8/16 bit gray scale png format causes rectified map to not be displayed in preview (Map#8161)

[xorxx]

Rectifying and masking this map pass without an obvious error and also some preview inside editor is displayed.

After changing to preview tab of this map interface displays the following message:

Whoops, the map needs to be rectified before you can view it
Sorry, you'll have to rectify a map before you can view it.

I can only guess that something is dangling around in some hidden cache?

[timwaters]

Interesting map.

When the clipping mask is applied I see this:

this is likely causing an error. It might be a palletetized png colour space or greyscale one. The software tries to handle these but prefers more colours.

When the mask is not applied, this is the result (although its not shown in preview)

[timwaters]

`
gdalinfo Schwetzingen-Schloss-1836.png
Driver: PNG/Portable Network Graphics
Files: Schwetzingen-Schloss-1836.png
Size is 1010, 930
Image Structure Metadata:
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 930.0)
Upper Right ( 1010.0, 0.0)
Lower Right ( 1010.0, 930.0)
Center ( 505.0, 465.0)
Band 1 Block=1010x1 Type=Byte, ColorInterp=Gray
Mask Flags: PER_DATASET ALPHA
Band 2 Block=1010x1 Type=Byte, ColorInterp=Alpha

`

[xorxx]

Ah now I see: The image is a 16 bit gray scale image in PNG format. This may be the reason. I just uploaded a newer version to Wikimedia that was converted to 8 bit gray scale. The original image was 8 bit information anyway, but saved as 16 bit. Now I don't know how to force the maps server to fetch the new image version.

[timwaters]

It's good to keep these kind of images so that the software can work around them:
https://commons.wikimedia.org/wiki/File:Schwetzingen-Schloss-1836.png

Will change the title