Beyond Grading

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Blender Compositing and Post Processing

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Learn the techniques to create believable and stunning visuals with Blender Compositor with this book and ebook

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by Mythravarun Vepakomma | January 2014 | Open Source

This article written by Mythravarun Vepakomma, the author of the book Blender Compositing and Post Processing, deals with advanced compositing beyond grading. These techniques emphasize alternate methods in Blender Compositing for some specific 3D render requirements that can save lots of render time, thereby also saving budgets, in making a CG film. Following is the list of contents that will be presented in this article:

  • Relighting
  • Fresnel effect in compositing

(for more resources related to this topic, see here.)

Kudos

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

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.

Summary

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:


Further resources on this subject:


Blender Compositing and Post Processing Learn the techniques to create believable and stunning visuals with Blender Compositor with this book and ebook
Published: January 2014
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About the Author :


Mythravarun Vepakomma

Mythravarun Vepakomma was born in Hyderabad, India, in 1983 and is currently working as a CG Supervisor at Xentrix Studios Pvt Ltd, India. Though he graduated in Electrical and Electronics Engineering in 2004, he has always had a great passion for comics and cartoons. During his studies, his passion got him attracted to web designing and 3D animation.

Mythravarun always believed in transforming his passion into a career. He decided to go for it and started learning 3D graphics and web designing on his own. He also started working as a part-time illustrator and graphic designer. After consistent efforts, he finally moved into the field of 3D animation in 2005 to chase his dream of making it his career.

He has a decade of experience in several TV series, theme park ride films, and features. He now deals with creating and setting up CG lighting and compositing pipelines, providing a creative direction for CG Projects, research and development on several render engines to create a stable future for the studio, and many more things.

Midway through his career, Mythravarun encountered Blender and was fascinated by its features and the fact that it was an open source software. This made him dig deeper into Blender to get a better understanding. Now he prefers to use Blender for many of his illustrations.

As a hobby and secondary interest, he composes music and writes blogs on social awareness. His online presence can be found at the following links:

                                                 www.youtube.com/thevroad
                                                 www.youtube.com/joyasysnthesis
                                                 http://joyasynthesis.blogspot.in

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