# OpenGL.GL.REND.screen_coordinates

OpenGL extension REND.screen_coordinates

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

Overview (from the spec)

This extension allows the specification of screen coordinate vertex
data. Screen coordinate vertices completely bypass transformation,
texture generation, lighting and frustum clipping. It also allow for
fewer floating point computations to the performed by OpenGL.

If we get screen coordinate inputs then in order to perspectively
correct data (eg texture), the input data currently has to be
specified in one of the following manners

1. Specify all the data normally
eg.
glTexture2T(s, t);
and the coordinates as
glVertex4T(x*w, y*w, z*w, w);
or
2. Divide each data by w
eg.
glTexture4T(s/w, t/w, r/w, q/w);
and the coordinates as
glVertex3T(x, y, z);

Most hardware already performs some form of correction of the
coordinate data with respect to the w term prior to interpolation.
This is normally in the form of a multiplication of the terms by the
inverse w. It would be much more efficient to simply specify screen
coordinates as shown in the following example
glTexture2T(s, t, r, q);
and the coordinates as
glVertex4T(x, y, z, w);
and allow the hardware to bring the interpolated terms into a linear
screen space.

Additionally if the application derives screen coordinates it is
also highly likely that the 1/w term may already be computed. So it
would be advantageous to be able to specify 1/w directly instead of
w in the input screen coordinates.

For hardware that linearly interpolates data, the hardware
interpolates the following data:
s/w, t/w, r/w, q/w, x, y, z
If the input w represents the original 1/w, then the hardware can
avoid the division and instead interpolate:
s*w, t*w, r*w, q*w, x, y, z

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

## Functions

## Constants

GL_INVERTED_SCREEN_W_REND (33937)

GL_SCREEN_COORDINATES_REND (33936)