Le 13/05/2012 15:41, Jörg 'Yadgar' Bleimann nous fit lire :
> Hi(gh)!
> 
> With a new computer finally at hand, I wanted to upgrade my POVSolar
> (and, thus, also POVEarth) project, replacing the simple 2048 x 1024
> surface and clouds texture by a more sophisticated one. After adding
> separate textures for surface (i. e. land, sea and sea ice) and clouds
> (on a slightly larger sphere to make them float at 5 kms above the
> surface), I also want to add a "city lights" texture showing only up on
> the night side of Earth.
> 
> To achieve this, I added the "city lights" image_map as a new texture
> layer beneath the daylight surface image_map - but to make it appear on
> the night side only, I'll also somehow have to add a gradient pattern
> for the alpha channel of the daylight surface texture. Unfortunately,
> the official manual is silent about this - it only contains notes on
> assigning alpha values to index colours or using the alpha channel
> provided with the image file...
> 
> So, how can I add an alpha gradient to my daylight image_map?
> 

gimp ?

Add a alpha canal, edit as convenient that layer, save as flattened png.

Now, the hard part is that excepted for equinoxe, the day vs night line
does not go through the poles and the projection used to make the map
does not help.

Equinox is easy: rectangular, full height. The edge on left & right
might be large enough for whatever needed gradient.

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