Maya Rendering Guides

High Dynamic Range Images HDRI

High Dynamic Range Images (HDRI) can better represent the light value present in real life by recording the light intensity value for different exposure levels and thus provide for a high dynamic range of light stored in an image. Real-life light values are not limited by any range, such as the typical 0 to 1 range in a computer. For example, when we look into the sun, we must immediately look away because the light intensity is so great, while at a cave mouth, light will fall away into complete darkness. The ability to retrieve the contribution of light based on its intensity in real life and use it with CG opens the door for more realistically simulating real-world light. For example, HDR images are used to sample light from outdoor environments that are then applied as lighting values onto 3D vehicles, providing an almost hyper-real quality to the cars. This has become common practice for car commercials and a feature of many special-effects-intensive films.

Let's explore the IBL node and Final Gathering.

1. Open the IBL_Light01 scene from the CD and familiarize yourself with it. The objects represent different types of materials, from fully reflective to simple Lambert shaders. We will examine how HDRI with IBL can be used to light the scene.
2. Make sure that the default light is off, and create an IBL node by clicking the Create button in the Image Based Lighting tab in the Render Settings window. A wireframe sphere representing the IBL node appears in the viewport. If you do not see the sphere, you can scale it down; the scale is irrelevant because we are using Infinite mode.
3. Select the IBL node and open the Attribute Editor. In the Type attribute drop-down list, select Texture and then change the Texture attribute from black (default) to white. Then render, which sets the environment to white.

Figure 14.31 The Physical Sun And Sky simulator showing an early evening lighting condition

1. Notice that the white background reflects on reflective objects. Let's add lighting with final gathering. Enable Final Gathering with Point Interpolation of 20. (See the section "Final Gathering" earlier in this chapter for details.) Render a frame.
2. Notice that with final gather, all the colors come out and there is also a nice degree of shading, as seen in Figure 14.32. To increase the light intensity (irradiance), in the Image Based Lighting Attributes tab (for the mentalraylblShapel node) click the Color Gain color, increase the V (HSV) value to 2, and then render. This increases the light intensity and with HDR images acts as a means for controlling the exposure level.

Now let's look at how IBL, Final Gathering, and HDRI work together.

6. From the Type attribute drop-down list, select Image File, which enables mapping an image file to the IBL node. Using the Image Name drop-down list, load the image lat_long_ldr.tiff from the CD. If you zoom out in the viewport, you can see the image displayed (in shaded mode) on the IBL node; you can control this display with the Hardware Filter and Alpha attributes. Reset the Color Gain value to 1 and render. As Figure 14.33 shows, the chrome ball in particular now shows the image surrounding it.

The Mapping attribute (set to Spherical) provides two methods for mapping images onto the IBL node. The difference is in how images are wrapped around the IBL node. Both angular and spherical images can contain a 360° panoramic view, representing an environment. We used a latitude-longitude image (as with a globe), which is used with spherical mapping. An angular map looks like a reflective chrome ball and is mapped using the angular method. Both image formats are displayed in Figure 14.34.

Figure 14.33

Results of using a texture map for the Image Based Lighting environment

Figure 14.32

Image Based Lighting on three spheres

Figure 14.33

Results of using a texture map for the Image Based Lighting environment

Figure 14.34 Latitude-longitude vs. angular HDR images. Both types can be used with the IBL node and wrap around the scene, providing environmental light influence.

The image is visible in reflections and provides some lighting for the scene. Save a snapshot. We will now replace this image with its HDR version.

Load lat_long.hdr into the texture file and rerender. The dynamic range is visible on diffusive surfaces and reflective surfaces. Notice the differences in the sky and clouds on the reflective sphere. The image is dark, so increase the Color Gain value to 2 and render. Figure 14.35 shows the resulting image.

Replace the HDR image with ANGULAR.hdr and change Mapping to Angular. Render and see how the scene reacts to the new environment. Tumble around in the scene and notice how the angular image wraps around the IBL node. Compare this with the spherical mapping method in the viewports.

Figure 14.34 Latitude-longitude vs. angular HDR images. Both types can be used with the IBL node and wrap around the scene, providing environmental light influence.

Figure 14.35 IBL with a High Dynamic Range image as texture

jJL=Al

Figure 14.36 You use the Light Emission options to simulate Directional lights influenced by color and intensity captured from the image used with an IBL node.

0 0