OpenGL.GL.SGIX.sprite

OpenGL extension SGIX.sprite

This module customises the behaviour of the OpenGL.raw.GL.SGIX.sprite to provide a more Python-friendly API

Overview (from the spec)

This extension provides support for viewpoint dependent alignment of geometry, in particular geometry that rotates about a point or a specified axis to face the eye point. The primary use is for quickly rendering roughly cylindrically or spherically symmetric objects, e.g. trees, smoke, clouds, etc. using geometry textured with a partially transparent texture map.

Rendering sprite geometry requires applying a transformation to primitives before the current model view. This matrix includes a rotation which is computed based on the current model view matrix and a translation which is specified explicitly (SPRITE_TRANSLATION_SGIX). The current model view matrix itself is not modified.

Primitives are first transformed by a rotation, depending on the sprite mode:

SPRITE_AXIAL_SGIX: The front of the object is rotated about an axis so that it faces the eye as much as the axis constraint allows. This is used for roughly rendering cylindrical objects such as trees in visual simulation.

SPRITE_OBJECT_ALIGNED_SGIX: The front of the object is rotated about a point to face the eye with the remaining rotational degree of freedom specified by aligning the top of the object with a specified axis in object coordinates. This is used for spherical objects and special effects such as smoke which must maintain an alignment in object coordinates for realism.

SPRITE_EYE_ALIGNED_SGIX: The front of the object is rotated about a point to face the eye with the remaining rotational degree of freedom specified by aligning the top of the object with a specified axis in eye coordinates. This is used for rendering sprites which must maintain an alignment on the screen, such as 3D annotations.

The axis of rotation or alignment, SPRITE_AXIS_SGIX, can be an arbitrary direction to support geocentric coordinate frames in which "up" is not along X, Y or Z.

Sprite geometry is modeled in a canonical frame: +Z is the up vector. -Y is the front vector which is rotated to point towards the eye. In the discussion below, the eye vector is the vector to the eye from the origin of the model view frame translated by the sprite position.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/SGIX/sprite.txt