Unite 2010GDC ChinaAsia Game Show 2010GDC 2011
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The Third DimensionYou need a good familiarity with 3-D geometric principles to program Direct3D applications. The first step in understanding these principles is understanding the transformation pipeline as part of the rendering pipeline: The figure shows the Direct3D pipeline.
I focus here on the marked T & L Pipeline: You can think of the process of texturing and lighting as an assembly line in a factory, in which untransformed and unlit vertices enter one end, and then several sequential operations are performed on them. At the end of the assembly line, transformed and lit vertices exit. A lot of programmers have implemented their own transformation and lighting algorithms. They can disable parts of this pipeline and send the vertices that are already transformed and lit to Direct3D.
But in most cases it will be best to use the Direct3D T&L pipeline, because it's really fast, especially with the new T&L graphics drivers, which are provided with the Geforce and Savage 2000 chipsets. These graphic processors have gained an important role in the last couple of years. Most tasks in the rendering pipeline are now computed by the graphics processing unit:
In the old days, transform and lighting functions of the 3D pipeline have been performed by the CPU of the PC. Since 1999, affordable graphic cards with dedicated hardware T&L acceleration have been available. With these cards higher graphics performance is possible, because they can process graphic functions up to four times the speed of the leading CPUs. On the other side, the CPU can now be better utilized for functions such as sophisticated artificial intelligence (AI), realistic physics and more complex game elements. So the new generation of cards will provide a lot of horsepower for the new game generation. It's a great time for game programmers :-) Transformation PipelineIt's a complex task to describe and display 3D graphics objects and environments. Describing the 3D data according to different frames of reference or different coodinate systems reduce the complexity. These different frames of reference are called "spaces" such as model space, world space, view space, projection space. Because these spaces use different coordinate systems, 3D data must be converted or "transformed" from one space to another.
The world transformation stage transforms an object from model or object to world space. Model space is the coordinate space in which an object is defined, independant of other objects and the world itself. In model space, the points of the model or object are rotated, scaled and translated to animate it. For example, think of a Quake 3 model, which rotates his torso and holds his weapon in your direction. With model space it's easier and faster to move an object by simply redefining the transformation from model space to world space than it would be to manually change all the coordinates of the object in world space. For example, to rotate a sphere or cube around its center looks more natural and is much easier when the origin is at the center of the object, regardless of where in world space the object is positioned. Worldspace is the abolute frame of reference for a 3-D world; all object locations and orientations are with respect to worldspace. It provides a coordinate space that all objects share, instead of requiring a unique coordinate system for each object. To transform the objects from model to world space each object will be rotated about the x-axis, y-axis, or z-axis, scaled (enlarging or shrinking the object) and translated, by moving the object along the x-axis, y-axis, or z-axis to its final orientation, position and size. Direct3D uses a left-handed coordinate system, in which every positive axis (x, y or z) is pointing away from the viewer:
The view transformation stage transforms the objects from world space into camera space or view space. This transformation puts the camera at the origin and points it directly down the positive z-axis. The geometry has to be translated in suitable 2D shapes. It's also useful for lighting and backface culling. The light coordinates, which are specified in world space, are transformed into camera space at this stage and the effect of light is calculated and applied to the vertices.
The projection transformation takes into consideration the camera's horizontal and vertical fields of view, so it applies perspective to the scene. Objects which are close to the front of the frustrum are expanded and objects close to the end are shrunk. It warps the geometry of the frustrum of the camera into a cube shape by setting the passed in 4x4 matrix to a perspective projection matrix built from the field-of-view or viewing frustrum, aspect ratio, near plane or front clipping plane, and far plane or back clipping plane. This makes it easy to clip geometry that lies both inside and outside of the viewing frustrum. You can think of the projection transformation as controlling the camera's internals; it is analogous to choosing a lens for the camera.
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The transformation pipeline transforms each vertex of the object from an abstract, floating-point coordinate space into pixel-based screen space, taking into account the properties of the virtual camera used to render the scene. This transform is done with three transformation matrices: the world-, view- and projection-matrix. The use of world-, view- and projection- transformations ensures that Direct3D only has to deal with one coordinate system per step. Between those steps, the objects are oriented in a uniform manner.
