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Tessellation

Tessellation is the process of breaking a high-order primitive (which is known as a patch in OpenGL) into many smaller, simpler primitives such as triangles for rendering. OpenGL includes a fixed-function, configurable tessellation engine that is able to break up quadrilaterals, triangles, and lines into a potentially large number of smaller points, lines, or triangles that can be directly consumed by the normal rasterization hardware further down the pipeline. Logically, the tessellation phase sits directly after the vertex shading stage in the OpenGL pipeline and is made up of three parts: the tessellation control shader, the fixed-function tessellation engine, and the tessellation evaluation shader.

Tessellation Control Shaders

The first of the three tessellation phases is the tessellation control shader (TCS; sometimes known as simply the control shader). This shader takes its input from the vertex shader and is primarily responsible for two things: the determination of the level of tessellation that will be sent to the tessellation engine, and the generation of data that will be sent to the tessellation evaluation shader that is run after tessellation has occurred.

Tessellation in OpenGL works by breaking down high-order surfaces known as patches into points, lines, or triangles. Each patch is formed from a number of control points. The number of control points per patch is configurable and set by calling glPatchParameteri() with pname set to GL_PATCH_VERTICES and value set to the number of control points that will be used to construct each patch. The prototype of glPatchParameteri() is

void glPatchParameteri(GLenum pname,
                       GLint value);

By default, the number of control points per patch is three. Thus, if this is what you want (as in our example application), you don’t need to call it at all. The maximum number of control points that can be used to form a single patch is implementation defined, but is guaranteed to be at least 32.

When tessellation is active, the vertex shader runs once per control point, while the tessellation control shader runs in batches on groups of control points where the size of each batch is the same as the number of vertices per patch. That is, vertices are used as control points and the result of the vertex shader is passed in batches to the tessellation control shader as its input. The number of control points per patch can be changed such that the number of control points that is output by the tessellation control shader can differ from the number of control points that it consumes. The number of control points produced by the control shader is set using an output layout qualifier in the control shader’s source code. Such a layout qualifier looks like this:

layout (vertices = N) out;

Here, N is the number of control points per patch. The control shader is responsible for calculating the values of the output control points and for setting the tessellation factors for the resulting patch that will be sent to the fixed-function tessellation engine. The output tessellation factors are written to the gl_TessLevelInner and gl_TessLevelOuter built-in output variables, whereas any other data that is passed down the pipeline is written to user-defined output variables (those declared using the out keyword, or the special built-in gl_out array) as normal.

Listing 3.7 shows a simple tessellation control shader. It sets the number of output control points to three (the same as the default number of input control points) using the layout (vertices = 3) out; layout qualifier, copies its input to its output (using the built-in variables gl_in and gl_out), and sets the inner and outer tessellation level to 5. Higher numbers would produce a more densely tessellated output, and lower numbers would yield a more coarsely tessellated output. Setting the tessellation factor to 0 will cause the whole patch to be thrown away.

The built-in input variable gl_InvocationID is used as an index into the gl_in and gl_out arrays. This variable contains the zero-based index of the control point within the patch being processed by the current invocation of the tessellation control shader.

Listing 3.7: Our first tessellation control shader

#version 450 core

layout (vertices = 3) out;

void main(void)
{
    // Only if I am invocation 0 ...
    if (gl_InvocationID == 0)
    {
        gl_TessLevelInner[0] = 5.0;
        gl_TessLevelOuter[0] = 5.0;
        gl_TessLevelOuter[1] = 5.0;
        gl_TessLevelOuter[2] = 5.0;
    }
    // Everybody copies their input to their output
    gl_out[gl_InvocationID].gl_Position =
        gl_in[gl_InvocationID].gl_Position;
}

The Tessellation Engine

The tessellation engine is a fixed-function part of the OpenGL pipeline that takes high-order surfaces represented as patches and breaks them down into simpler primitives such as points, lines, or triangles. Before the tessellation engine receives a patch, the tessellation control shader processes the incoming control points and sets tessellation factors that are used to break down the patch. After the tessellation engine produces the output primitives, the vertices representing them are picked up by the tessellation evaluation shader. The tessellation engine is responsible for producing the parameters that are fed to the invocations of the tessellation evaluation shader, which it then uses to transform the resulting primitives and get them ready for rasterization.

Tessellation Evaluation Shaders

Once the fixed-function tessellation engine has run, it produces a number of output vertices representing the primitives it has generated. These are passed to the tessellation evaluation shader. The tessellation evaluation shader (TES; also called simply the evaluation shader) runs an invocation for each vertex produced by the tessellator. When the tessellation levels are high, the tessellation evaluation shader could run an extremely large number of times. For this reason, you should be careful with complex evaluation shaders and high tessellation levels.

Listing 3.8 shows a tessellation evaluation shader that accepts input vertices produced by the tessellator as a result of running the control shader shown in Listing 3.7. At the beginning of the shader is a layout qualifier that sets the tessellation mode. In this case, we selected the mode to be triangles. Other qualifiers, equal_spacing and cw, indicate that new vertices should be generated equally spaced along the tessellated polygon edges and that a clockwise vertex winding order should be used for the generated triangles. We will cover the other possible choices in the “Tessellation” section in Chapter 8.

The remainder of the shader assigns a value to gl_Position just like a vertex shader does. It calculates this using the contents of two more built-in variables. The first is gl_TessCoord, which is the barycentric coordinate of the vertex generated by the tessellator. The second is the gl_Position member of the gl_in[] array of structures. This matches the gl_out structure written to in the tessellation control shader given in Listing 3.7. This shader essentially implements pass-through tessellation. That is, the tessellated output patch is exactly the same shape as the original, incoming triangular patch.

Listing 3.8: Our first tessellation evaluation shader

#version 450 core

layout (triangles, equal_spacing, cw) in;

void main(void)
{
    gl_Position = (gl_TessCoord.x * gl_in[0].gl_Position +
                   gl_TessCoord.y * gl_in[1].gl_Position +
                   gl_TessCoord.z * gl_in[2].gl_Position);
}

To see the results of the tessellator, we need to tell OpenGL to draw only the outlines of the resulting triangles. To do this, we call glPolygonMode(), whose prototype is

void glPolygonMode(GLenum face,
                   GLenum mode);

The face parameter specifies which type of polygons we want to affect. Because we want to affect everything, we set it to GL_FRONT_AND_BACK. The other modes will be explained shortly. mode says how we want our polygons to be rendered. As we want to render in wireframe mode (i.e., lines), we set this to GL_LINE. The result of rendering our one triangle example with tessellation enabled and the two shaders of Listing 3.7 and Listing 3.8 is shown in Figure 3.1.

Figure 3.1

Figure 3.1: Our first tessellated triangle

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