(for more resources related to this topic, see here.)
As the final look of the frame is achieved during the compositing stage, there will always be numerous occasions where there is a requirement for more render passes to finalize the image. This results in extra 3D renders, along with more time and money. Also, few inevitable applications that give life to an image, such as lens effects (Defocus, Glares, and motions blur), are render intensive. Blender Compositor provides alternate procedures for these effects, without having to go back to 3D renders. A well planned CG pipeline can always provide sufficient data to be able to use these techniques during the compositing stage.
Relighting is a compositing technique that is used to add extra light information not existing in the received 3D render information. This process facilitates additional creative tweaks in compositing. Though this technique can only provide light without considering shadowing information, additional procedures can provide a convincing approach to this limitation.
The Normal node
Relighting in Blender can be performed using the Normal node. The following screenshot shows the relighting workflow to add a cool light from the right screen. The following illustration uses a Hue Saturation Value node to attain the fake light color. Alternatively, any grading nodes can be used for similar effect. The technique is to use the Dot output of the Normal node as the factor input for any grade node.
The following screenshot shows relighting with a cyan color light from the top using the Normal node:
The light direction can be modified by left-clicking and dragging on the diffused sphere thumbnail image provided on the node.
This fake lighting works great when used as secondary light highlights. However, as seen on the vertical brick in the preceding screenshot, light leaks can be encountered as shadowing is not considered. This can often spoil the fun. A quick fix for this is to use the Ambient Occlusion information to occlude the unwanted areas.
The following screenshot illustrates the workflow of using the Ambient Occlusion pass along with the normal pass to resolve the light leak issue. The technique is to multiply the dot output of the Normal node with Ambient Occlusion info from the rendered image using Mix or Math nodes. As it can be observed in the following screenshot, the blue light leaks on the inside parts of the vertical brick is minimized by the Ambient Occlusion information. This solution works as long as relighting is not the primary lighting for the scene.
Another issue that can be encountered while using the Normal node is negative values. These values will affect the nonlight areas, leading to an unwanted effect. The procedure to curb these unwanted values is to clamp them from the Dot output of the Normal node to zero, before using as a mask input to grade nodes.
The following screenshot illustrates the issue with negative values. All pixels that have an over-saturated orange color are a result of negative values.
The following screenshot shows the workflow to clamp the negative values from the dot information of a normal pass. A map value is connected between the grade node and Normal node, with the Use Minimum option on. This makes sure that only negative values are clamped to zero and all other values are unchanged.
The Fresnel effect
The Fresnel option available in shader parameters is used to modify the reflection intensity, based on the viewing angle, to simulate a metallic behavior. After 3D rendering, altering this property requires rerendering. Blender provides an alternate method to build and modify the Fresnel effect in compositing, using the Normal node.
The following screenshot illustrates the Fresnel workflow. In this procedure, the dot output of a Normal node is connected to the Map Range node and the To Min/ To Max values are tweaked to obtain a black-and-white mask map, as shown in the screenshot. A Math node is connected to this mask input to clamp information to the 0-1 range.
The 3D-combined render output is rebuilt using the diffuse, specular, and reflection passes from the 3D render. While rebuilding, the mask created using the Normal node should be applied as a mask to the factor input of the reflection Add node. This results in applying reflection only to the white areas of the mask, thereby exhibiting the Fresnel effect. A similar technique can be used to add edge highlights, using the mask as a factor input to the grade nodes.
This article dealt with advanced compositing techniques beyond grading. These techniques emphasize alternate methods in Blender Compositing for some specific 3D render requirements that can save lots of render times, thereby also saving budgets in making a CG film.
Resources for Article:
- Introduction to Blender 2.5: Color Grading [article]
- Blender Engine : Characters [article]
- Managing Blender Materials [article]