Class NVPathRendering


  • public class NVPathRendering
    extends java.lang.Object
    Native bindings to the NV_path_rendering extension.

    Conventional OpenGL supports rendering images (pixel rectangles and bitmaps) and simple geometric primitives (points, lines, polygons).

    This extension adds a new rendering paradigm, known as path rendering, for rendering filled and stroked paths. Path rendering is not novel but rather a standard part of most resolution-independent 2D rendering systems such as Flash, PDF, Silverlight, SVG, Java 2D, Office drawings, TrueType fonts, PostScript and its fonts, Quartz 2D, XML Paper Specification (XPS), and OpenVG. What is novel is the ability to mix path rendering with arbitrary OpenGL 3D rendering and imaging.

    With this extension, path rendering becomes a first-class rendering mode within the OpenGL graphics system that can be arbitrarily mixed with existing OpenGL rendering and can take advantage of OpenGL's existing mechanisms for texturing, programmability, and per-fragment operations.

    Unlike geometric primitive rendering, paths are specified on a 2D (non-projective) plane rather than in 3D (projective) space. Even though the path is defined in a 2D plane, every path can be transformed into 3D clip space allowing for 3D view frustum & user-defined clipping, depth offset, and depth testing in the same manner as geometric primitive rendering.

    Both geometric primitive rendering and path rendering support rasterization of edges defined by line segments; however, path rendering also allows path segments to be specified by Bezier (cubic or quadratic) curves or partial elliptical arcs. This allows path rendering to define truly curved primitive boundaries unlike the straight edges of line and polygon primitives. Whereas geometric primitive rendering requires convex polygons for well-defined rendering results, path rendering allows (and encourages!) concave and curved outlines to be specified. These paths are even allowed to self-intersect.

    When filling closed paths, the winding of paths (counterclockwise or clockwise) determines whether pixels are inside or outside of the path.

    Paths can also be stroked whereby, conceptually, a fixed-width "brush" is pulled along the path such that the brush remains orthogonal to the gradient of each path segment. Samples within the sweep of this brush are considered inside the stroke of the path.

    This extension supports path rendering through a sequence of three operations:

    1. Path specification is the process of creating and updating a path object consisting of a set of path commands and a corresponding set of 2D vertices.

      Path commands can be specified explicitly from path command and coordinate data, parsed from a string based on standard grammars for representing paths, or specified by a particular glyph of standard font representations. Also new paths can be specified by weighting one or more existing paths so long as all the weighted paths have consistent command sequences.

      Each path object contains zero or more subpaths specified by a sequence of line segments, partial elliptical arcs, and (cubic or quadratic) Bezier curve segments. Each path may contain multiple subpaths that can be closed (forming a contour) or open.

    2. Path stenciling is the process of updating the stencil buffer based on a path's coverage transformed into window space.

      Path stenciling can determine either the filled or stroked coverage of a path.

      The details of path stenciling are explained within the core of the specification.

      Stenciling a stroked path supports all the standard embellishments for path stroking such as end caps, join styles, miter limits, dashing, and dash caps. These stroking properties specified are parameters of path objects.

    3. Path covering is the process of emitting simple (convex & planar) geometry that (conservatively) "covers" the path's sample coverage in the stencil buffer. During path covering, stencil testing can be configured to discard fragments not within the actual coverage of the path as determined by prior path stenciling.

      Path covering can cover either the filled or stroked coverage of a path.

      The details of path covering are explained within the core of the specification.

    To render a path object into the color buffer, an application specifies a path object and then uses a two-step rendering process. First, the path object is stenciled whereby the path object's stroked or filled coverage is rasterized into the stencil buffer. Second, the path object is covered whereby conservative bounding geometry for the path is transformed and rasterized with stencil testing configured to test against the coverage information written to the stencil buffer in the first step so that only fragments covered by the path are written during this second step. Also during this second step written pixels typically have their stencil value reset (so there's no need for clearing the stencil buffer between rendering each path).