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Table Of Contents

Introduction

Tessellation is a computer graphics technique that can make a coarse, low-polygon mesh render smooth. This is achieved through polygonal subdivision, which happens at render-time. Working with low-polygon meshes and letting Redshift do the subdivision during rendering has certain advantages:

  • Low-polygon meshes can be simpler to manage for animation reasons
  • The 3D program itself doesn't have to maintain large numbers of polygons which can be expensive in terms of system memory

It can be more memory-efficient (which is important for GPUs) when combined with view-dependent and/or adaptive subdivision. Small or distant objects, for example, can render smooth with fewer subdivisions.

Redshift subdivides quad polygons using the Catmull-Clark algorithm. For triangles, it uses the Loop algorithm. Redshift supports both screen-space and world-space adaptive tessellation for improved memory usage.
Displacement is a technique typically combined with tessellation. It allows the user to add extra detail on their meshes through shader networks, i.e. textures, noise shader nodes, etc.

The benefits of displacement include:

  • Manipulating textures and shader networks for certain displacement effects (such as a brick wall) is much easier than manipulating lots of vertices in a 3D program
  • Sculpting apps like ZBrush and Mudbox are easier to use compared to polygon modeling when creating organic geometry. A displacement (or vector displacement) map allows this sculpted detail to be applied on a fairly low-resolution mesh.

Because displacement can happen on adaptively tessellated meshes, it can be more memory efficient than using a full-detail, full-tessellation mesh in memory at all times and irrespective of its size or viewpoint.

Redshift supports both heightfield (displacing along the vertex normal) and vector displacement maps. The vector displacement maps can be in object or tangent space. Importantly, any displacement detail that couldn't be represented given the existing tessellation settings is represented, instead, using bump mapping – therefore a good level of surface detail can be present even in fairly low-quality tessellation settings.

How To Enable


 

Tessellation Settings

Subdivision Rule

Redshift supports two different algorithms for polygon subdivision: "Loop", which is used for triangles and "Catmull-Clark", which is used for quads. These algorithms are also called "Subdivision Rules".

The "CC+Loop" subdivision rule uses Catmull-Clark for quads and Loop for triangles. On the other hand, the "CC Only" option uses Catmull-Clark for triangles too, by first splitting each triangle into three quads. The "CC Only" mode should be used when Redshift is combined with other software that doesn't support Loop subdivision.

Screen Space Adaptive

Enabling screen-space adaptive tessellation means that objects that are further away from the camera will be subdivided less and will, therefore, use fewer polygons and less GPU memory. If this option is disabled, then subdivision becomes "world space adaptive". This option affects the unit used for the "minimum edge length" setting, as explained below.

Smooth Subdivision

This controls whether Redshift should subdivide quads and triangles using the Catmull-Clark and Loop algorithms respectively or whether it should do a simple linear subdivision instead. If you are adding displacement on simple angular meshes (such as walls or a box) and don't want them turned into smooth, curvy objects, disabling smooth subdivision might be the right option for you.


Smooth subdivision enabledSmooth subdivision disabled

Minimum Edge Length

Adaptive subdivision keeps dividing quads/triangles while their edges are longer than this setting. If you are using screen-space adaptive subdivision, this length is measured in screen pixels. If you are not using screen-space adaptive subdivision, this means "world space adaptive subdivision" so the length is measured in world-space units. The smaller this value, the more tessellation will be applied to the mesh. If you set the value to zero, tessellation will continue until "maximum subdivisions" (see below) has been reached.

The following pictures show a tessellated cube which becomes this spherical-like shape with smooth tessellation. Notice how there is more tessellation when the min edge length becomes smaller. This is showing screen-space adaptive subdivision so 8 means "8 pixels" while 2 means "2 pixels".

Screenspace adaptive with min edge length of 8.0

Screenspace adaptive with min edge length of 2.0

Maximum Subdivisions

Subdivision happens in 'passes'. Each pass can turn single quad/triangle into 4 quads/triangles respectively. This means that the number of polygons can grow extremely quickly with this option. It is a "power of four".

  • A setting of 1 can turn 1 quad into 4 quads
  • A setting of 2 can turn 1 quad into 16 quads
  • A setting of 3 can turn 1 quad into 64 quads
  • A setting of 4 can turn 1 quad into 256 quads
  • A setting of 5 can turn 1 quad into 1024 quads
  • A setting of 6 can turn 1 quad into 4096 quads
  • A setting of 7 can turn 1 quad into 16384 quads
  • A setting of 8 can turn 1 quad into 65536 quads

So a mesh containing only 1000 quads, using a "minimum edge length" of 0.0 and a "maximum subdivisions" 8, could become a 65 million quad mesh (warning) which could take a long time to generate and would consume lots of memory! For this reason, great care has to be applied when adjusting both "maximum subdivisions" and "minimum edge length".


Subdivision disabledOne LevelTwo LevelsThree Levels

Out-Of-Frustum Tessellation Factor

This option allows objects that outside the camera frustum (i.e. object that are not directly visible to the camera) to be tessellated to a lesser degree. The larger this value, the lesser the tessellation will be. This setting can help save a lot of memory by tessellating "unimportant" objects less. However, sometimes an object might be outside the camera frustum but still very clearly visible through reflections. Or it might be casting a well-defined shadow within the camera frustum. For these key objects, smaller numbers should be used. A setting of 0.0 disables this optimization.

Limit Out-Of-Frustum Tessellation / Max Out-Of-Frustum Subdivs

When "limit out-of-frustum tessellation" is enabled, you can specify the maximum number of subdivisions that should happen outside the camera frustum. This setting is useful when the "out of frustum tessellation factor" setting can still yield excessive tessellation. This condition can happen when the mesh is using large displacements. 

Displacement Settings

Maximum Displacement

This parameter tells Redshift what is the maximum length the displacement shaders/textures will be displacing the vertices by. For example, if you're adding two displacement textures in the shader graph and each displacement texture can push the vertices by 1 unit, then both of them can push the vertices by a maximum of 2 units, so a setting of 2.0 should be used for this setting. Unfortunately, due to flexible nature of shaders, it's not currently possible to compute this value automatically. Settings similar to this can be found on other renderers, too. They might be called "bounds padding" or "min/max bound".

If the value for this setting is set too low, you will see a 'ceiling' on your displacements, i.e. the maximum displacement will be clamped. If, on the other hand, this value is set too high, there won't be any visual artifacts but the performance could suffer. It is, therefore, advisable to use a value that is as low as possible before seeing any artifacts.

Displacement Scale

This scales the displacement results, which has the effect of accentuating or toning down the displacement. While it is possible to scale the displacement in the shader graph itself, this setting was added for the case where the same displacement shader is used on different meshes but different levels of displacement 'strength' are required among these meshes.

Below is a sphere displaced with a fractal shader.

Scale 0.05Scale 0.1

Scale 0.3

Enable Auto Bump Mapping

Very high-detail surface detail can require very high tessellation levels to be captured sufficiently, otherwise the result might look too soft. However, this can mean longer rendering times and higher memory usage!

The 'Enable Auto Bump Mapping' option effectively emulates what would happen if you were to tessellate your geometry to a sub-pixel level and modifies the surface normals accordingly, as if they were bump-mapped.

The following two spheres were rendered with exactly the same tessellation setitngs but the sphere on the right uses auto bumpmapping. Notice how it's able to capture more surface detail.

Without auto bump mappingWith auto bump mapping

How To Use Displacement

After configuring the tessellation settings, the displacement shader should be set. Please click here for more information.

UV Smoothing

When "Smooth subdivision" is enabled, Redshift will smooth not only the vertex positions but also the UV coordinates and tangent space vectors. Smoothing UV coordinates means that the UVs will be shifted to remove any 'zig-zagging' and 'UV breaks' during tessellation and maintain smooth UV-space curves. In the majority of cases, this is the desirable way to treat UVs. However, there are cases where strict UV layouts (such as with when UV tiles are aligned to quads) need to be preserved and not smoothed.

For this reason, Redshift supports enabling/disabling UV smoothing:
In XSI, Redshift uses the option in the Texture Editor window, menu File -> UV Properties -> Smooth when subdividing
In Maya, Redshift uses the "Smooth UVs" attribute which belongs to the shape's Smooth Mesh -> Extra Controls set of attributes.

Out Of Frustum Tessellation

The following images show what "out of frustum tessellation factor" does for polygons that are outside the camera frustum.

The test scene is simple: it's just a quad with a soft fractal displacement. The camera is looking at the quad (slightly tilted down) from a close position.

Scene setup


We use a wireframe shading node so visualize tessellation. In the images below, we froze tessellation in the RenderView and pulled the camera back so we could better see the effect of tessellation. As you can see, the polygons that are inside the camera frustum tessellated the most. The polygons that were away from the camera frustum tesselated less and depending on the "out-of-frustum tessellation factor".

Out-of-frustum tessellation factor 8

Out-of-frustum tessellation factor 16
Out-of-frustum tessellation factor 32

While "out-of-frustume tessellation factor"

Unsupported Features

Redshift doesn't currently support edge/vertex crease values in Softimage. They are supported in Maya.