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can you invert a background shader?

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jim
can you invert a background shader?
by
on Jun 2, 2006 at 11:28:00 am

Hi, Is it possible to invert the background shader to reveal instead of hide.

ie I have a model that needs to reveal from the ground up if i cover it with a block that has the background shader i can reveal nicley from top to bottum but not the other way around as you just get a striaght mask.

if however I could tell maya to render every thing that is in the block and ignore whats out side the block I would be laughing!

the last time I had to do a reveal from the ground up I used Boolean whichis not something I want to repeat, and this model is alot more detailed so the renders would be too painful.

cheers

Jim


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Steve Sayer
Re: can you invert a background shader?
on Jun 2, 2006 at 3:01:01 pm

I'm not sure I understand exactly what you're after, but rather than 'inverting the Use Background shader' (which I don't think it's possible to do), what about using a large block (one that covers all of the camera's view) with a hole in it to reveal what you want, instead of a single block in the centre? It seems to me that that would be the equivalent of inverting the shader.

If your question is more about alpha channels, note that the Matte Opacity attributes let you exert control over this.

Hope that was helpful.

-Steve



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jim
Re: can you invert a background shader?
by
on Jun 2, 2006 at 3:14:37 pm

I have changed the angle of the shot to work using the background shader technique although this is not the answer it will get me to the deadline.

my original question worded a bit better is. I want to make something appear as if masking it on but in true 3d.
one method would be boolean
i.e. create a boolean difference between a block and the model and animate the block up in Y to reveal the model.

I was looking for other methods because the boolean technique is slow and messy.

cheers for your response.


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Steve Sayer
Re: can you invert a background shader?
on Jun 2, 2006 at 3:34:42 pm

Ah, okay, now I understand.

I think you probably could have done it by combining two shaders--the regular shader and the Use Background--with some kind of two-colour texture, such as a ramp or a checker, projecting that texture onto the object and then animating it so it transitioned from one to the other.

Things like shadows would have to be dealt with separately, though...

Sorry I couldn't be of more help.

-Steve



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jim
Re: can you invert a background shader?
by
on Jun 2, 2006 at 3:39:01 pm

interesting.
I will have to try that. Ive got a hunch it wont be quite right, but I will let you know.

thanks for your help.


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Ben Insler
Re: can you invert a background shader?
on Jun 2, 2006 at 5:30:15 pm

Jim,

It seems to me that there is one major problem that you're interested in solving here (that said however, I may be interpreting this completely wrong). I would be in total agreement with all of Steve's abovementioned methods, except you mentined that you previously used booleans to achieve this effect. With the 'use background' shader, maya renders transparency in the alpha wherever the 'use background object' is visible to the camera. So basically, if we have a series of 5x5 pillars being revealed by a 'use background' shaded box, the pillars that are occluded by the box just get cut off as the box renders - we see no perspective down the top of the pillar line (as we try to look above the front pillars to the next line of pillars, for example) because nothing is rendered behind this 'use background' box. If we were to combine all the pillars and then apply a difference boolean to them with this box instead (animated, of course, to reveal the pillars from bottom to top), the pillars will appear to grow. The will have caps on the top, and we will see down the entire line of pillars in perspective as they grow. If this were a more complicated object, lke a car for example, it might give the illusion that the car was being teleported in, from bottom to top, and that each component of the car was solid (akin to the way the skull of Lilu is built in The Fifth Element, if you've seen it). Regardless of what method you use with the 'use background' shader, the fact reamins that once the camera sees an object with this shader, it simply renders the alpha as transparent and nothing will appear there (but I really like the idea of using an animated ramp with 'use background' in it - nice idea Steve). I'm not sure how you would achieve this without booleans, but booleans get painful pretty quickly and cause a lot of trouble with complicated objects. Back to the car example, a car model might be way to complex to combine the car and do one box boolean for the whole car, but you might be able to make one box boolean for each individual object in the car and then animate/parent/constrain them all together so that the boolean transitions for each object appear to happen on one plane. You might also be able to achieve this with some complicated form of a lattice (but I wouldn't want to be the person to try it).

Hope I was on track with that. Interesting problem. Good luck.

-Ben


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