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selection.cpp

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// Polygon selection functions by Alan Baylis 2003

// SelectPolygon() creates a ray from the current camera coordinates to a point half a unit into the screen that is coincident
// with the mouse coordinates passed in and checks for an intersection along the line of this ray with the array of 
// polygons passed in.
// If there is an intersection then the function returns the zero based index of the closest polygon intersected or -1
// if there was no intersection.
// The polygons passed in must have between 3 to 29 edges, be convex, non-degenerate and have their normals set. 
// MouseX and MouseY are relative to the top-left corner of the window.

#include "selection.h"
#include "camera.h"
#include "general.h"
#include "collision.h"

extern CAMERA* camera;
extern int currentCamera;

int SelectPolygon(HWND hWnd, POLYGON* Polygons, int numPolys, int MouseX, int MouseY)
{
    POLYGON tempPolygon;
    VECTOR CollisionPoint, p0, pN, a, b, c;
    int tempPolygonNumber;  // Returned index
    VECTOR tempVect;
    float Distance;
    VERTEX tempVertex;
    VECTOR WorldPos;
    float ShortestDistance = 10000000;  // Set to an initial distance further than all polygons
    bool flag = false;  // Flag indicating intersection status

    // Temporary variables for polygon tessellation
    int numNewPolygons = 0;
    POLYGON newPolygon[30];

    // Get the modelview and projection matrices 
    double WorldPosX, WorldPosY, WorldPosZ, MousePosX, MousePosY, MousePosZ;
    double ModelMatrix[16]; 
    glGetDoublev(GL_MODELVIEW_MATRIX, ModelMatrix);
    double ProjMatrix[16];
    glGetDoublev(GL_PROJECTION_MATRIX, ProjMatrix);

    // Get the current viewport
    int Viewport[4];
    glGetIntegerv(GL_VIEWPORT, Viewport);

    if (MouseX >= Viewport[0] && MouseX <= Viewport[2] && MouseY >= Viewport[1] && MouseY <= Viewport[3])
    {
        // Set the end point of ray in windows coordinates
        MousePosX = MouseX;
        MousePosY = Viewport[3] - MouseY; // invert mouse Y coordinate
        MousePosZ = 0.5; // near clip plane depth

        // Get unprojected end point
        gluUnProject
        (
            MousePosX, 
            MousePosY, 
            MousePosZ, 
            ModelMatrix, 
            ProjMatrix,
            Viewport,
            &WorldPosX,
            &WorldPosY,
            &WorldPosZ
        );
        WorldPos.x = WorldPosX;
        WorldPos.y = WorldPosY;
        WorldPos.z = WorldPosZ;

        // Check for line polygon collision with all polygons
        for (int outerloop = 0; outerloop < numPolys; outerloop++)
        {
            // Make a copy of the polygon to work with
            tempPolygon.Vertex[0] = Polygons[outerloop].Vertex[0];
            tempPolygon.Vertex[1] = Polygons[outerloop].Vertex[1];
            tempPolygon.Vertex[2] = Polygons[outerloop].Vertex[2];
            tempPolygon.numVertices = Polygons[outerloop].numVertices;
            // Get a point on the polygon
            p0 = tempPolygon.Vertex[0].coords;
            // Get the polygons normal
            pN = tempPolygon.Vertex[0].normal;
            // Find collision point on the plane
            CollisionPoint = line_plane_collision(&camera[currentCamera].Position, &WorldPos, &tempPolygon);
            // If the collision point is outside the frustum (indicating a collision behind the camera) then continue
            if (!CheckClipPlanes(camera[currentCamera], CollisionPoint)) // Function is in the general.cpp source file
                   continue;

            // Subdivide polygon into new polygons
            numNewPolygons = 0;
            for (int innerloop = 2; innerloop < tempPolygon.numVertices; innerloop++)
            {
                numNewPolygons++;
                newPolygon[innerloop - 2].Vertex[0] = tempPolygon.Vertex[0];
                newPolygon[innerloop - 2].Vertex[1] = tempPolygon.Vertex[innerloop - 1];
                newPolygon[innerloop - 2].Vertex[2] = tempPolygon.Vertex[innerloop];
            }

            // If collision point is within the polygon
            for (int innerloop = 0; innerloop < numNewPolygons; innerloop++)
            {                
                a = newPolygon[innerloop].Vertex[0].coords;
                b = newPolygon[innerloop].Vertex[1].coords;
                c = newPolygon[innerloop].Vertex[2].coords;
                if (CheckPointInTriangle(CollisionPoint, a, b, c)) // Function is in the collision.cpp source file
                {
                    // Find distance to the collision point
                    tempVect = CollisionPoint - camera[currentCamera].Position;
                    Distance = tempVect.GetMagnitude();
                    // If this intersection is closest
                    if (Distance < ShortestDistance)
                    {
                        tempPolygonNumber = outerloop;
                        ShortestDistance = Distance;
                        flag = true;    
                    }
                }
            }                    
        }
        if (flag)
            return tempPolygonNumber;
        else
            return -1;
    }
}

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