_GRAPHICS PROGRAMMING COLUMN_ by Michael Abrash [LISTING ONE] /* New header file entries related to texture-mapped polygons */ /* Draws the polygon described by the point list PointList with a bitmap texture mapped onto it */ #define DRAW_TEXTURED_POLYGON(PointList,NumPoints,TexVerts,TexMap) \ Polygon.Length = NumPoints; Polygon.PointPtr = PointList; \ DrawTexturedPolygon(&Polygon, TexVerts, TexMap); #define FIXED_TO_INT(FixedVal) ((int) (FixedVal >> 16)) #define ROUND_FIXED_TO_INT(FixedVal) \ ((int) ((FixedVal + DOUBLE_TO_FIXED(0.5)) >> 16)) /* Retrieves specified pixel from specified image bitmap of specified width. */ #define GET_IMAGE_PIXEL(TexMapBits, TexMapWidth, X, Y) \ TexMapBits[(Y * TexMapWidth) + X] /* Masks to mark shading types in Face structure */ #define NO_SHADING 0x0000 #define AMBIENT_SHADING 0x0001 #define DIFFUSE_SHADING 0x0002 #define TEXTURE_MAPPED_SHADING 0x0004 /* Describes a texture map */ typedef struct { int TexMapWidth; /* texture map width in bytes */ char *TexMapBits; /* pointer to texture bitmap */ } TextureMap; /* Structure describing one face of an object (one polygon) */ typedef struct { int * VertNums; /* pointer to list of indexes of this polygon's vertices in the object's vertex list. The first two indexes must select end and start points, respectively, of this polygon's unit normal vector. Second point should also be an active polygon vertex */ int NumVerts; /* # of verts in face, not including the initial vertex, which must be the end of a unit normal vector that starts at the second index in VertNums */ int ColorIndex; /* direct palette index; used only for non-shaded faces */ ModelColor FullColor; /* polygon's color */ int ShadingType; /* none, ambient, diffuse, texture mapped, etc. */ TextureMap * TexMap; /* pointer to bitmap for texture mapping, if any */ Point * TexVerts; /* pointer to list of this polygon's vertices, in TextureMap coordinates. Index n must map to index n + 1 in VertNums, (the + 1 is to skip over the unit normal endpoint in VertNums) */ } Face; extern void DrawTexturedPolygon(PointListHeader *, Point *, TextureMap *); [LISTING TWO] /* Draws a bitmap, mapped to a convex polygon (draws a texture-mapped polygon.) "Convex" means that every horizontal line drawn through the polygon at any point would cross exactly two active edges (neither horizontal lines nor zero-length edges count as active edges; both are acceptable anywhere in the polygon), and that the right & left edges never cross. Nonconvex polygons won't be drawn properly. Can't fail. */ #include #include #include "polygon.h" /* Describes the current location and stepping, in both the source and the destination, of an edge */ typedef struct { int Direction; /* through edge list; 1 for a right edge (forward through vertex list), -1 for a left edge (backward through vertex list) */ int RemainingScans; /* height left to scan out in dest */ int CurrentEnd; /* vertex # of end of current edge */ Fixedpoint SourceX; /* current X location in source for this edge */ Fixedpoint SourceY; /* current Y location in source for this edge */ Fixedpoint SourceStepX; /* X step in source for Y step in dest of 1 */ Fixedpoint SourceStepY; /* Y step in source for Y step in dest of 1 */ /* variables used for all-integer Bresenham's-type X stepping through the dest, needed for precise pixel placement to avoid gaps */ int DestX; /* current X location in dest for this edge */ int DestXIntStep; /* whole part of dest X step per scan-line Y step */ int DestXDirection; /* -1 or 1 to indicate way X steps (left/right) */ int DestXErrTerm; /* current error term for dest X stepping */ int DestXAdjUp; /* amount to add to error term per scan line move */ int DestXAdjDown; /* amount to subtract from error term when the error term turns over */ } EdgeScan; int StepEdge(EdgeScan *); int SetUpEdge(EdgeScan *, int); void ScanOutLine(EdgeScan *, EdgeScan *); int GetImagePixel(char *, int, int, int); /* Statics to save time that would otherwise pass them to subroutines. */ static int MaxVert, NumVerts, DestY; static Point * VertexPtr; static Point * TexVertsPtr; static char * TexMapBits; static int TexMapWidth; /* Draws a texture-mapped polygon, given a list of destination polygon vertices, a list of corresponding source texture polygon vertices, and a pointer to the source texture's descriptor. */ void DrawTexturedPolygon(PointListHeader * Polygon, Point * TexVerts, TextureMap * TexMap) { int MinY, MaxY, MinVert, i; EdgeScan LeftEdge, RightEdge; NumVerts = Polygon->Length; VertexPtr = Polygon->PointPtr; TexVertsPtr = TexVerts; TexMapBits = TexMap->TexMapBits; TexMapWidth = TexMap->TexMapWidth; /* Nothing to draw if less than 3 vertices */ if (NumVerts < 3) { return; } /* Scan through the destination polygon vertices and find the top of the left and right edges, taking advantage of our knowledge that vertices run in a clockwise direction (else this polygon wouldn't be visible due to backface removal) */ MinY = 32767; MaxY = -32768; for (i=0; i MaxY) { MaxY = VertexPtr[i].Y; MaxVert = i; } } /* Reject flat (0-pixel-high) polygons */ if (MinY >= MaxY) { return; } /* The destination Y coordinate is not edge specific; it applies to both edges, since we always step Y by 1 */ DestY = MinY; /* Set up to scan the initial left and right edges of the source and destination polygons. We always step the destination polygon edges by one in Y, so calculate the corresponding destination X step for each edge, and then the corresponding source image X and Y steps */ LeftEdge.Direction = -1; /* set up left edge first */ SetUpEdge(&LeftEdge, MinVert); RightEdge.Direction = 1; /* set up right edge */ SetUpEdge(&RightEdge, MinVert); /* Step down destination edges one scan line at a time. At each scan line, find the corresponding edge points in the source image. Scan between the edge points in the source, drawing the corresponding pixels across the current scan line in the destination polygon. (We know which way the left and right edges run through the vertex list because visible (non-backface-culled) polygons always have the vertices in clockwise order as seen from the viewpoint) */ for (;;) { /* Done if off bottom of clip rectangle */ if (DestY >= ClipMaxY) { return; } /* Draw only if inside Y bounds of clip rectangle */ if (DestY >= ClipMinY) { /* Draw the scan line between the two current edges */ ScanOutLine(&LeftEdge, &RightEdge); } /* Advance the source and destination polygon edges, ending if we've scanned all the way to the bottom of the polygon */ if (!StepEdge(&LeftEdge)) { break; } if (!StepEdge(&RightEdge)) { break; } DestY++; } } /* Steps an edge one scan line in the destination, and the corresponding distance in the source. If an edge runs out, starts a new edge if there is one. Returns 1 for success, or 0 if there are no more edges to scan. */ int StepEdge(EdgeScan * Edge) { /* Count off the scan line we stepped last time; if this edge is finished, try to start another one */ if (--Edge->RemainingScans == 0) { /* Set up the next edge; done if there is no next edge */ if (SetUpEdge(Edge, Edge->CurrentEnd) == 0) { return(0); /* no more edges; done drawing polygon */ } return(1); /* all set to draw the new edge */ } /* Step the current source edge */ Edge->SourceX += Edge->SourceStepX; Edge->SourceY += Edge->SourceStepY; /* Step dest X with Bresenham-style variables, to get precise dest pixel placement and avoid gaps */ Edge->DestX += Edge->DestXIntStep; /* whole pixel step */ /* Do error term stuff for fractional pixel X step handling */ if ((Edge->DestXErrTerm += Edge->DestXAdjUp) > 0) { Edge->DestX += Edge->DestXDirection; Edge->DestXErrTerm -= Edge->DestXAdjDown; } return(1); } /* Sets up an edge to be scanned; the edge starts at StartVert and proceeds in direction Edge->Direction through the vertex list. Edge->Direction must be set prior to call; -1 to scan a left edge (backward through the vertex list), 1 to scan a right edge (forward through the vertex list). Automatically skips over 0-height edges. Returns 1 for success, or 0 if there are no more edges to scan. */ int SetUpEdge(EdgeScan * Edge, int StartVert) { int NextVert, DestXWidth; Fixedpoint DestYHeight; for (;;) { /* Done if this edge starts at the bottom vertex */ if (StartVert == MaxVert) { return(0); } /* Advance to the next vertex, wrapping if we run off the start or end of the vertex list */ NextVert = StartVert + Edge->Direction; if (NextVert >= NumVerts) { NextVert = 0; } else if (NextVert < 0) { NextVert = NumVerts - 1; } /* Calculate the variables for this edge and done if this is not a zero-height edge */ if ((Edge->RemainingScans = VertexPtr[NextVert].Y - VertexPtr[StartVert].Y) != 0) { DestYHeight = INT_TO_FIXED(Edge->RemainingScans); Edge->CurrentEnd = NextVert; Edge->SourceX = INT_TO_FIXED(TexVertsPtr[StartVert].X); Edge->SourceY = INT_TO_FIXED(TexVertsPtr[StartVert].Y); Edge->SourceStepX = FixedDiv(INT_TO_FIXED(TexVertsPtr[NextVert].X) - Edge->SourceX, DestYHeight); Edge->SourceStepY = FixedDiv(INT_TO_FIXED(TexVertsPtr[NextVert].Y) - Edge->SourceY, DestYHeight); /* Set up Bresenham-style variables for dest X stepping */ Edge->DestX = VertexPtr[StartVert].X; if ((DestXWidth = (VertexPtr[NextVert].X - VertexPtr[StartVert].X)) < 0) { /* Set up for drawing right to left */ Edge->DestXDirection = -1; DestXWidth = -DestXWidth; Edge->DestXErrTerm = 1 - Edge->RemainingScans; Edge->DestXIntStep = -(DestXWidth / Edge->RemainingScans); } else { /* Set up for drawing left to right */ Edge->DestXDirection = 1; Edge->DestXErrTerm = 0; Edge->DestXIntStep = DestXWidth / Edge->RemainingScans; } Edge->DestXAdjUp = DestXWidth % Edge->RemainingScans; Edge->DestXAdjDown = Edge->RemainingScans; return(1); /* success */ } StartVert = NextVert; /* keep looking for a non-0-height edge */ } } /* Texture-map-draw the scan line between two edges. */ void ScanOutLine(EdgeScan * LeftEdge, EdgeScan * RightEdge) { Fixedpoint SourceX = LeftEdge->SourceX; Fixedpoint SourceY = LeftEdge->SourceY; int DestX = LeftEdge->DestX; int DestXMax = RightEdge->DestX; Fixedpoint DestWidth; Fixedpoint SourceXStep, SourceYStep; /* Nothing to do if fully X clipped */ if ((DestXMax <= ClipMinX) || (DestX >= ClipMaxX)) { return; } if ((DestXMax - DestX) <= 0) { return; /* nothing to draw */ } /* Width of destination scan line, for scaling. Note: because this is an integer-based scaling, it can have a total error of as much as nearly one pixel. For more precise scaling, also maintain a fixed-point DestX in each edge, and use it for scaling. If this is done, it will also be necessary to nudge the source start coordinates to the right by an amount corresponding to the distance from the the real (fixed-point) DestX and the first pixel (at an integer X) to be drawn) */ DestWidth = INT_TO_FIXED(DestXMax - DestX); /* Calculate source steps that correspond to each dest X step (across the scan line) */ SourceXStep = FixedDiv(RightEdge->SourceX - SourceX, DestWidth); SourceYStep = FixedDiv(RightEdge->SourceY - SourceY, DestWidth); /* Clip right edge if necessary */ if (DestXMax > ClipMaxX) { DestXMax = ClipMaxX; } /* Clip left edge if necssary */ if (DestX < ClipMinX) { SourceX += SourceXStep * (ClipMinX - DestX); SourceY += SourceYStep * (ClipMinX - DestX); DestX = ClipMinX; } /* Scan across the destination scan line, updating the source image position accordingly */ for (; DestX