OpenGL® has no innate ability to display text in a 3D scene, leading to the creation of independent libraries. Three methods for creating OpenGL® text are texture mapping, geometry, and bitmaps, each with advantages and disadvantages. The easiest method is using bitmap text, while geometry allows for easy extrusion of 3D letters. Texture maps are effective for static text.
OpenGL® text describes any computer font that, through a number of different methods, appears as an element within an OpenGL® scene. There is no innate ability for OpenGL® to display text within a three-dimensional (3D) scene, and there are really no intrinsic ways to overlay system text within a framebuffer. This has led to the creation of several independent libraries to try and make working with OpenGL® text as easy as possible. There are three basic methods for creating OpenGL® text: texture mapping, geometry, and bitmaps. Each has advantages and disadvantages, with the biggest disadvantage for almost all OpenGL® text being a reduction in rendering speed.
There is no mechanism for displaying text in OpenGL®, mainly because the purpose of the library is to provide hardware-accelerated 3D graphics. Displaying characters or other text is not considered a necessary part of the OpenGL® function library. Instead, software and operating system developers have provided independent libraries in an effort to make it easier to use text within a 3D environment.
There are a few ways to use OpenGL® bitmap text, but it’s by far one of the easiest methods to implement. This process begins with a single bitmap image of the entire character set that will be used. When a word needs to be displayed on the screen, the individual letters are copied from the bitmap and arranged on an OpenGL® texture. This texture, which now contains a complete string, is then mapped to a quadrilateral and placed in front of the scene to overlap the rest of the 3D objects.
You can also use geometry to actually construct OpenGL® text. This involves using a font that contains two-dimensional (2D) vector information about how to draw each letter. These vectors are translated into vertices and triangles, making each letter its own pattern. To display a text string with geometric characters, the models just need to line up in the correct order in the scene. An advantage of this method is that text patterns can be easily extruded to form block-like 3D letters.
The last common method that can be used to display OpenGL® text is to use a simple pre-made texture map. This is an image where the desired text string is already drawn as it should appear within the scene. The image can then be applied to any object, showing the text when the object is rendered. Texture maps are mostly effective for static text, like words on the side of a 3D cardboard box model. It’s one of the easiest ways to display text with proper spacing and kerning.
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