strange thing on Iray Matte

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Here I put a boy and an iray matted plane in the scene, together with a HDRI environment.

If I hide the boy, only left the plane in the scene and render, the result is as the left, everything is fine, the plane is fully hided.

But when unhide the boy (he was putted behind the plane), the rendered picture shows a edge of the plane, as the blue arrow pointed, that is the plane edge.

Why there shows the edge? What I can do to let the edge disappear?

markusmatern

Try to increase the width of the plane. Maybe it's the shadow of the boy showing, because the plane is not wide enough?

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markusmatern said:

Try to increase the width of the plane. Maybe it's the shadow of the boy showing, because the plane is not wide enough?

No, it is exactly the plane's edge, unless widen the plane out of the viewport (which means can not see the edge). If put the plane behind the boy to catch shadow, the edge shows more clear. 

Richard Haseltine

The plane isn't just a shadow catcher, it will also catch reflections etc. It is important to have its surface properties be fairly close to those of the surface it is catching for. I'm not sure that is all that is happening, but I do think it is at least part of what you are seeing.

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Richard Haseltine said:

The plane isn't just a shadow catcher, it will also catch reflections etc. It is important to have its surface properties be fairly close to those of the surface it is catching for. I'm not sure that is all that is happening, but I do think it is at least part of what you are seeing.

But there is a product in the store: Iray HDRI Toolkit , I saw its picture didn't contain the plane edge and just want to repeat it. How does that toolkit do?

PixelSploiting

Your plane reflected the light. Iray matte shadow catchers will show reflections if they are reflective enough. If you want it to vanish completely set all the possible reflections (specular, glossy) to 0, all the specular/glossy roughness to the max, reflectivity to 0 in the surface properties tab. All specular/reflection colors set to either pitch black or dull gray.

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PixelSploiting said:

Your plane reflected the light. Iray matte shadow catchers will show reflections if they are reflective enough. If you want it to vanish completely set all the possible reflections (specular, glossy) to 0, all the specular/glossy roughness to the max, reflectivity to 0 in the surface properties tab. All specular/reflection colors set to either pitch black or dull gray.

Well, not this reason. It is just a simple plane, I already set all glossy to zero, roughness to 1, and all color to black, just no use.

As mentioned at beginning, the same plane, with people it shows edge, without people it didn't, so it is not a reflection problem.

I checked that Iray HDRI Toolkit again and found a tricky, all its matte planes are very long, out of the viewport, maybe that's their way to avoid edge.

PixelSploiting

Try if a very flat cube or cylinder also shows edges. Might be an issue with Iray and 2-dimensional planes with no actual Z-depth.