It is possible to define a patch using the “GL_PATCHES” primitive. As mentioned before, this is a sorta of geometric compression: the amount of vertices sent through the pipeline is fewer than the amount of vertices of the geometry which will be displayed. It is important to notice that the number of vertices provided for the patch can be considerably lower than the number of vertices of the final geometry. It can be thought as a set of control vertices constraining the final geometry. A patch contains just the vertices sufficient to define all the other vertices of the geometry going to be tessellated. The tessellation process is applied to the new “patch” primitive (added in OpenGL Version 4.0). Surely you can find more exhaustive explanation in the OpenGL Red Book or in the OpenGL Tessellation Wiki, but the aim of this post is just to give an insight of how tessellation shaders work. The following lines will briefly describe tessellation shaders. a shader can be very easily imported in several applications.the OpenGL application handles the curves at higher abstraction level.it is a kind of geometric compression, because just the curve control points are sent to the GPU, instead of a high resolution poly-line.This can be effective for several reason: It allows the developer to create a set of “ custom graphics primitives“, which requires as input only mathematical parameters, and let the GPU do the rasterization, like a normal OpenGL primitive. TES is mandatory, while TCS can be omitted.įrom a developer point of view, the development of a tessellator shader for a particular curve could be very effective. This stage is only executed if a TES is active in the current program or program pipeline. Tessellator instead, is a fixed function stage, responsible for creating a set of new primitives from the input patch. Tessellation Control Shader (TCS) and Tessellation Evaluation Shader (TES) can be programmed and customized. Shaders stage is between Vertex Shader and Geometry Shader. The blue boxes are programmable shader stages. Tessellationĭiagram of the Rendering Pipeline. The picture on the right describes the OpenGL 4 Rendering Pipeline. In the following lines I will describe my technique based on tessellation shaders. Further, they also provide geometry compression. Using such a high level primitive also simplify the software development process, preventing the coding of ad-hoc data structures to handle geometry data. Potentially, you can build your own library of primitives. Moreover, using tesellation shaders, it is possible to raise the abstraction level of the OpenGL graphics primitives. Tessellation Control Shaders and Tessellation Evaluation Shaders, introduced in the OpenGL Core Functionalities since version 4.0, really fit well for curve rasterization tasks. They offer a lot of functionalities, but their use does not fit properly to Curve Rasterization, even if they can be used also for this task. Then, with OpenGL 3, Geometry Shaders arrived. In the earlier versions of OpenGL, built-in tessellators and evaluators unburdened the developer’s efforts to sample curve points properly but, although they were easy to use, they were not much flexible. Uniform Sampling is the most used and straightforward technique, but obviously more effective techniques exist. define the resolution of the sampling and the curvilinear abscissa of the points This technique requires to sample a set of points of the curve, which is a two steps process: If you want to draw a curve using OpenGL, basically you have to draw a poly-line, or more properly, a polygonal chain. Moreover, it seems that future releases of OpenGL APIs will not enrich the set of primitives, since Khronos group strives to keep OpenGL API as minimal as possible: as a matter of fact, primitives like quads and polygons have been removed from the modern core profile. Rasterization of Parametric Curves using Tessellation Shaders in GLSLĭrawing parametric curves is a common issue for OpenGL developers, because there is no graphics primitive which can directly rasterize them.
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