, and the image looks exactly as in the visualization window of Siril.

Maybe a GIMP problem?

I did not quite understand your last sentence, though. You mean to change the histogram in, say, GIMP? I can brighten the image in Siril using “Image Processing > Histogram transformation…”. Maybe it is a good idea to use this instead of GIMP’s because the 32 vs 16 bits?


Yes I meant the histogram transformation tool of Siril. I don’t know if it’s better than Gimp’s, but at least you get a decent exported image, which is a good thing to start off with.

If the image looks sometimes like in siril, maybe it’s because you have used this tool or a few others that actually modify the image before exporting it.

Good. I will use the histogram transformation in Siril then. At least closer to a decent looking image.

No idea why sometimes GIMP shows a good image. I will keep an eye on this.