Re: Workaround for this bug?

[Frank C. <dev...@ch...>]

On 21-Jan-05, at 12:11 PM, Keith Wiley wrote:

> On Fri, 21 Jan 2005, James W. Walker wrote:
>
>> On Jan 21, 2005, at 12:10 AM, Keith Wiley wrote:
>>
>>> I am trying to use an alpha channel to make a soft sun that fades 
>>> into the background sky.  It works perfectly except at the horizon, 
>>> where the square shape of the plane the sun texture is put on shows 
>>> up as a translucent square.  This problem vanishes if I turn off 
>>> fog.  I am aware of the discussions pointing out that fog does not 
>>> work properly with alpha channeled textures...and I assume you will 
>>> agree that this bug is the problem I am experiencing.  My question 
>>> is, does anyone havea reccomendation for how to work around it?
>>
>> Why not use a disk instead of a square for the sun?
>
> The only reason I used a square was to reduce the number of triangles. 
>  I considered a circle and thought it was "bad desig" to make shapes 
> with triangles that could be made with transparent textures instead.

Indeed...

> I'll try it with a disk...although I would still like the alpha 
> texture to work properly if I want the sun to have a fuzzy edge.

A similar effect can be achieved by using a subdivided disk with 
per-vertex colour/transparency attributes that fade out towards the 
outer edges, but there is a solution for some additive blended 
textures...

First ensure the alpha in your art is not pre-multiplied (likely the 
case already) then put the sun shape in a display group under a fog 
object with its visibility set to false. This will ensure no fog is 
drawn on the sun, which is typically a desired effect on all celestial 
bodies. Also place a Lambert shader before the sun geometry to turn off 
any potential specular highlights, since they may display a similar 
artifact as fog.

HTH!
Frank.
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