计算机图形学D图形表示方式.pptVIP

Viewing In 3D The basic questions of 3D graphics 如何在二维平面上显示三维物体 表示问题 遮挡关系的反映 真实感问题 3D viewing process Conception Projections transform points in a coordinate system of dimension n into points in a coordinate system of dimension less than n. Projection Definition: The projection of a 3D object is defined by straight projection rays (called projectors) emanating from a center of projection, passing through each point of the object, and intersecting a projection plane to form the projection. Projections Projection’s Parameter : projection center projection plane The class of projections Perspective Projection The distance between the center of projection and the projection plane is finite. Parallel Projection The distance between the center of projection and the projection plane is infinite. The class of projections The subclasses of  Planar Geometric Projections. Three Orthographic Projections Three Orthographic Projections(三视图） 正平行投影（三视图） Front Elevation Top Elevation Side Elevation Perspective Projection Vanishing Point. The perspective projections of any set of parallel lines that are not parallel to the projection plane converge to a vanishing point. Axis Vanishing Point If the set of lines is parallel to one of the principal axes, the vanishing point is called an axis vanishing point. The style of Perspective Projection Perspective projections are categorized by their number of principal vanishing points and therefore by the number of axes the projection plane cuts The style of Perspective Projection One-point perspective projection Projection-plane cuts only one of three axes. Two-point perspective projection Projection-plane cuts only two of three axes at the same time. Three-point perspective projection Projection-plane cuts all of three axes at the same time. One-point perspective projection Two-point and Three-point Perspective Projection The