Particles Compositing - Black Edges Problem

[vinz9]

Hi,

When rendering particles with the hardware renderer and geometry mask on, when I put back the particles and the objects togeter, I always get a little black edge around the object. The alphas don't exactly match. I know it's because of the antialiasing, is there a proper way to fix this ?

I have the same problem with fluid compositing, which are often in front and behind objects, and require using matte for the objects.

Left is particles and objects rendered separately, right is all rendered with hardware renderer.

Thanks a lot.

[joojaa]

QUOTE(Vinz9 @ 02/24/09, 02:05 PM) [snapback]301581[/snapback]
I know it's because of the antialiasing, is there a proper way to fix this ?

Actually its not, its about alpha interpretation. So you know wrong.

See trasparent pixels do have values and how you read thos values can be doen in 2 ways:

premutiplied, where the pixel colors are multiplied with alpha (and bg color usually black)
Straight where they are not, this is yet tobe done by the app

Straight reading, premutiplied with black yelds this. premultiplied reading straight would appear to have hard edges.

Most comp applications have a switch to say wether footage is straight or premutiplied. So what application do you use for compositing. So proper way to fix this is to tell the application what kind fo alpha you have. (exept in photoshop where you convert the data form premultipled to straight)

[vinz9]

Thank you for your answer, but I know about alphas and premultiplication.
These are coming from maya, so they are premultiplied. And I checked the correct option in after effects.

I had made another test, with a torus around box.
Let's say I render the box alone, and then the torus with a black matte material assigned to the box.
Now if I comp these two together, putting the torus (with black where the box is) over the box, there is a same problem.
Where the edges meet the background shows through. Both coming straight from maya with premultiplied alphas.

I've tested it, and seen about it in a gnomon workshop dvd about compositing.

For the torus and box case, I don't need them rendered separately so it's no big deal, but for particles and objects I do.

With no antialiasing, there are no transparent pixels on the edges so no problem.

[joojaa]

QUOTE
Let's say I render the box alone, and then the torus with a black matte material assigned to the box.
Now if I comp these two together, putting the torus (with black where the box is) over the box, there is a same problem.

Ok i did this no problem. No gap. Also with particles no gap.

But yes this is classic sign of false interpretation.
QUOTE
hese are coming from maya, so they are premultiplied.

Ok, see maya doesn't HAVE to be premutiplied there's a option in render settings to choose what you want,and is the premult color in AE right. Also chek the bg color is set to black.

o oods are the other is premultiplied while the other might not be.

QUOTE
With no antialiasing, there are no transparent pixels on the edges so no problem.

Yes offcourse no alpha interpretation error.

But yes it might be aa issue among other things, it would happen If you would choose to use mentalray and use a higher order filter, use box (in maya software dont use multipixel filtering). But then if you use a nurbs sphere youd still be off a bit since the tessellation is different in the renderers.

Also chek that the famebuffers are linear, if you adjust gamma the aa isnt going to match.

Then theres the small fact that mentalray isn't guaranteed to have same rounding as maya. this is why you can render particles in mentalray.

Personally i dont do it thisway, as theres a better option of using zcomp inside maya hardware renderer. Or in shake (but good luck with this in AE)

QUOTE
And I checked the correct option in after effects.

Please re chek. I have had this duiscussion about 50 times and it was still wong because after effects didnt do whatyou tough you told it. Because in few cases the global was set and in other not.