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