Am 26.11.2014 16:34, schrieb Stephen:
> On 26/11/2014 15:02, Thomas de Groot wrote:
>> What is a pallet-based transparency? The term is unfamiliar to me.
>
> Not surprised. I made it up.
>
>  From the top of my head:
> It is an index of colours used in the image. In Pov we can set the
> transparency and filter values in the image_map definition. Each entry
> in the index can be set to a transparency, individually. I think that
> pallet based images are limited to 8 bit images.
> So in a cloud map each grey could have a different level of transparency.
>
> rgb -> rgbft

Actually, with palette-based images you are limited to 1 bit of 
transparency: On or off. Because there's just one special colour among 
the 256 that denotes a transparent pixel.

Images with alpha channel (usually png) typically give you 8 bit of 
transparency information. If you need more, you can opt for one of the 
following:

A) 16-bit png images with alpha channel.

B) pigment_map, using the image's greyscale value to choose between an 
opaque and a transparent pigment. (Make sure to set a proper "gamma" 
parameter!). However, this will typically give you not much more than 8 
bit either, so you'll want to go for 16-bit images anyway.

C) high dynamic range images; OpenEXR can support an alpha channel, 
alternatively you can use pigment_map as described above. However, near 
100% its precision is a good deal less than that of 16-bit images 
(comparable to 12-bit images; Radiance HDR is even worse).

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