Le_Forgeron <lef### [at] freefr> wrote:
> Le 26/10/2010 23:49, Mr a écrit :
> > "Mr" <nomail@nomail> wrote:
> >> If I do that,  only the last entry of color map of my gradient will be used as
> >> if the the first entry had never existed. so it seems like the transparency is
> >> baked into the color for each entry. could there be an option to make it so that
> >> if the first entry has transmit 1 its color is still retained and all
> >> transparencies
> >> only applied in the alpha layer?
> >
> > Actually it's not the color of the last entry, it happened to be the color of my
> > background too. If I remove the background, I get a completely black skysphere!
> > So what is happening, Are spy sphere totally ignoring the separation between
> > colors and alpha?
> >
> >
>
> You are specifying a transmit of 1, which means that the ray must goes
> straight through the sky_sphere, without interacting with the colour.
> Hence it get to the background... whose default is black unless you
> specify another.
>
> I'm afraid we are entering the big discussion in image format: does a
> transparent pixel have a colour (or: is the rgb part of an alpha-enabled
> image format pre-multiplied or not in regard to alpha (and gamma, it's
> far more fun with gamma in the equation)).
>
> The problem is that different format have different answers.
> (and they might be all valid, up to their tenants)
>
> Maybe you want to replace rgbt with rgbf (it give a colored background
> with some transparency with +UA (and using a format which support alpha,
> such as png)) ?
>
> Or keep rgbt, but reduces t to 0.5 ?
>
> I do not get what you are trying to achieve with such transmitting
> sky_sphere.
>
> --
> Real software engineers work from 9 to 5, because that is<br/>
> the way the job is described in the formal spec.  Working<br/>
> late would feel like using an undocumented external procedure.

I'm working with PNG and I want my Blender exporter to give me the proper alpha
to allow compositing, and other things just like the internal renderer outputs:
it allows a gradient for color of the background sky (blend sky option) but the
alpha is  still cutting the whole sky as *fully* tranparent. So in Pov an alpha
of 0.5 will not solve the problem as I'll have only partial transparency.

And the first solution you suggest, won't work either, should I use Filter for
the background, I get no transparency at all. Maybe that's what is broken? So I
assume you meant transmit, and transmit 0 for BG as well as both sky sphere
color map entries gets me no transparency, transmit 1 for both color entries
of skysphere gives me only the flat color taken from background keyword, and
using transmit 1 for only one of the sky sphere colormap entries, namely the one
that is similar to the "background" to let background actually do it and sky
sphere only do the second color: then I get a gradient in the transparency
values.

Any other solution?

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