Re: [Gimp-user] grainy scanned images

[Ofnuts <ofnuts gmx com>]

On 08/08/17 18:37, Olivier wrote:
> I'm  using GIMP 2.9 on Ubuntu Studio with MATE window manager. I'm
> scanning black-and-white photos taken slightly less than 50 years ago.
> This is working generally well, although of course I have to rotate,
> crop, and sharpen the images, and use the Levels tool a lot. However,
> for two photos a weird thing happens: the photo looks rather good when
> displayed in the GIMP window, but it looks terribly grainy when
> displayed by "Eye of MATE Image Viewer".
>
> I placed the photo at the address http://olecarme.homelinux.net/page0022.jpg
>
> Clearly it's grainy, but not so much as in the image viewer, and in
> the GIMP window it looks acceptable at a scale of 33.33%, and even at
> a scale of 50%.
>
> Two questions:
> - May anybody provide an explanation of this phenomenon?
> - Is there a way to display the image correctly outside of GIMP?
>
> Thanks for any idea!

To zoom out the image the application has to interpolate pixels. 
Different applications can use different algorithms, that don't behave same
with noise (a.k.a grain). Some algorithms can even make the image look 
grainier than it is, when the zoom factor is close to an integer 
multiple of the grain size.

If you want the image to display well at a given size, then make a 
scaled copy of the image. Then the viewer will display it 
pixel-for-pixel and you won't have to worry about the interpolation 
algorithm.