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Topic: Reflections/refractions 101

posted by Flipside
archived on 17.1.2004

Ok, some clarification about reflections/refractions:

Reflection color: it replaces a number for the amount of reflection. White is fully reflective, black is non reflective at all. By using color instead of greyscale you get colored reflections (of course).

Fresnel: polygons facing the viewpoint will reflect less than polygons facing more away from the viewpoint. For example on a sphere, the edges will reflect more than the middle part. All materials actually have this property, so in theory you should always check this option. The ratio of amount reflection between the polygons facing away and polygons facing towards you, is controlled by the IOR. The lower you go, the less reflection there will be on the polygons facing towards you. So for glass, you should use 1.3-1.5 for IOR, but of course this also affects the look of the refractions! Metals for example are not transparant, so you can play with the ior as much as you like. They are however very reflective also on the polygons facing towards you, so you need to use high IOR values (15 for example), or you could just leave the fresnell unchecked, so that all polygons will reflect an equal amount.

Glossiness: the blurryness of the reflections. 1 is not blurry at all, 0 is very blurry.

The look (or the accuracy) of the blur is controlled by the subdivs. The higher the subdivs, the smoother the blur will be, but the longer the rendertimes! Usually you can use 3-8 subdivs (if you don't use the interpolation, see further down). An important factor here is also the AA settings. if you use good settings, the look of the blurred reflections will be better of course. So it's difficult to predict how your blur will look like if you test render with AA turned off. If you adjust the subdivs value while AA is turned off, you will most likely end up using way too high values in your final rendering when AA is turned on.

Interpolation is a way to speed up the calculation of glossies. It uses prepasses like with the irradiance map settings to calculate the glossies for a low resolution image first and then figueres out where it needs more 'attention' to further calculate more detail in the glossies. When using interpolation, you need to increase the subdivs drastically! Also the interpolation settings are very important here. The parameters are the same as for the irradiance map. Refer to vray.info or this forum for more info on these parameters.
Imo, the interpolation method isn't usefull in many cases, but sometimes you can benefit a lot from it. It depends on many factors like the shape of the surface, the amount of detail that is required etc... Sometimes you need to increase the interpolation settings so much, that you better don't use it and simply use low subdivs value. Especially for reflection glossies, I rarely use it.

Max depth is the number of times a ray bounces before reflection calculation stops. The old example is when you stand between two mirrors and you can see yourself an infinitive amount of times. You can limit this amount in vray by using the max depth value and this can speed up rendering.

The exit color is the color vray assigns to the ray when it reaches the max depth value.

All the same can be transposed to the refraction settings. The IOR of course controls the amount a ray is bent after it goes through a transparant surface.

The fog color is used to make thicker parts of a transparant object darker then thinner parts. The fog multiplier controls the amount of the effect, just play around with this, you'll notice the effect immediately. This usually requires low values (0.05 for example) otherwise your object will turn black.

Affect shadows will make your shadows colored, depending on your fog color and multiplier. Actually, these are fake caustics!!! This is very important to realize, because for this reason, when you turn on caustics for any object in your scene, the affect shadows option is neglected!!! Otherwise you would render double caustics (the real ones and the fake fog ones). Altough an option to not neglect the 'affect shadows' while using caustics would be great...

For more info about refraction glossies, refer to richard rosenmans excellent tutorial yo ucan find here:
www.richardrosenman.com/vray_absorption.htm

Mmmm that's kinda it I think. Also remember that glossy reflection replace the highlight setting you're used to in max materials. These are fake highlights. Also here, I would appreciate a fake highlight option for vray materials!

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