![]() Schematic of OpenGL tessellation Tessellation Primitive Modes This setup is shown in the schematic of Figure 8.1.įigure 8.1. This is stored in the gl_TessCoord input variable. When the tessellation engine is generating lines or triangles, those coordinates are simply a pair of normalized values indicating the relative position of the vertex. When the tessellator is generating triangles, those coordinates are barycentric coordinates. The only input to the tessellation evaluation shader generated by OpenGL is a set of coordinates indicating where in the patch the vertex lies. When the fixed-function tessellator runs, it generates a new set of vertices spaced across the patch as determined by the tessellation factors and the tessellation mode, which is determined using a layout declaration in the tessellation evaluation shader. If some of the data is common to all output vertices (such as the color of the patch), then that data may be marked as per patch. Besides the tessellation factors, the output of a tessellation control shader is a new patch (i.e., a new collection of vertices) that is passed to the tessellation evaluation shader after the patch has been tessellated by the tessellation engine. The tessellation factors are sent on to the fixed-function tessellation engine, which uses them to determine the way that it will break up the patch into smaller primitives. The position and other attributes for each output control point.The per-patch inner and outer tessellation factors.The tessellation control shader is responsible for generating three things: In the context of tessellation, the input vertices are often referred to as control points. The tessellation control shader operates on groups of up to 32 vertices 1 at a time, collectively known as a patch. When tessellation is active, incoming vertex data is first processed as normal by the vertex shader and then passed, in groups, to the tessellation control shader. ![]() ![]() Logically, these three stages fit between the vertex shader and the geometry shader stage. In OpenGL, tessellation is produced using three distinct stages of the pipeline - the tessellation control shader (TCS), the fixed-function tessellation engine, and the tessellation evaluation shader (TES). There are many uses for tessellation, but the most common application is to add geometric detail to otherwise lower fidelity meshes. TessellationĪs introduced in the section “Tessellation” in Chapter 3, tessellation is the process of breaking a large primitive referred to as a patch into many smaller primitives before rendering them. In this chapter, we’ll cover both tessellation and geometry shading, and investigate some of the OpenGL features that they unlock. Next, the geometry shader processes entire primitives (points, lines, and triangles) and runs once for each. First, the two tessellation shader stages and the fixed-function tessellator that they flank together process patches. The next few stages of the pipeline seem similar to vertex shaders at first, but can actually be considered primitive processing stages. A vertex shader runs once on each of the vertices you send OpenGL and produces one set of outputs for each. We’ve covered the vertex shader stage in some detail, including how its inputs are formed and where its outputs go. In the previous chapters, you’ve read about the OpenGL pipeline and have been at least briefly introduced to the functions of each of its stages. How to use geometry shaders to process whole primitives and create geometry on the fly.How to use tessellation to add geometric detail to your scenes. ![]()
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