29 The Power of Inheritance and Polymorphism

Inheritance and Polymorphism 1034line of sight. Its implementation uses the auxiliary private member functionsClearRow() etc.The algorithm for the ClearLineOfSight() function is the most complex inthe program. There are two easy cases; these occur when the two points are in thesame row or the same column. In such cases, it is sufficient to check each pointfrom start to end (or out to a specified maximum) making certain that the line ofsight is not blocked by a wall. Pseudo code for the ClearRow() function is:Dungeon::ClearRow(Pt p1, Pt p2, int max, Pt path[])delta = if p1 left of p2 then 1 else -1current point = p1for i < max docurrent point's x += delta;if(current point is not accessible) return failpath[i] = current point;if(current point equal p2) return successi++return failCases where the line is oblique are a bit more difficult. It is necessary to checkthe squares on the map (or screen) that would be traversed by the bestapproximation to a straight line between the points. There is a standard approach tosolving this problem; Figure 29.5 illustrates the principle.The squares shown by dotted lines represent the character grid of the map orscreen; they are centred on points defined by integer coordinates. The start pointand end point are defined by integer coordinates. The real line between the pointshas to be approximated by a sequence of segments joining points defined by integercoordinates. These points define which grid squares are involved. In Figure 29.5the squares traversed are highlighted by • marks.(11,5)(1,1)Figure 29.5A digitized line.