multimatte keying over AE "set-matte" gives border

[thomes]

hello,

i render out 4k footage with multimattes. if i key e.g. red colour over after effects´ "set Matte" and screw down/up e.g. the brightness i get a white AA one pixel borderline (moirée) .
noise thesh 0,005/ min1 /max24/ no image filter/ enable filtering in multimatte is "on"

any help here?

thanks a lot
thomes

[thomes]

bump...anyone from vray? lele, aleksandar ?

[nikoleta.garkova]

Hi Thomes,

Would it be possible to attach the scene files or send them to support@chaosgroup.com (make sure to mention this thread in the email) so we can have a look and test it further? Thanks.

[thomes]

thanks for looking into ! file sent

[fixeighted]

Just a thought - if you're rendering the matte with antialiasing but then you only key the red, are you not losing those antialiased values and so the matte is then not accurate?

[thomes]

as it seems "set Matte" in AE is also picking the less red pixels in the aa parts of the multimatte... so this looks principially good i would say...

[fixeighted]

Ah, ok...it's not that then

[thomes]

nicoleta? time to look into this little thing?

[nikoleta.garkova]

Thanks for the provided scene files, thomes,

We've tested this in both After Effects and Photoshop, and seems the expected behavior when it comes to anti-aliasing - as it is taking a variable number of samples per pixels and depends on the difference in the intensity of the neighboring pixels.

If you apply a Brightness effect to your mask and increase the sub-pixel difference drastically, it is normal to get fringes.

In this particular case, you have a 4K rendering but the building is far in the distance. The unwanted effect becomes more visible when zooming in the rendered output. If you either render a 4K image with the building closer to the camera OR keep it as is and increase the image resolution even more, this will help refine the anti-aliasing as it will produce the necessary pixels for proper sampling.

Poor anti-aliasing can be also prevented if a given object is rendered alone on a black background so that issues with alpha premultiplication are avoided.

Note that such effects are not necessarily a result of poor image sampling. In general, they appear simply because the image is discretized into square pixels. They can be reduced through the usage of different anti-aliasing filters as well, but are not completely avoidable.

Hope that makes it more clear.

[vaya_studio]

thomes Try puting a Levels adjustment after the Set Matte, change the Chanel to Alpha and you'll be able to adjust how "soft" the edge is. Also, put whatever adjustments you use to brighten/darken before the Set Matte effect.

[thomes]

thanks a lot both for your information ! i will give it a try the next days ,i´ll be back.