Now we come to the fun bit, lighting and rendering. HDRi lighting forms the basic environmental lighting setup for all of our scenes. Many of you will already use Peter Guthrie's excellent HDRi skies, available here, which are in my opinion the best skies available (but then I would say that) and no doubt you have followed Peter's instructions regarding their use for best results. I think, however, it is worth going over again here to illustrate some of the project specific settings and some of the immense variation we were able to achieve with only a single sky!
We start by loading a sky into a Vray HDRI map and renaming the map to correspond to the number of the sky. Make sure the mapping type is set to spherical. PG Skies, which we use exclusively, usually work with processing multipliers set to 1.0. Sometimes it can be useful to set the Overall multiplier to 0.1 and the Render multiplier to 10 giving exactly the same output but allowing the HDRI map to be visible in the material editor. You can of course adjust these values to increase or reduce the intensity of the sky. I usually use the Vray physical camera parameters to adjust light levels but adjusting the HDRI can be useful if for example you have a scene set up with lots of artificial interior lighting and you change the HDRI sky with a different light intensity, it is often quicker and easier to adjust the HDRI rather than all of the internal lights.
Set the Colour space Type to Inverse Gamma. The default value is 1.0. Lower values give greater contrast and higher values give a more washed out result. Most PG Skies work best with gamma set between 0.7 - 0.85 with the sweet spot often being around 0.75. More on Gamma settings below.
Next set up a Vray Dome Light and give it a name corresponding to the the HDRI. Set the intensity units to Image which is the default and make sure the multiplier is set to 1.0. The Multiplier can be used to adjust the intensity of the HDRI map but if using a Vray physical camera it is generally more accurate to leave this at 1.0.
Finally instance the HDRI map into the texture slot of the Dome light and lock to dome orientation.
Horizontal rotation of the sky i.e. where the sun is coming from is controlled in two ways; either in the horizontal rotation dialogue in the mapping section of the HDRI map which works in degrees or by rotating the dome light itself. I usually rotate the map rather than the dome light through habit. 0.0 rotation equates to East in the HDRI map as in the sun position is due East at 0.0 rotation rather than North as you may expect. Therefore when thinking of the rotation as points on a compass I usually think of a compass rotated by 90 degs clockwise. If using the rotate Dome Light method, remember to check 'lock to dome orientation' in the texture section of the dome light. In the example below we have drawn a simple triangle linked to the dome light pointing in the direction in which the sun is shining so as you rotate the dome light you have a quick visual reference. Apparently this is something which may be added to Vray dome lights in future releases.
PG Skies are of a sufficient resolution (particularly the new super hi-res releases such as the excellent new 1714) that we almost always keep the rendered sky in the final image. If however you wish to replace the sky with a back plate for crazy resolution images (10k plus) you will need an alpha chanel. Using an HDRI mapped onto a dome light will give you an alpha channel, provided you untick 'affect alpha' in the Dome Light options.
I tend to start by loading in several HDRI skies and assigning them to different dome lights each named occording to the sky number. I can then easily switch between skies using the Vray Light Lister and test multiple lighting setups quickly and easily.
Once all of the dome lights are set I like to start by isolating the building and the ground plane so that I can test different lighting setups quickly without having to wait for any complex geometry to upload or light cache to build. You can also do this with Vray RT if you have a decent graphics card although I generally prefer to do simple 1200px renders using the progressive image sampler and save them out in the frame buffer history.
In the example above a simple grey override material with a VrayEdgesTex is used to increase the speed of the render further as it is the sky and general lighting which is being tested. These are 1200 x 800px renders, progressive 50 passes approx which took about 30 seconds each.
As previously mentioned the gamma settings in the HDRI map can be adjusted to achieve differing results. Below are some examples of a moody sky 1853 which is one of my favorites on account of the broad range of results which can be obtained with different horizontal rotations.
Lower gamma values not only produce more contrast but also appear brighter which may require camera exposure adjustments. There also seems to be a correlation between lower gamma valuse and increased noise resulting in longer render times. I'm not sure what the technical reason is for this but I often find that if a scene is rendeing slowly at Gamma 0.7, increasing it to 0.75 can make a big difference in speeding it up.
If a higher gamma value is used but more contrast is required we often use a simple 'S curve' in the Vray Frame buffer. This is particularly necessary when using our standard colour mapping settings of gamma 2.2 with a burn value of 0.05.
It's always worth testing each sky you load in to a scene with different rotations and different gamma settings as you often find drastically different moods can be created with a single sky.
In the next tutorial we'll have a look at some of the artificial internal light settings and the excellent Vray lens effects.