[IPOL discuss] Vignette image compression in archive

[Miguel Colom]

Quoting Bertrand Kerautret <bertrand.kerautret at univ-lorraine.fr>:
> Dear All,
> In the test of the demonstration 68, I see that the vignette images  
> of the archive gallery appear with low quality in comparison to the  
> original ones.
> In particular it is visible on image representing a distance map in  
> gray scale format. For instance you can see it here:
> http://dev.ipol.im/~kerautre/ipol_demo/68/archive/
> The vignette images look as if it was near binary images while the  
> original are more in grayscale.
> The format used is png, and by testing it with jpeg it looks better  
> (see last archive).
> Did there is a simple way to define the vignette quality in the  
> app.py file ? (else it is not a problem we can use jpeg i think)
> Thank for your help.
> All the best.
> Bertrand

I've been looking at this and it seems that the problem is because we  
use PIL to process the images (the crop the input images, to create  
thumbnails, etc).

When the image is encoded in 8 bits, everything works files.
However, when it's 16 bits (as in your case), PIL is unable to deal  
with them properly.

It seems that it took your 16-bit grayscale image and "converted" it  
into 8 bits by saturating all pixels whose intensity is over 255.  
That's why it looks like a binary image.

We should change PIL to something else or add some kind of 16 bits  
extension. It's something that we know we have to do, but we haven't  
had the time to start it yet.

Best,
Miguel