OpenGL.GLX.SGIS.blended_overlay

OpenGL extension SGIS.blended_overlay

This module customises the behaviour of the OpenGL.raw.GLX.SGIS.blended_overlay to provide a more Python-friendly API

Overview (from the spec)

This extension augments the set of transparency types for GLX visuals (defined by the EXT_visual_info GLX extension). A new transparency type designated BLENDED_RGBA_SGIS is defined for overlay windows supporting an alpha color component. Instead of using a particular transparent pixel or color value to indicate transparency, a window created with a BLENDED_RGBA_SGIS transparency type visual blends with the lower frame buffer layers based on the overlay window's per-pixel alpha component.

The overlay blend function is:

if ( Cu is color index pixel type ) then

if ( Ao > 0.0 ) then Cd = Co /* Non-zero overlay alpha simply uses overlay color */ else Cd = Cu /* Note: later CI cmap turns Cd index into true color */ endif

else /* Cu is an RGBA, RGB, L, or LA pixel type */

/* blend overlay color with normal color based on overlay alpha */ Cd = Co * (Ao,Ao,Ao,Ao) + Cu * ( (1,1,1,1) - (Ao,Ao,Ao,Ao) )

endif

where Cd is the resulting displayed color. Co is the RGBA quadruplet for the overlay RGBA components of the overlay window pixel. Ao is the alpha component for the overlay alpha component of the overlay window pixel. Cu is the RGBA quadruplet for the RGBA components of the displayed pixel if no overlay where present in the overlay window's layer (or any higher level). Think of Cu as the displayable color "under" the BLENDED_RGBA_SGIS overlay pixel.

In the case of extended range frame buffer formats, the overlay blend function is applied post-clamping to the [0,1] range.

Here are some of the uses for blended RGBA overlays:

Render antialiased lines, points, and polygons in the overlays with GL_LINE_SMOOTH, GL_POINT_SMOOTH, and GL_POLYGON_SMOOTH.

Alpha blended antiliasing could be used on the overlay boundaries of overlay control panels and boat hulls.

Clean blending of overlaid mattes for live video. Live video stream could be overlaid by computer generated overlaid graphics.

Swank user interface support for sweeping out irregular screen regions by painiting the region in the overlay with a fractional alpha to blend the sweeped region with some constant color in the overlay.

Swank brush shapes for 3D painting that let you see the blended pixels underneather the overlay brush shape.

Really smooth (ie, subpixel positioned), full-color, antialiased cursors (application drawn into a blended RGBA overlay).

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