Why do we have to do a dirt comp ?

[tct70]

In the past 6-9 months, I dont think ive made a single (non-glass/liquid) material that doesnt have a vray dirt in the diffuse. Much prefer the control you get over doing on a per-material basis, because then you can use a different bitmap in the dirty area or make some areas more subtle/spread out than others.

Would you mind elaborating on your process of adding dirt in the diffuse - I'm a little confused (although that's fairly standard).

Thanks

[joconnell]

It's literally putting a copy of the vray dirt map in the diffuse slot. The major difference between this and not using it is that the vray dirt map has controls to let you tweak the distance and speed of falloff between surfaces where as it's quite difficult to control GI in the same way - you do get an option of using GI bounce mutipliers but it's a single simple control as opposed to the finesse of vray dirt. Also rather than using black and white or simple colour swatches, cubicle seems to be using a normal version of his map in the clean slot and a darkened version in the dirt slow so not only does he have control of how fast or slow these light and shadow areas work together but also has a levels control to tweak how bright or dark certain areas of the same material are.

For some this might be an overcomplicated method of doing things but on the other hand having such fine control is great if there's one specific thing you want to tweak in an image - instead of having to changes lights and materials and re-render you only have to tweak a single material setting with more predictable results.

[tct70]

[quote=joconnell;360819]It's literally putting a copy of the vray dirt map in the diffuse slot.quote]

But the bitmap for the main material is in the diffuse slot - I must be missing something here!

[dlparisi]

Put the diffuse bitmap in the occluded/unoccluded color slot at the bottom of the vraydirt map. Make the occluded version a darker version or just keep it black depending on your needs.

[tct70]

Ahhhhhhhhhh............I see.

Thank you.

[Neilg]

cubicle seems to be using a normal version of his map in the clean slot and a darkened version in the dirt

I am for most, but you can set up multiple levels of a dirt too - have one setup where it stretches out 1.5m, but put another vray dirt in the occluded set to 250mm or something, in unoccluded of that have the darker version of your diffuse and a really deep mossy colour in the occluded and a decent/messy dirtmask image in the radius.

Can also use them as masks in mix materials, should you want to get normal/speculars involved, but i wouldnt reccomend doing that on more than one main object per view, it's a little slow

[dbuchhofer]

Fun stuff, i haven't gotten that deep into it myself. I tend to just use dirtmaps as an 'enhancer' of silly details like Reveals in exterior architectual renders, where, we ALL know you really wont see them from THIS far away, but the arch insists you should Basically lets the detail still come through in lower res, lower AA, or lower GI settings.. and with the radius cutout, it doesn't add TOO much to the render times even.

[dlparisi]

Put the diffuse bitmap in the occluded/unoccluded color slot at the bottom of the vraydirt map. Make the occluded version a darker version or just keep it black depending on your needs.

BTW: a quick way of setting it up is to keep the occluded color black and just instance the map into the both the occluded/unoccluded slots. Adjust the spinner for the occluded one to darken it up as needed.

[Morbid Angel]

I'd like to know more about exactly how VRay calculates the GI. Mind sharing?
If it's traced from the camera, how do you get the effect of first bounce objects that aren't visible or are out of frame?

I almost never use dirt but that's because I prefer painting in things like that in post. I usually make stills though.

A ray is fired from a camera, when it intersects a triangle its angled based on the triangles normal creates new direction and can take the ray off screen, or bounce to another object. These are called eye rays. They also subdivide into other catigories, direct rays and indirect rays. Shooting rays outside of camera does not mean that objects out side of the frame will not be calculated...welcome to raytracing. If you have 10000000000 million polygons offscreen but reflected in a plane, you will not be able to render it fast, or will run out of memory.
With vray, because of tracing rays from camera, there is no need for extra rays, like mental ray used to have for example.

[Shimakaze]

How is the light information calculated? How does v-ray know which object is being directly lit and which is being indirectly lit by bounced light?
Does it keep tracing in all directions until it hits a light, an object or nothing? Or does it calculate the initial light information based on the lights and then do the GI separately?
Also, can't a triangle be affected by bounced light from all directions? Wouldn't you have to trace in all directions and not just one based on the normal angle?

[vlado]

This is done in the same way in which a typical raytracer calculates, for example, reflections. Only for GI, the reflection direction is not uniquely determined by the viewing direction, but is randomly generated over the hemisphere at the intersected point.

Best regards,
Vlado

[Shimakaze]

Yea that's what I thought.
Thanks for the responses. Time to go back on topic

[Morbid Angel]

No you don't

Dirt it is an ambient occlusion pass - fake, GI illusion. It is one bounce QMC (DMC, brute force - whatever ). It is used mainly for a hardcore compositing needs. When you don't need or can't afford pure GI (dirt and ambient occlusion - it is not a GI, as GI uses more than one bounce).

Cheers

Paul actually occlusion is not a fake gi, and its not a 1 bounce brute force.
Amb occlusion is simply a mesure of ray distance between objects from closest to furtherest points of ray intersections. Which can be controlled with falloff.
It is not a 1 bounce brute force, because brute force (or any gi for that matter) can have a direction of emmition, which can affect everything like gi shadow direction. Amb occlusion, without modification has uniform falloff direction. Which may resemble the look of a uniformly lit dome light.
GI, actually is suppose to illuminate on the first bounce, not darken, which is what occlusion does. Thats why its not suppose to be used in interrios, on thinds like walls as vlado explained.
So you cannot mix the two
just a clarification :P

[werticus]

silly details like Reveals in exterior architectual renders, where, we ALL know you really wont see them from THIS far away, but the arch insists you should

ahahha yes! that is exactly one thing i use for dirt now. I just to extrude the 2cm reveal to 100cm+ to get them to show in the past :P

[sv]

GI, actually is suppose to illuminate on the first bounce, not darken, which is what occlusion does. Thats why its not suppose to be used in interrios, on thinds like walls as vlado explained.
So you cannot mix the two
just a clarification :P

that's why i never understood the mental ray tutorials that come with max, that teach you to combine final gather with ambient occlusion on the walls. purposefully darkening the corners, where actually the final gather result without AO was far more realistic. As vlado mentions in this thread white walls wouldn't be darkening in the corners which is what the AO does.

I guess it's just teaching you techniques though...........but i think it could be done in a much better way