Designing Games with Game Maker - YoYo Games

This might look a bit complicated. We look from the camera position (x,y), 10 above the
ground. To determine a point in the correct direction we need to do a little arithmetic. This
point is indicated by the next three parameters. Finally we use the up vector as above.
One important remark! When Game Maker starts drawing a room it will set the view point
back to the default position. So the first thing you must do when drawing the scene is set is to
the projection you want. This must be done in a drawing event!
There is also an extended version of the function above:
d3d_set_projection_ext(xfrom,yfrom,zfrom,xto,yto,zto,xup,yu
p,zup,angle,aspect,znear,zfar) An extended version of this function in
which you also specify the angle defining the field of view, the aspect ratio
between horizontal and vertical size of the view, and the near and far clipping
planes.
The additional parameters work as follows. If you specified the camera position, point to look
at, and up vector, you can still change how wide the lens of the camera is. This is called the
field of view. A reasonable value is 45 degrees and this is what is default taken. But you can
change this if you like. Next you can specify the aspect ratio between the horizontal and
vertical projection. Normally you want to use the same as the aspect ration of the room or
view, e.g. 640/480. Finally you can indicate the clipping planes. Objects that are closer than
znear to the camera are not drawn. Similar for objects further than zfar. It can be important
to set these parameters to reasonable values because they also influence the precision of the
z-comparisons. If you make the range too large the precision gets worse. Default we use 1
and 32000. znear must be larger than 0!
Sometimes you temporarily need a normal orthographic projection as is used when there is no
3D. Or you want to return to the default perspective projection. For this you can use the
following functions:
d3d_set_projection_ortho(x,y,w,h,angle) Sets a normal
orthographic projection of the indicated area in the room, rotated over the
indicated angle.
d3d_set_projection_perspective(x,y,w,h,angle) Sets a normal
perspective projection of the indicated area in the room, rotated over the
indicated angle.