OpenGL Insights

An Introduction to Tessellation

Shaders

Philip Rideout and Dirk Van Gelder

6.1 Introduction

Tessellation shaders open new doors for real-time graphics programming. GPU-

based tessellation was possible in the past only through trickery, relying on multiple

passes and misappropriation of existing shader units.

OpenGL 4.0 finally provides first-class support for GPU tessellation, but the new

shading stages can seem nonintuitive at first. This chapter explains the distinct roles

of those stages in the new pipeline and gives an overview of some common rendering

techniques that leverage them.

GPUs tend to be better at “ streamable” amplification; rather than storing an

entire post-subdivided mesh in memory, tessellation shaders allow vertex data to be

amplified on the fly, discarding the data when they reach the rasterizer. The system

never bothers to store a highly-refined ver tex buffer, which would have an impractical

memory footprint for a GPU.

Pretessellation graphics hardware was already quite good at rendering huge

meshes, and CPU-side refinement was often per fectly acceptable for static meshes.

So why move tessellation to the GPU?

The gains are obvious for animation. On a per-frame basis, only the control

points get sent to the GPU, greatly alleviating bandwidth requirements for high-

density surfaces.

Animation isn’t the only killer application of subdivision surfaces. Displacement

mapping allows for staggering geometric l evel-of-detail. Previous GPU techniques

87

6

88 IDiscovering

required multiple passes over the geometry shader, proving awkward and slow. Tes-

sellation shaders allow displacement mapping to occur in a single pass [Casta

˜

no 08].

Tessellation shaders can also compute geometric level-of-detail on the fly, which

we’ll explore later in the chapter . Previous techniques required the CPU to resubmit

new vertex buffers when changing the level-of-detail.

6.1.1 Subdivision Surfaces

One of the most compelling uses of GPU tessellation is efficiently rendering Catmull-

Clark subdivision surfaces. Most of these techniques use tessellation shaders to eval-

uate a parametric approximation of the limit surface rather than performing iterative

subdivision. Iterative subdivision can still be done on the GPU but is often better

suited for CUDA or OpenCL.

Pa rametric approximation of Catmull-Clark surfaces (ACC) arose from Charles

Loop’s research at Microsoft in 2008 [Loop and Schaefer 08], and was subsequently

enhanced to support creases [Kovacs et al. 09]. An excellent overview of the state of

the art can be found in [Ni et al. 09]. This includes a report from Valve, the first

major game developer to use tessellation shaders in this way.

6.1.2 Smoothing Polygonal Data

Catmull-Clark surfaces are not the only way to make good use of tessellation shaders;

game developers may find other surface definitions more attractive. For example,

tessellation can be used to simply “smooth out” traditional polygonal mesh data.

PN triangles are a popular example of this. An even simpler application is Phong

tessellation, the geometric analogue of a Phong lighting.

6.1.3 GPU Compute

OpenCL or CUDA can be used in conjunction with tessellation shaders for various

techniques. The compute API can b e used for simulation, e.g., hair physics, [Yuksel

and Tariq 10], or it can be used to perform a small number of iterative subdivisions

to “ clean up” the input mesh, removing extraordinary vertices before submitting the

data to the OpenGL pipeline [Loop 10].

6.1.4 Curves, Hair, and Grass

Tessellation shaders can also be applied to lines with isoline tessellation, which opens

up several possibilities for data amplification. One is tessellating a series of line seg-

ments into a smooth cubic curve. In this way, application code works only with a

small number of points. Smooth curves are generated entirely on the GPU, either for

3D applications like hair or rope or for 2D applications such as B´ezier curves from a

drafting tool. Isoline tessellation can also be used to generate multiple curves from a

single cur ve.

6. An Introduction to Tessellation Shaders 89

Figure 6.1. Hairy teapot; lines grown from patches.

Geometry shaders can be used in conjunction with isoline tessellation, which can

be useful for applications such as grass and hair. Figure 6.1 is a screenshot from the

accompanying sample code in which a surface is tessellated into many small polygons,

then extruded into hairs using a geometry shader.

6.1.5 Other Uses

There are also many less obvious uses for tessellation. If a post-tessellated mesh is

sufficiently refined, its geometry can be deformed to simulate lens distortion. These

effects include pincushion warping and panoramic projection. Because GPU raster-

izers can only perform linear interpolation, traditional techniques relying on post-

processing often result in poor sampling.

Figure 6.2 depicts an example of cylindrical warping using tessellation shaders

applied to a cubescape. The vertex buffer sent to the GPU is extremely light because

each cube face is a 4-vertex patch.

Figure 6.2. Cylindrical distortion using tessellation shaders.