Rendering : can max4.2/5 do ambient occlusion passes

[Equinoxx]

heya Arch baby . . . sup man

what exactly is an ambient occlusion pass ??

not really familliar with the term

[robert east]

ok for those of you who dont know about ambient occlusion ,

http://www-viz.tamu.edu/students/bmoyer/617/ambocc/


and to see it in action look here http://www.xsibase.com/forum/index.php?board=6;action=display;threadid=992

regards rob

[ssalo]

I might be a dumb, but it looks same as GI/skylight to me....A different term maybe?

[Aearon]

well this looks like a skylight blurred skylight with low sample rate to me ;)

you could do similar things with brazil, i don't know about rendertimes though

*edit* rendered with brazil in 12 secs

http://www.k-dump.net/files/cgtalk/skylight.jpg

oh and it should work with the built-in max 5 scanline renderer too

[gaggle]

As far as I can tell Brazil, and Skylight in R5, does their sampling thing by shooting rays into the scene from the camera's perspective. Or something to that effect.

It seems an Occlusion Shader works entirely different, it's more of a hack, but what you get is a "grayscale texture" of sorts, that defines how much a surface is lit. It seems to mean it's light-independent, you can take your Occlusion Shaded model of a statue, and light it like you want, and the areas that needs to be dark (armpits for instance) will get dark automatically. At least that's as far as I can tell.

That means that, yes, you can mock up something with Brazil that looks like the occlusion shaded images, or even looks better, but the point seems to be that occlusion shaders ends up being much more lightweight for production purposes: animation and high polycounts and such. Or something. I'm not neccessairly entirely clear on what exactly is going on yet :)

I don't think you can do this with MAX. Or, rather, not like they've done it with XSI. We don't have that kind of superp control over our shaders I don't think. It's likely doable by writing your own shader though, or plugin, or whatever is required.

I personally consider it incredibly cool that some guy read up on the occlusion shading theory, and then devised his own way of making that happen using the standard nodes available in XSI. Speaks volumes on the potential power available in XSI and Maya I think. But I digress.

[Rice]

You can acheive the ambient occlusion pass in max. If your running Brazil be sure to check out the render pass controls. A close, but not exact, method of doing this is to use the White Plaster Override Material with a white Skylight to achieve a deep shadow look. You can also create your own Material to acheive more accurate results with a custom light rig. The white plaster works pretty well though.

Then comp in whatever compositing program your using to increase or decrease this effect.

Cheers

[Rice]

Actually, let me rephrase one thing. You can cheat the ambient occlusion pass in max! It's not exact and it's not acheived in the same manner as in XSI. If I have time I'll post the differences between the two.

[ZeBoxx]

I could be mistaken, but...

As far as I know, Ambient Occlusion itself is not really different from skylighting.
Basically you take a look at the surrounding, and the 'average value' of that surrounding determines how much light hits the point where you were looking from.*

One difference may be that with some Ambient Occlusion settings, you only deal with the model itself - e.g. it doesn't take into account any nearby (or far away) other models.
Ambient Occlusion probably also only stores one thing : "Does light hit this point, and if so - by how much."
Whereas a full Skylight render also takes into account "Does the light get filtered by any transparent objects, and if so - how much. And what color/intensity is this light anyway - is the skylight a single color, or a gradient, or a HDR image ?" etc.

That said - Brazil r/s, and I'm assuming max5, can do this in a 'pass' just as softimage etc. do.

If you're talking about actually -storing- this information in either :
A. Vertices
B. Bitmap(s)

Then no, Brazil r/s doesn't do this currently. Max 5 lighttracer, I believe, does.

As a sidenote, this sort of thing is essentially not much (if at all) different from the various 'dirt' plugins that exist for 3ds max right now.

DirtyReyes - stores ambient occlusion information for a single model in a texture map texture map that is mapped to the object analytically (or actually the other way around).

QuickDirt - stores ambient occlusion information in vertex colors.

TrueGrit - stores ambient occlusion information in vertex colors, or a texture map based on existing UV coordinates.

* This could be in one of many ways.
For Brazil r/s' skylight, and most ambient occlusion calculation methods, this is by shooting out a number of rays into the surrounding in a hemispherical area around the surface normal. If the rays hit an object - that's a shadow. If the rays don't hit an object - that's light.
For realtime applications, hemicube renders can be generated instead, and the pixel values weighted in.
Some may simply render a hemispherical angular fisheye lens render of the surrounding, and average the values in the inscribed circle of the square resulting bitmap.

[robert east]

thankyou very much for these replys ... its good to see so many experts giving me thier views ...
it has been asummed in the office that by using a raytrace material and ajusting its falloff it creates a ambient occlusion pass. i rejected this idea. i shall look into using quick dirt for a pass. if anyone has any examples please let me know. i shall post up some results.

regards robert east

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