

 
threedkit(7) 
Svgalib User Manual 
threedkit(7) 
threedkit  a set of functions for 3D support.
The 3dkit consists mainly of the following triangle functions
gl_striangle(3), gl_swtriangle(3), gl_triangle(3),
gl_trigetcolorlookup(3), gl_trisetcolorlookup(3),
gl_trisetdrawpoint(3), gl_wtriangle(3).
Beware, these functions are not a direct part of the svgalib
library. Instead their source is part of svgalib and can be found in the
threeDkit/ subdirectory of the original svgalib distribution.
However, it is not installed in the system by default, s.t. it is unclear
where you can find it if your svgalib was installed by some linux
distribution.
In case of any such problem, simply get an svgalib distribution
from the net. You even don't need to install it. Just make in the
threeDkit/ subdirectory. As of this writing,
svgalib1.2.12.tar.gz is the latest version and can be retrieved by
ftp from sunsite.unc.edu at /pub/Linux/libs/graphics and
tsx11.mit.edu at /pub/linux/sources/libs which will most
probably be mirrored by a site close to you.
The functions are defined in the tri.o and triangl.o
files (or their resp. sources) which you must link to your program.
This is main engine for 3D rendering.
Program flow:
 1.
 The function called from outside of 3dkit.c is TD_drawsolid.
This first calculates the rotation matrix from the camera rotation angles
(see below for more details). It then allocates memory for the temporary
array for holding temporary coords in subsequently called functions. It
also sorts the surfaces from furthest to closest; according to the
distance of the centre gridpoint of each surface from the camera.
It also establishes whether ROTATE_OBJECT option is on
and zero's the camera position if so  this is for displaying the
object at the screen centre like in a 3D CAD package, as apposed to
virtual reality where the object can be anywhere and the actual camera
position can move.
In the case of ROTATE_OBJECT being on, although the
camera position is zero, some distance has to be placed between the
camera and the object (or else it would appear to be infinitely large on
the screen). This is done using the variable s_cam which is
initialized to distance which is set by the calling application.
It then loops through each surface (ordering them in the way they were
just sorted  i.e. according to sortarray indexing) and calls
one of five graphic routines to write the 3D surface to the
hardware.
 2.
 Assume that TD_drawsolid then calls TD_drawmesh. Here, each
surface grid point is first TD_translate'd into a 2D screen point
and stored in the temp array. There are obviously w(idth)*h(eight)
points in the grid.
Following, each line from the 2D temp array is drawn on the
screen. To draw the surface, the corner wishbone (two lines) from each
grid square is drawn while advancing across and the down. After
completing the scan, the furthest two edges of the surface must then be
filled in, vis.:
_ _ _ _ _ _
______
______
______
______
______
     
To understand the object rotation, a knowledge of matrix
multiplication is required. I once derived a camera rotation before I
learned matrix computation. It amounted to the same thing, but was
unnecessarily complicated to optimise.
 3.
 TD_translate called from TD_drawmesh (and others) converts
from the 3D grid point coordinate to the 2D screen coordinate using:
(a) the three camera position coordinates, (or the single camera
distance value, s_cam, if ROTATE_OBJECT is set), and
(b) the three camera rotation angles. However, the three camera
rotation angles have already been converted into a rotation matrix when
TD_calc_rotation_matrix was called by TD_draw_solid.
To convert from a 3D coordinate to a 2D screen coordinate, the
camera position (or more correctly, the position of the object from the
camera) must first be added to each of the 3D grid coordinates. If the
user has chosen to use 32 bit values for the discription of the surface,
then these must be right shifted to the same size as the 16 bit
case.
x, y and z now hold the 3D position of
the object relative to the camera centre (or in these terms, the centre
of the video screen RIGHT ON the screen). The vector [x
y z] must now be multiplied by the rotation matrix. The
xt value must also have the camera distance, s_cam, added
to it in case the ROTATE_CAMERA is set (in which case
x_cam, y_cam and z_cam (the camera position) will
be zero and instead s_cam will have a value to provide the
necessary objectcamera distance). A test is also made as to whether
this value is zero or negative. In the case, the point is too close to
the camera, or behind the camera, and must not be drawn.
After the multiplication, the resulting vector [xt
yt zt] has been rotated to be aligned with screen. The vector
is now adjusted for perspective by dividing the yt and zt
values (horizontal and vertical respectively) by the xt value
(into the screen). Division is done by muldiv64 because the
intermediate product is larger than 32 bits. xscale and
yscale are factors that scale the image to size. posx and
posy is just the centre of the screen, or more precisely:
The exact position of the pinhole camera viewing the
object.
 4.
 TD_calc_rotation_matrix calculates the nine entries of the 3 by 3
matrix used in TD_translate. In order that only integer arithmetic
is performed, these values are stored and used as integers. Since this
matrix's entries are always between 1 and +1, they have to be integer
left shifted to give them accuracy. TD_MULCONSTANT scales them to
sufficient bits of accuracy before they are converted to integers.
This also means that results (of multiplications with them)
have to be scaled down by the same amount. This scaling is inherent in
the final multiplication and division (muldiv64) done in the
TD_translate function, so an extra division is not consumed.
The rotation matrix effectively rotates the vector by the
Eulerian angles alpha, beta and gamma. These angles
represent successive rotations about each of the 3D axes. You can test
which angles do what by looking at the calling application. Their
precise definitions are not all that important since you can get the
keyboard to do the right thing with a little trial and error.
Intrisics of drawing nontransparent surfaces...
to be continued ?!
vgagl(7), svgalib(7), gl_striangle(3),
gl_swtriangle(3), gl_triangle(3),
gl_trigetcolorlookup(3), gl_trisetcolorlookup(3),
gl_trisetdrawpoint(3), gl_wtriangle(3), plane(6),
wrapdemo(6).
This manual page was edited by Michael Weller
<eowmob@expmath.uniessen.de>. The demos, the initial documentation and
the whole threedkit stuff was done by Paul Sheer <psheer@icon.co.za>.
Paper mail:
Paul Sheer
P O BOX 890507
Lyndhurst
Johannesburg 2106
South Africa
Donations (by check or postal order) will be appreciated and will
encourage further development of this software. However this is strictly on
a voluntary basis where this software falls under the GNU LIBRARY GENERAL
PUBLIC LICENSE.
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