/* Draws all visible faces (faces pointing toward the viewer) in the
   specified object.  The object must have previously been transformed
   and projected, so that the ScreenVertexList array is filled in. */
#include "polygon.h"

void DrawVisibleFaces(struct Object * ObjectToXform)
{
   int i, j, NumFaces = ObjectToXform->NumFaces, NumVertices;
   int * VertNumsPtr;
   struct Face * FacePtr = ObjectToXform->FaceList;
   struct Point * ScreenPoints = ObjectToXform->ScreenVertexList;
   long v1,v2,w1,w2;
   struct Point Vertices[MAX_POLY_LENGTH];
   struct PointListHeader Polygon;

   /* Draw each visible face (polygon) of the object in turn */
   for (i=0; i<NumFaces; i++, FacePtr++) {
      NumVertices = FacePtr->NumVerts;
      /* Copy over the face's vertices from the vertex list */
      for (j=0, VertNumsPtr=FacePtr->VertNums; j<NumVertices; j++)
         Vertices[j] = ScreenPoints[*VertNumsPtr++];
      /* Draw only if outside face showing (if the normal to the
         polygon points toward the viewer; that is, has a positive
         Z component) */
      v1 = Vertices[1].X - Vertices[0].X;
      w1 = Vertices[NumVertices-1].X - Vertices[0].X;
      v2 = Vertices[1].Y - Vertices[0].Y;
      w2 = Vertices[NumVertices-1].Y - Vertices[0].Y;
      if ((v1*w2 - v2*w1) > 0) {
         /* It is facing the screen, so draw */
         /* Appropriately adjust the extent of the rectangle used to
            erase this page later */
         for (j=0; j<NumVertices; j++) {
            if (Vertices[j].X > EraseRect[NonDisplayedPage].Right)
               if (Vertices[j].X < SCREEN_WIDTH)
                  EraseRect[NonDisplayedPage].Right = Vertices[j].X;
               else EraseRect[NonDisplayedPage].Right = SCREEN_WIDTH;
            if (Vertices[j].Y > EraseRect[NonDisplayedPage].Bottom)
               if (Vertices[j].Y < SCREEN_HEIGHT)
                  EraseRect[NonDisplayedPage].Bottom = Vertices[j].Y;
               else EraseRect[NonDisplayedPage].Bottom=SCREEN_HEIGHT;
            if (Vertices[j].X < EraseRect[NonDisplayedPage].Left)
               if (Vertices[j].X > 0)
                  EraseRect[NonDisplayedPage].Left = Vertices[j].X;
               else EraseRect[NonDisplayedPage].Left = 0;
            if (Vertices[j].Y < EraseRect[NonDisplayedPage].Top)
               if (Vertices[j].Y > 0)
                  EraseRect[NonDisplayedPage].Top = Vertices[j].Y;
               else EraseRect[NonDisplayedPage].Top = 0;
         }
         /* Draw the polygon */
         DRAW_POLYGON(Vertices, NumVertices, FacePtr->Color, 0, 0);
      }
   }
}