Wasn't it Nicolas Alvarez who wrote:
>Nicolas Alvarez wrote:
>> It will supersample even if there is a completely color.
>
>OK that got messed up... I meant "It will supersample even if there is a 
>completely plain color." Even if your whole image is the exact same 
>color, +a0.0 will supersample it...

It is sometimes useful to supersample images for which the colour is
exactly the same at integer pixel positions, but which have sub-pixel
features.

For example:

camera {location  <0,0,-10> look_at 0}

plane {z,0 
  pigment {gradient x
    colour_map {[0.005 rgb 0][0.005 rgb 1]}
  }
  finish {ambient 1}
}

If you render that with any non-zero antialias threshold then it will
miss some of the lines where the line happens to squeeze between pixels.
How many lines are missed depends on the image size: at 320x240 all the
lines are missed.

If you render it with +a0.0 then it finds all the lines.

That's a bit of an extreme example, but it's possible to encounter the
same sort of effect in real scenes.

-- 
Mike Williams
Gentleman of Leisure

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