Compositing + Alpha +MB

[rikou]

Hello and Happy New Year to everyone !


I have a question about alpha and compositing...
I have some animations to do and I need to render all my frames on a black background.
All backgrounds will be manage in post.


I have some Motion Blur and I don't know how can I do to avoid any problems with MB in postprod (I'm not the one who do the postprod on this project).


1- Do I have to save in particulary image file format ?? PNG ? TIFF ?? EXR ?
2- Another thinking, can I use the multimattes and my buddy could extract what he want (I think that it will be done in After Effect)
Anyway, could you please give me some advices for this ??


thanks a lot !

[joconnell]

You'll have no issues in after effects, any of those formats will work fine, you just tell after effects that the images are premultiplied and that they're premultiplied against black.

[rikou]

You'll have no issues in after effects, any of those formats will work fine, you just tell after effects that the images are premultiplied and that they're premultiplied against black.

Thanks !! I'll try with PNG.

And what about the multimattes ?? he did some tests but he can't have a nice selection from differents multimattes... there is like a frange
Is it better to use ID matte ? multimatte ? Or are there differents techniques to have a full access to selection in AE ??

[joconnell]

Right!

So multimattes can cause fringing in pretty much any application if you force things really far, here's why (and it's very similar to how alphas work):

When you render something against an empty background (let's say we choose black as our bg colour), the alpha channel will tell a compositing program how much of the semi transparent edge pixels are the foreground object and how much is the empty black background. If you've got a mid grey pixel in the alpha then it's half of the foreground object and half of the empty black background. When you bring in your rendered image to a compositing program and put your image over a new background to replace the empty black part, the program will look at the colours of the pixels along the edge and then look at the alpha channel to find out how much of the pixel we want to keep. Let's say we have a red foreground object rendered on black and along the soft edges it's mid grey or 50% transparent. That means 50% of our foreground red colour, 50% of our black background colour. The compositing program understands that for the edge pixels, it has to unmix the 50% red and 50% black to temporarily make the edge pixel 100% solid red. Now when we put in a new colour underneath our render, the program will take our 100% red colour, take our new background colour and use whatever grey value is in the alpha to know how much it needs to mix the two.

The problem that you get with multimattes is that they're often a selection for something inside your render - like a middle bit of an object that has other parts of the object around it. It isn't something simple like an object rendered against pure black which is really easy to unmix, now they bit that you want to colour correct might be on top of something that has lots of different colours along the edges where they meet. Now the compositing program can't as easily just unmix a bit of black from everything because of all of the colour differences so you can get lots of fringing problems :/

[rikou]

Right!

So multimattes can cause fringing in pretty much any application if you force things really far, here's why (and it's very similar to how alphas work):

When you render something against an empty background (let's say we choose black as our bg colour), the alpha channel will tell a compositing program how much of the semi transparent edge pixels are the foreground object and how much is the empty black background. If you've got a mid grey pixel in the alpha then it's half of the foreground object and half of the empty black background. When you bring in your rendered image to a compositing program and put your image over a new background to replace the empty black part, the program will look at the colours of the pixels along the edge and then look at the alpha channel to find out how much of the pixel we want to keep. Let's say we have a red foreground object rendered on black and along the soft edges it's mid grey or 50% transparent. That means 50% of our foreground red colour, 50% of our black background colour. The compositing program understands that for the edge pixels, it has to unmix the 50% red and 50% black to temporarily make the edge pixel 100% solid red. Now when we put in a new colour underneath our render, the program will take our 100% red colour, take our new background colour and use whatever grey value is in the alpha to know how much it needs to mix the two.

The problem that you get with multimattes is that they're often a selection for something inside your render - like a middle bit of an object that has other parts of the object around it. It isn't something simple like an object rendered against pure black which is really easy to unmix, now they bit that you want to colour correct might be on top of something that has lots of different colours along the edges where they meet. Now the compositing program can't as easily just unmix a bit of black from everything because of all of the colour differences so you can get lots of fringing problems :/

Thanks again for this great explanation. I understand now how it works.


We did a test and it's ok for what we have to do but I saw just one problem that I never met before ...
If you look at my multimatte, you can see some artefact in AA along the edges...It's one object with 2 materials (RED effectID 1 / GREEN effectID 2)
It seems to happen only on this case... No problem with AA if it's 2 differents objects.


What is the problem with my multimatte on this object with 2 differents materials ?

[joconnell]

Ah okay - this comes up a lot.

If you post the same section from the RGB render what I'll bet is where the edges of those two objects meet, I'd say their materials are very similar. The quality of your multi mattes is based on the quality of your beauty render's anti aliasing. Vray uses contrast in the beauty render to decide if it needs more AA along an edge or not so if the place where two objects meet is very low contrast, vray won't need to do as much work cleaning up that edge which means it's using less AA. The problem here is that the multimatte also gets sampled with that same low AA and your edge isn't as nice.

Here's the sneakiest trick in the world to force vray to do something nicer though. What we need to happen is vray to see lots of contrast along the edges of our object's edges so it'll do a nice job of sampling them for the multi matte. So what we can do is add in a vray_extratex render element and put in a vray edges tex map into it. This will render an element of black and white lines for our objects edges. The vray extratex map has a setting called "consider for anti aliasing" and what that'll do is as well as vray using the contrast in the beauty render to choose what level of aa you get, it'll also use the contrast in the vray extratex map to control AA too. The black and white edges you get from the extra tex provides huge amounts of contrast so vray will ramp up the AA quite a lot and you'll get a much smoother AA line on your mattes.

CAUTION!!!!

The only problem with this method is that if you force up the quality of your AA just to make matte lines nicer, you'll also end up getting more quality in your beauty render which you may not need - the beauty probably looked fine before we added in this trick so you'll be making vray work harder than it did before and your renders will be slower.

[rikou]

Ah okay - this comes up a lot.

If you post the same section from the RGB render what I'll bet is where the edges of those two objects meet, I'd say their materials are very similar. The quality of your multi mattes is based on the quality of your beauty render's anti aliasing. Vray uses contrast in the beauty render to decide if it needs more AA along an edge or not so if the place where two objects meet is very low contrast, vray won't need to do as much work cleaning up that edge which means it's using less AA. The problem here is that the multimatte also gets sampled with that same low AA and your edge isn't as nice.

Here's the sneakiest trick in the world to force vray to do something nicer though. What we need to happen is vray to see lots of contrast along the edges of our object's edges so it'll do a nice job of sampling them for the multi matte. So what we can do is add in a vray_extratex render element and put in a vray edges tex map into it. This will render an element of black and white lines for our objects edges. The vray extratex map has a setting called "consider for anti aliasing" and what that'll do is as well as vray using the contrast in the beauty render to choose what level of aa you get, it'll also use the contrast in the vray extratex map to control AA too. The black and white edges you get from the extra tex provides huge amounts of contrast so vray will ramp up the AA quite a lot and you'll get a much smoother AA line on your mattes.

CAUTION!!!!

The only problem with this method is that if you force up the quality of your AA just to make matte lines nicer, you'll also end up getting more quality in your beauty render which you may not need - the beauty probably looked fine before we added in this trick so you'll be making vray work harder than it did before and your renders will be slower.

AAhhh !! ok this is a tricky situation !!
Thanks for that trick.


My problem, in fact, is that it's one piece.. one object so there is only one material for this one but I need to separate those 2 parts (to change color or anything else in AE, don't know yet)
I will try with a vray_extratex render element and let you know.


Thanks again, I really appreciate your help

[joconnell]

As an alternative, if you're using material ID's to make your multimatte, you can do a similar trick where you make a multisubtex map, change all of the colour swatches to be very different colours and then set that to "Face ID" mode. Again put this into an extratex map and you'll hopefully get a render with loads of weird rnadom colours on your object - consider for anti aliasing will again make vray work harder.