A stencil buffer is a per-pixel buffer used in computer graphics programming to achieve different effects or mask areas of a scene. It can be used to draw shadows, halos, and detect overlapping objects. Originally, it was a 2D mask applied to a 3D scene, but modern graphics cards can hold a full byte value at each pixel location. One popular use is creating volume shadows by rendering a dimly lit scene and transferring shapes to the stencil buffer.
In computer graphics programming, a stencil buffer is an area of memory that can be drawn and used in relation to other graphics buffers to achieve different effects or to mask areas of a scene so they are not processed or rendered. A stencil buffer is a per-pixel buffer similar to a black and white image, where each location in the buffer can hold a value that is up to 1 byte long but can be as small as 1 bit. The buffer was originally designed to act as a true or false mask, just like a real-world stencil, so areas of a scene could be blacked out or ignored by setting the values of different regions of the stencil buffer to a non-zero value . There are numerous uses for the buffer beyond just masking, including drawing a volume of shadow, implementing halos around objects, or locating areas where objects overlap each other. In many cases, the buffer is kept in graphics memory on a graphics card so that it can be accessed quickly when rendering.
The concept of a stencil buffer was originally a way to create a two-dimensional (2D) mask that could be applied to a three-dimensional (3D) scene as vertices moved through the graphics pipeline. If the 3D vertex position, once rasterized, was related to a stencil buffer area that was not set to zero, then that vertex could be ignored and the rest of the pipeline processing skipped. This allowed programmers to render only small parts of a scene, or to render an image in an irregularly shaped window, such as a reflection in a small mirror within a larger scene, where the mirror shape is drawn as a stencil buffer and then the reflection is drawn across that shape.
Memory issues led to the original form of a stencil buffer which was a grid of single-bit values where each pixel in the buffer could only be true or false. As graphics cards developed, most buffers became capable of holding a full byte value at each pixel location, allowing programmers to have more complex values in one location. This can allow the buffer to be used to detect areas where objects overlap, which can be done by increasing the buffer value to one location if an object occupies that pixel on the screen when rasterized. After this, the value of each pixel position in the buffer will be equal to the number of objects that pass through that pixel on a screen.
One of the most popular uses for a stencil buffer is creating volume shadows. This can be done by first rendering a scene in very dim lighting so that everything appears to be in shadow. Next, calculations are made to determine the position of all polygonal surfaces in the scene lit by a light source, and these shapes are transferred to the stencil buffer. The scene is again rendered as if each object is fully lit and blends through the stencil shapes onto the shadowed scene, effectively creating an image that appears to contain objects with shadows.
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