reflection of light got alias problem

[davidkwan]

Hello, I'm using vray 1.5 with maya 2011

For a basic vray material, I find that the reflection of light (e.g. rect light) appear on surface have "alias" problem, the edge of light (appear on reflection) never got anti-alias until I turn down the value of hilight glossiness so that make the edge a little blur. I can't make perfect reflection of light (glosiness = 1) without alias

Is there something I can do with this??

Thanks,

[davidkwan]

can anyone help?

[CerpinTaxt]

Can you post a picture?
A bright reflection with pixel values above 1 can result in an anti-aliased edge that still has pixel values above 1 and look aliased. Take a bright pixel with a value of 3 and a dark pixel with a value near 0. If you blend these two together you get a pixel with a value of 1.5 that still appears as a pure white pixel.
You have two options to solve this. The first is to clamp your image and get rid of those high pixel values. Look under Color Mapping in the globals. The second option is to deal with this problem in compositing and add post effects like e.g. glow to your image.

[davidkwan]

I have attached an image showing the alias problem of the light casting on a reflection surface.
As you can see, the brown surface is a perfect one which have a high reflection value, which reflected the 2 Rect_light above it.
One of the Rect_light with intensity 1 while the other's intensity is 50.
You can clearly see that the reflection of light with value 1 has a much better alias problem (it still have a little zigzag on edge) while the 50 one got serious zigzag and black edge around.
I have try rendering this image with different passes (reflection, diffuse, specular, etc) and do a simple compositing but the black edge still appear

So what's the problem exactly?

[davidkwan]

oops.... the attachment never appear in my last reply
here is the attach photo

[vlado]

Try turning on the "Subpixel mapping" and "Clamp output" options in the Color mapping section; this should sort out the issue.

Best regards,
Vlado

[snivlem]

Actually doesn't this phenomenon happen in real life with cameras too, but they naturally show a halo effect which hides this aliasing? Think I may have read this somewhere

[vlado]

It does happen, but actual cameras usually spill a little into neighboring pixels (due to bloom and glare), so this effect is not as pronounced.

Best regards,
Vlado