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>Mike Williams wrote:
>> 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.
>> 
>       In that case, render at a higher resolution with +a0.1, then
>downscale the image with an external tool. This will be faster than
>+a0.0 for an equivalent quality.

Depends how large you want the final image. If you want a 640*480 image
of the above example, then it's faster to use +a0.0 at 640*480. On my
machine it takes 4.8s to render at 640*480 +a0.0, but in order to see
all the lines at +a0.1 it needs to be scaled up to about 2300*1725 which
takes 9.4s.

-- 
Mike Williams
Gentleman of Leisure

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