1. A shading circuit for shading a plurality of triangular polygons which constitute a solid model to be displayed on a screen defined by X, Y and Z coordinate axes, each orthogonal to the other, comprising: preprocessing means including means for receiving three vertex data, each containing the intensity and X, Y, and Z coordinates of a corresponding vertex of a corresponding one of the plurality of triangular polygons, and means for defining two points at which a selected one of a plurality of scan lines disposed substantially parallel to the X coordinate axis intersects a corresponding one of the polygons, obtaining the intensity and Z coordinate corresponding to each of said two points, and obtaining a unit change .DELTA.I/.DELTA.X of intensity with respect to each unit change of the X coordinate axis and a unit change .DELTA.Z/.DELTA.X of the Z coordinate axis with respect to each unit change of the X coordinate axis, based on the intensities and Z coordinates of said two points; and shading section, responsive to said received three vertex data and the obtained changes .DELTA.I/.DELTA.X and .DELTA.Z/.DELTA.X, for sequentially obtaining the intensity and Z coordinate corresponding to each of a plurality of points disposed on a selected one of the sides of a corresponding one of the polygons to define a starting intensity and Z coordinate for a corresponding one of said scan lines, and for sequentially obtaining the intensity and Z coordinate of each of a plurality of points disposed inside a corresponding one of the polygons on a corresponding one of the scan lines, based on the corresponding starting intensity and Z coordinate thereof and the obtained changes .DELTA.I/.DELTA.X and .DELTA.Z/.DELTA.X.