On-Line Computer Graphics Notes
WORLD AND IMAGE SPACE
COORDINATE SYSTEMS

A model is reduced to a picture by systematic transformation through several coordinate systems. The user's coordinate system, commonly called world space is the highest level, while the device space is the lowest. We define here the spaces through which a polygon must pass to be reduced from the user's coordinate system to the device coordinate system.

• device space - the actual coordinate system by which points are plotted on the graphics device. This is usually measured in pixels. On the SGI machines, the lower left corner of your window is (0,0), with x values increasing to the right and y values increasing upward.

• Screen Space - defined to be the square $-1 \leq x,y \leq 1$ on the $xy$ plane. Most drivers for specific graphics devices are written to map this space onto device space.

• Image Space - the 3-dimensional coordinate system containing Screen Space. To allow for depth calculations, this space is usually defined by $-1 \leq x,y,z \leq 1$. Points in image space are mapped onto the screen by projecting them, via a parallel projection, onto the plane $z=0$ - thus, for example, any point $(0,0,z)$ in image space will project to the center of the display screen.

• Object Space - the local coordinate system of an object being created. After objects are created, they can be transformed, duplicated, etc. and moved about a larger coordinate system, which is commonly referred to as world space.

• World Space - the coordinate system of the scene that is to be projected into image space for viewing. A mapping is usually constructed that maps an area of world space into the image space volume $-1 \leq x,y,z \leq 1$.

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