Tips & Tricks Pipeline Techniques Level: High Date: 10th June 2008
Pipeline Techniques : Bend Normals We often talk about solutions within particular software, however there are times when it is easier to work in complimenting software, and speed up the production. This needs technical information across both or multiple softwares. One such case it the used of Render Passes. To those who are not familiar with render passes, just think of any rendered final image, in this final image you may not be happy with the shadow or the specular and will feel the need to re‐render the shot only because of that. For that reason, if we render the animation in passes, we can then composite these passes, with the intensity we choose, to get our final look. The common render passes used are: 1) Beauty Pass : The main render having the diffuse illumination and texture maps 2) Specular Pass 3) Reflection Pass 4) Shadow Pass 5) Depth Pass 6) Effects pass There are more passes as well, depending on the complexity and flexibility that is needed. Hence, another two option passes are also commonly used. 7) Lighting Pass 8) Ambient Occlusion Pass We have to remember here that compositing softwares have also developed a great deal and hence the interoperability of softwares with each other has also increased. For this exercise, we are going to concentrate on Ambient Occlusion.
P a g e | 1
Tips & Tricks Pipeline Techniques Level: High Date: 10th June 2008 Ambient Occlusion in its basic form is to scale down light from ambient areas, and functions more like a diffuse occlusion. However, using Ambient Occlusion we can achieve much more than just this. We are going to focus on one attribute of Ambient Occlusion i.e. Bend Normals Create a scene file of you own.
Create render passes. i.e at least a Diffuse Pass and an Ambient Occlusion Pass (with output Mode 2).
In the Parameters Set change the Output mode to 2 as we want bend normals.
P a g e | 2
Tips & Tricks Pipeline Techniques Level: High Date: 10th June 2008 Output Mode 0 : Ambient Occlusion Output Mode 1 : Diffuse, environment sample, Occlusion Output Mode 2 : bent normal in world space Output Mode 3 : bent normal in camera space Output Mode 4 : bent normal in object space As this is a pipeline, we will not go into individual Output Modes except what we need i.e bend normal Now the question why do we need bend normals ? Before answering that, we should know what normals are. In any 3D package, the 3D models render based on the surface normals. If the normals are facing camera, the object will render, if it is facing away from the camera, it will not (unless, backface rendering is on). By default the object normals are “outwards” and “perpendicular”. However, due to various modeling techniques used they may not be so. Hence, rotating the normals to suit shading needs (making the part/section of the object dull) is a common trick. In simple terms, the normals is how the object shows up based on the light’s position. But isn’t that inside of the 3D software ? why should one render this as a render pass ? Rendering a Bend Normal Pass will allow the compositor to tweak the rendered layer based on the light inside the compositing system. As we know more high end compositing have lights and each layer can have a “surface properties” like specular, shadows etc. In conventional terms these layer being rendered images are flat i.e 2D hence the light does not affect this in the way it would if these were real 3D.
That’s where Occlusion Pass with Bend Normals come into great use, as the image now holds the information of the original 3D object. Hence manipulation of the light inside the compositing software, changes the look (“shading”) of the rendered layer.
P a g e | 3
Tips & Tricks Pipeline Techniques Level: High Date: 10th June 2008
Notice the change in the shading across the images. A very watchfull eye will also notice “highlights” in the sphere and the floor, these “highlights” are not specular of the objects themselves, but created due to the light in the composite. This is very vital specifically when we need to match the shading of our Computer Graphics to real life footage.
Prem Moraes
Digitally signed by Prem Moraes DN: cn=Prem Moraes, o=Autodesk India Ltd., ou=M&E, email=prem.moraes@autodesk. com, c=IN Date: 2008.08.19 17:13:55 +05'30'
P a g e | 4