hdri shadow casting

[Jonathan]

Im playing around with hdri at the moment and am using a custom made panorama (mirror ball) whilst following Christian Bloch's excellent 'HDRI Handbook' as a guide. Im having issues with shadows. Is it possible to use the environment map as a source of direct lighting (i.e. shadow casting)? I dont want to include any other lights but is this unavoidable.

 

One other issue_ how can a matte/shadow/reflection be made to work when exposure values/physical scale etc have allready been set in mental ray?

 

Thanks for reading, and hopefully posting!

[Stephen Thomas]

The quality of shadows depends entirely on the HDRI you are using. There are 2 factors that affect this. One is the dynamic range of the image, i.e. the difference in exposure values between the darkest and lightest parts of the image; and secondly the size of the brighest parts. Just as a large area light gives softer shadows than a point light source, so a large area of backlit clouds e.g. will give softer shadows than a small, sharp sun. Or in the case of interior HDRIs, a large window will give softer shadows than a ceiling downlighter.

 

I'm not sure if mental ray has an equivalent to the vray dome light, but I've always found that to be more effective at producing sharp shadows. The environment slot also tends to throw up weird artefacts when using HDRIs with a very big dynamic range.

 

Since I don't use mental ray, I can't comment on the last part of your question.

[Jonathan]

Thanks Stef. Yes my panorama has fairly diffuse and subtle lighting yet surely I should be able to acheive a similar level of shadow density as other (real) objects within the hdri? Im not sure about an equivilent to the VRay domelight, 3Dats 'Advanced to Expert' book suggests an Environment/Background switcher in MR which is what I have been using. Perhaps a linked gamma/gain material would help?

[Stephen Thomas]

In theory, yes. Assuming the whole dynamic range of the scene was captured accurately. The key to having contrasty shadows is there being a big difference between the sky and the sun intensity. Try loading your HDRI into HDRShop or similar and measure the relative intensities. If your sun is around 4 or something, then you can forget shadows. If it is around 30+ then you should be fine.

 

Of course the way around this is to supplement the HDRI with a direct light source. There is nothing wrong with this method, and indeed it gives you much more control. I would really only worry about getting shadows directly from the HDRI if it contained multiple light sources that would be hard to replicate in max.

[pixelperfectg]

Im having issues with shadows. Is it possible to use the environment map as a source of direct lighting (i.e. shadow casting)? I dont want to include any other lights but is this unavoidable.

If you're using FG then it's going to be very difficult to get directional shadows from a HDR/EXR file. You'll get better HDR/EXR shadow results with Importons/IP and/or using iRay.

 

Ex: http://mrmaterials.com/jeffs-blog/91-irradiance-particles-hdrs-and-you.html

 

I'm not sure if mental ray has an equivalent to the vray dome light

Unfortunately no, it does not.