It's possible to add an alpha gradient to an image map like this:

pigment {
    gradient y
    pigment_pattern {
        [0 image_map { <your image> }]
        [1 image_map { <your image> transmit all 1 }]
    }
}

So the gradient blends between opaque and transparent versions of the map.

--
Tek


=?ISO-8859-1?Q?J=F6rg_=27Yadgar=27_Bleimann?= <yaz### [at] gmxde> wrote:
> 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?
>
> See you in Khyberspace!
>
> Yadgar

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