This tutorial covers how to set up an HDRI image in Maya.
Download this HDRI testing scene file.
Download the HDRI you intend to use (see chapter below).
Set the menu set to rendering.
Set the shelf to rendering.
HDRIs let you light images using photographs. By using contrast information on an HDRI image, you can generate light based on it. Since the light would be from a photograph, renderings using HDRI images have a tendency to be more photo realistic.
When an HDRI is created, the photographer takes pictures at different exposures of non-moving objects. Then, using software (Example: Adobe Photoshop), merges them all together into a 32 bit image which contains contrast information. You can create HDRIs yourself; basically you'll need a modern camera, a chrome ball, and some software package to merge them all together. The chrome ball is used to capure a spherical image of the enviromnent. Making an HDRI won't be covered in this tutorial.
There are two types of HDRI images used for lighting a digital scene: spherical and angular. Angular is the type that would be produced when using a chrome ball. Spherical HDRIs are more like panoramic images. Spherical HDRIs work the best.
If you don't intend to make your own HDRIs, you can buy them or get them free online.
Lists of free sites:
Notes On Selecting Good HDRI Images
1. Look for HDRIs that have a direct light source.
2. The highest resolution HDRI might not always be the best. A lot of times it's just adding unnecessary render time. If you intend to use your HDRI as a background for your model or a large reflection, then resolution is important.
3. Purchased HDRIs have higher resolution and are better quality (not saying the free ones aren't any good).
4. Check the license on the HDRIs. A lot of the free ones are for non-commercial use. In other words, you can't use it to make profit.
5. Avoid HDRIs that have too much indirect light.
6. Avoid grainy HDRIs.
7. Chrome balls are the easiest HDRI to make, but they have less quality. So watch out for chrome balls that have specks of lint on them or dust.There's little you can do to clean this up. Also make sure the chrome ball was cropped correctly.
HDRIs In Maya
There are two ways of using an HDRI in Maya:
Using final gather and a sphere.
Using a mental ray IBL node.
Using Final Gather
Step one - Create a sphere. Scale it so it's larger than your scene.
Step two - Assign a surface shader material to it.
Step three - Click on the checkerboard next to the out color to open the create render node window and create a file. This will connect the output of the file node to the input of the out color attribute.
Step four - Beside image name, click on the folder icon and, when the file browser comes up, locate where you saved the HDRI image.
Mental Ray is needed to render HDRI images. Maya software and Maya hardware renders can't render them.
To load Mental Ray:
Step one - Go to window>settings/preferences>plug-in manager.
Step two - Roll down to the Mayatomr.mll; check loaded and auto load.
Step three - Go to window>render editors>render settings.
Step four - Set the "render using" to mental ray.
Now that we have mental ray loaded, we need to enable final gather.
Step one - On the common tab, roll down to render options and uncheck the "enable default light" option.
Step two - Go to the mental ray tab of the render settings.
Step three - Under secondary effects, check final gather.
Step four - Roll down to the final gather tag and display it.
Step five - The three most important attributes are accuracy and point density and scale. Increasing the accuracy will increase the render quality. Increasing the point density will increase the render quality, but can wash out the detail. Scale acts as a mulitplier for the final output from the final gather. A darker color will darken the rendering; a light tinted color will tint the rendering, etc.
Now for a test render.
Tweaking The Rendering
Reselect your sphere and open the render settings for the shader.
Step one - Click on the arrow beside the out color; this will take you to the file node with the HDRI image.
Step two - To change the contrast of the rendering, use the color gain attribute. If the rendering is underexposed, increase the value (V) to something like 2 or 3, etc.
Using The IBL Node
The IBL node in mental ray lets you generate directional light from contrast information on the HDRI.
Step one - Repeat steps one through four in the mental ray section in the above chapter.
Step two - Go to the mental ray tab and roll down to the enviromnent tag.
Step three - Click the create image based lighting button.
As you can see it creates a sphere.This is where the HDRI will be projected.
Step four - Select it and open up the attribute editor (window>attributes editor).
Step five - Once again, click on the folder next to image name and, when the file browser opens, look up where you saved the HDRI image.
If your HDRI is a chrome ball HDRI, set the mapping to angular. If it's a paneramic style HDRI, set the mapping to spherical.
Step six - Roll down to the light emision tag.
Step seven - Check emit light.
Step eight - The attributes quality U and V are the amount of directional light that will be created along the U and V coordinates of the image. For test rendering, the quality value of 150 each will work for most renderings. For the final rendering, depending on the HDRI resolution, I recommend a value of over 300.
Now you can render.
Step one - If the rendering seems to be overblown or underexposed, in the mental ray IBL shapes attributes, check adjust light emmision color effects.
Step two - Use the color gain attribute to adjust the exposure. If the rendering is underexposed, increase the value (V) to something like 2 or 3, etc.
Step three - To increase shadow quality, increase the ray depth limit.
Step four - The way the sampling system works is when mental ray is sampling the HDRI image, it won't sample all of it. It will try to sample the most important information. The sample attributes define how many times it will do this: more samples, better quality.
Low samples, according to the manual, should be set to 1\8th of the samples. Mental ray wil use this as the minimun sampling rate.
Final Gather Vs. IBL Node
In my personal experience I find that the IBL node is better suited for HDRI rendering. Final gather shadows are less realistic and there's less control. You can enable final gather when using an IBL node so you get the best of both worlds, butt he render time wil be noticably higher.
I hope this tutorial has taught you the fundamentals of using HDRIs in Maya.