On 28/11/2014 12:24, clipka wrote:
> 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).

Thanks Clipka, I thought that I was finished with this. But what you've 
written need digesting. :-)


I must find another editor I've been using PaintShop Pro out of "it's 
good enough and I've always used it". What you can do with pallets and 
Alpha is basic.

Interestingly the 24 bit Tiff Cloud Map, I downloaded from Blue Marble. 
Has only 253 different colours.

-- 

Regards
     Stephen

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