clipka <ano### [at] anonymousorg> wrote:
> Obviously, if you use these values in the above formula, you get:

>    ResultColor = (R;G;B) * A^2 + Background * (A-1)

> which of course gives a different result color (except for A=0.0 and 
> A=1.0). One symptom is that gradients from opacity to transparency, 
> while appearing smooth in the preview, will exhibit pretty hard edges in 
> the image file.

  I don't see that happening. Just to test this, I rendered this scene
with POV-Ray 3.6, without +ua and then with +ua:

//----------------------------------------------------------------------
background { rgb 1 }
camera { right x location -z look_at 0 angle 90 }
plane
{ -z, 0
  pigment
  { gradient y color_map
    { [0 rgb 0]
      [.5 rgb y transmit .5]
      [1 rgb y transmit 1]
    }
    scale 2 translate -y
  }
  finish { ambient 1 }
}
//----------------------------------------------------------------------

  I put the two images side-by-side here:

http://warp.povusers.org/images/gradient_test.html

  The image on the left uses no alpha channel, and it's just a gradient
from black to white, going through green. The image on the right uses
alpha channel and goes from opaque black to transparent green.

  There is no visible difference. You can't even tell where the border
between the two images is.

  To corroborate that the image on the right indeed has alpha-transparency,
contrast with this (the only difference being the background color of the
page):

http://warp.povusers.org/images/gradient_test2.html

-- 
                                                          - Warp

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