void polygon3d(BITMAP *bmp, int type, BITMAP *texture, int vc, V3D *vtx);
void polygon3d_f(BITMAP *bmp, int type, BITMAP *texture, int vc, V3D_f *vtx);
Draw 3d polygons onto the specified bitmap, using the specified rendering
mode. Unlike the regular polygon() function, these routines dont support
concave or self-intersecting shapes, and they cant draw onto mode-X
screen bitmaps (if you want to write 3d code in mode-X, draw onto a
memory bitmap and then blit to the screen). The width and height of the
texture bitmap must be powers of two, but can be different, eg. a 64x16
texture is fine, but a 17x3 one is not. The vertex count parameter (vc)
should be followed by an array containing the appropriate number of
pointers to vertex structures: polygon3d() uses the fixed point V3D
structure, while polygon3d_f() uses the floating point V3D_f structure.
These are defined as:
typedef struct V3D
fixed x, y, z; - position
fixed u, v; - texture map coordinates
int c; - color
typedef struct V3D_f
float x, y, z; - position
float u, v; - texture map coordinates
int c; - color
How the vertex data is used depends on the rendering mode:
The x and y values specify the position of the vertex in 2d screen
The z value is only required when doing perspective correct texture
mapping, and specifies the depth of the point in 3d world coordinates.
The u and v coordinates are only required when doing texture mapping,
and specify a point on the texture plane to be mapped on to this vertex.
The texture plane is an infinite plane with the texture bitmap tiled
across it. Each vertex in the polygon has a corresponding vertex on the
texture plane, and the image of the resulting polygon in the texture plane
will be mapped on to the polygon on the screen.
We refer to pixels in the texture plane as texels. Each texel is a block,
not just a point, and whole numbers for u and v refer to the top-left
corner of a texel. This has a few implications. If you want to draw a
rectangular polygon and map a texture sized 32x32 on to it, you would use
the texture coordinates (0,0), (0,32), (32,32) and (32,0), assuming the
vertices are specified in anticlockwise order. The texture will then be
mapped perfectly on to the polygon. However, note that when we set u=32,
the last column of texels seen on the screen is the one at u=31, and the
same goes for v. This is because the coordinates refer to the top-left
corner of the texels. In effect, texture coordinates at the right and
bottom on the texture plane are exclusive.
There is another interesting point here. If you have two polygons side
by side sharing two vertices (like the two parts of folded piece of
cardboard), and you want to map a texture across them seamlessly, the
values of u and v on the vertices at the join will be the same for both
polygons. For example, if they are both rectangular, one polygon may use
(0,0), (0,32), (32,32) and (32,0), and the other may use (32,0), (32,32),
(64,32), (64,0). This would create a seamless join.
Of course you can specify fractional numbers for u and v to indicate a
point part-way across a texel. In addition, since the texture plane is
infinite, you can specify larger values than the size of the texture.
This can be used to tile the texture several times across the polygon.
The c value specifies the vertex color, and is interpreted differently
by various rendering modes. Read the beginning of chapter "Polygon
rendering" for a list of rendering types you can use with this function.