An OpenGL quad is a 3D shape with four sides and four points, useful for surface mapping images without distortion. It is commonly used for 2D graphics in a 3D environment, but can have disadvantages such as being broken into triangles for efficiency reasons. Some programmers suggest avoiding its use entirely.
An OpenGL® quadrilateral, or quad, in computer programming and graphics is a three-dimensional (3D) shape, also called a polygon, that has four sides and four points. Each 3D point that represents a single corner of the OpenGL® quad is known as a vertex and is defined with three numbers relative to its coordinate position in virtual space. The vertices of a quad don’t have to follow any real guidelines, except that there must be four; otherwise the shape is known as a polygon, triangle or line depending on the actual number. Most digital images are actually rectangular, so a quad OpenGL® can be a useful shape because the image can be surface mapped without a great deal of distortion and without the need for complex texture mapping techniques. Using a quad OpenGL® has its challenges, however, and there are a number of programmers who suggest avoiding its use entirely.
One of the primary uses of an OpenGL® quad is to draw two-dimensional (2D) graphics within a 3D environment. This can be done to have a static user interface overlaid on a 3D scene or to emulate 2D graphics with OpenGL®. Although OpenGL® was designed to render complex 3D graphics, it is often used in 2D graphics applications due to abstract programming interface (API) optimizations and flexibility. A quad becomes very important in rendering 2D graphics, because it is a natural 3D representation of a digital image, which means that a quad can be scaled to exactly match the size of the image. Once the aspect ratio matches, the image can be texture mapped, or projected, onto the quad’s surface without any sort of distortion, wrap, or gutter gap.
One advantage of using an OpenGL® quad, according to some programmers, is that it’s easier to think about how the quads fit together, rather than visualizing how the triangles fit together. This is especially true for simple 3D shapes like a cube where each quad is a single side; with a triangle mesh, two triangles must be combined to make a quad that forms only one side. Quads can also be easily manipulated to create perspective or other effects within a tiled or grid-aligned environment.
There are some disadvantages to using a quad OpenGL®, mainly due to the algorithms used to rasterize or render an image within the viewer window. One of the most encountered problems is that the rendering or graphics card could, at any time, break the quad into two triangles for efficiency reasons. This can cause an otherwise smooth surface to suddenly have a visible distortion at an angle across the quad where the edges of the triangles meet.
Another more common problem than you sometimes think has to do with clipping an OpenGL® quad. When part of the quad is outside the viewing window, off screen, the renderer will crop the quad so that only the visible part is rendered. This means that a quad will be cut geometrically. If the quad was converted to two triangles before the crop occurred, each triangle becomes a quad, and each of those quads is converted to two triangles. This leads to a situation where a truncated quad suddenly consists of four triangles instead of a smooth quad.
The complexity and unpredictability of converting quads to triangles leads to undesirable results. These include distortions in textures, inaccuracies or artifacts related to vertex lighting, and sometimes missing polygonal surfaces. For these reasons, some programmers avoid using quads altogether.
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