OpenGL.GL.NV.blend_equation_advanced

OpenGL extension NV.blend_equation_advanced

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

Overview (from the spec)

This extension adds a number of "advanced" blending equations that can be used to perform new color blending operations, many of which are more complex than the standard blend modes provided by unextended OpenGL. This extension provides two different extension string entries:

Some implementations may support NV_blend_equation_advanced without supporting NV_blend_equation_advanced_coherent.

In unextended OpenGL, the set of blending equations is limited, and can be expressed very simply. The MIN and MAX blend equations simply compute component-wise minimums or maximums of source and destination color components. The FUNC_ADD, FUNC_SUBTRACT, and FUNC_REVERSE_SUBTRACT multiply the source and destination colors by source and destination factors and either add the two products together or subtract one from the other. This limited set of operations supports many common blending operations but precludes the use of more sophisticated transparency and blending operations commonly available in many dedicated imaging APIs.

This extension provides a number of new "advanced" blending equations. Unlike traditional blending operations using the FUNC_ADD equation, these blending equations do not use source and destination factors specified by BlendFunc. Instead, each blend equation specifies a complete equation based on the source and destination colors. These new blend equations are used for both RGB and alpha components; they may not be used to perform separate RGB and alpha blending (via functions like BlendEquationSeparate).

These blending operations are performed using premultiplied colors, where RGB colors stored in the framebuffer are considered to be multiplied by alpha (coverage). The fragment color may be considered premultiplied or non-premultiplied, according the BLEND_PREMULTIPLIED_SRC_NV blending parameter (as specified by the new BlendParameteriNV function). If fragment color is considered non-premultiplied, the (R,G,B) color components are multiplied by the alpha component prior to blending. For non-premultiplied color components in the range the corresponding premultiplied color component would have values in the range [0*A,1*A .

Many of these advanced blending equations are formulated where the result of blending source and destination colors with partial coverage have three separate contributions: from the portions covered by both the source and the destination, from the portion covered only by the source, and from the portion covered only by the destination. The blend parameter BLEND_OVERLAP_NV can be used to specify a correlation between source and destination pixel coverage. If set to CONJOINT_NV, the source and destination are considered to have maximal overlap, as would be the case if drawing two objects on top of each other. If set to DISJOINT_NV, the source and destination are considered to have minimal overlap, as would be the case when rendering a complex polygon tessellated into individual non-intersecting triangles. If set to UNCORRELATED_NV (default), the source and destination coverage are assumed to have no spatial correlation within the pixel.

In addition to the coherency issues on implementations not supporting NV_blend_equation_advanced_coherent, this extension has several limitations worth noting. First, the new blend equations are not supported while rendering to more than one color buffer at once; an INVALID_OPERATION will be generated if an application attempts to render any primitives in this unsupported configuration. Additionally, blending precision may be limited to 16-bit floating-point, which could result in a loss of precision and dynamic range for framebuffer formats with 32-bit floating-point components, and in a loss of precision for formats with 12- and 16-bit signed or unsigned normalized integer components.

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