Image viewer and scaling


when i subtract one image (e.g. Bias) from another image the result can contain negative pixel values. This works fine in Siril :wink:
But when i display the image and try to e.g. stretch the range about zero to see the remaining noise in the image i cannot restrict the display to e.g. -50 to 50 ADU because the slider/input field does not accept negative values.
Am i missing something? Or is it impossible?
Or is this just a bug in siril? The range of the slider should be MIN to MAX ADU instead of 0 to MAX ADU as currently shown.

Ciao, Udo

The management of the negative value of a pixel is essentially necessary for the internal calculation, so as not to lose information. However, there is no point in allowing us to display it. Indeed, in the final image we must have positive values. All negative pixels will be clipped.

So for me it is not a bug, but the wanted behavior.


for the final image this is maybe true. But to view intermediate results you should be able to scale the pixel values to see everything.
And even for final images it would be nice if the sliders both adopt to the actual range of pixel values. The top slider seems to do so. The bottom slider stays at zero.

Negative values easily can occur if i subtract a bias image as the remaining noise can have positive and negative values. As far as i know fits files are capable of storing neagive pixel values.

Ciao, Udo

P.S.: I did not find a way to add a constant value to all pixels - that would help with the negative values as i could shift them to positive values.

Hello Udo, indeed there is no way to set the cut-off cursors to negative values, as well as values above the maximum. For 16 bit images it’s normal, for 32 bit images, we can have negative values and values above 1. Both of them are considered as outside the normal range. We keep them because they provide some information and are saved in the FITS file. But we won’t provide a way to display them with the cursors because it’s too complicated to manage. Maybe the cut checkbox could display in white the negative values, but it’s also a lot of work.

They are accounted for by photometry, so by existing they don’t create energy from nothing in the images. If a processing results in many negative values, it’s a problem that should be corrected by doing a proper calibration, as explained here.

You have the pixel math tool and/or the offset command.

I totally do agree with @vinvin


thanks for the replies.


I still think that it should be possible to slide to negative values. If an image e.g. has values of +/-100 (noise around zero) and i put the upper slider to 100 and the lower slider to -100 the ADU range -100 to 100 should be mapped to the display as 0-255 (on an 8 bit display). It should not be a huge problem to add this.
I agree: In “normal” images this is not needed.
But ok: With offset this should be possible to solve (for me). I am relativly new to Siril and could not dig deep enough to figure out how commands like offset or pixel math work. Still stuck with the mouse pointer :wink:

Ciao, Udo

Currently it is. We handle this as ushort. ushort doesn’t allow negative values.