Vertices,
Primitives, Fragment and Pixels
1.1
3D Graphics Coordinate Systems
OpenGL adopts the Right-Hand
Coordinate System (RHS). In the
RHS, the x-axis is pointing right, y-axis is pointing up, and z-axis is
pointing out of the screen. With your right-hand fingers curving from the
x-axis towards the y-axis, the thumb is pointing at the z-axis. RHS is counter-clockwise (CCW). The 3D Cartesian Coordinates is a RHS.
Some graphics software (such as
Microsoft Direct3D) use Left-hand System (LHS), where the z-axis is inverted. LHS is
clockwise (CW). In this article, we shall adopt the RHS and CCW used in OpenGL.
1.2 Primitives
The inputs to the Graphics
Rendering Pipeline are geometric primitives (such as triangle, point, line or quad),
which is formed by one or more vertices.
OpenGL supports three classes of
geometric primitives: points, line segments, and closed polygons. They are specified via vertices. Each
vertex is associated with its attributes such as the position, color, normal
and texture. OpenGL provides 10 primitives as shown. Sphere, 3D box and pyramid
are not primitives. They are typically assembled using primitive triangle or
quad.
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