glBlendEquation

Name

glBlendEquation — specify the equation used for both the RGB blend equation and the Alpha blend equation

C Specification

void glBlendEquation( GLenum mode);

Parameters

mode: specifies how source and destination colors are combined. It must be GL_FUNC_ADD, GL_FUNC_SUBTRACT, GL_FUNC_REVERSE_SUBTRACT, GL_MIN, GL_MAX.

Description

The blend equations determine how a new pixel (the ''source'' color) is combined with a pixel already in the framebuffer (the ''destination'' color). This function sets both the RGB blend equation and the alpha blend equation to a single equation.

These equations use the source and destination blend factors specified by either glBlendFunc or glBlendFuncSeparate. See glBlendFunc or glBlendFuncSeparate for a description of the various blend factors.

In the equations that follow, source and destination color components are referred to as $(R_{s}, G_{s}, B_{s}, A_{s})$ and $(R_{d}, G_{d}, B_{d}, A_{d})$ , respectively. The result color is referred to as $(R_{r}, G_{r}, B_{r}, A_{r})$ . The source and destination blend factors are denoted $(s_{R}, s_{G}, s_{B}, s_{A})$ and $(d_{R}, d_{G}, d_{B}, d_{A})$ , respectively. For these equations all color components are understood to have values in the range $[0, 1]$ .

Mode	RGB Components	Alpha Component
`GL_FUNC_ADD`	$Rr = R_{s} s_{R} + R_{d} d_{R}$ $Gr = G_{s} s_{G} + G_{d} d_{G}$ $Br = B_{s} s_{B} + B_{d} d_{B}$	$Ar = A_{s} s_{A} + A_{d} d_{A}$
`GL_FUNC_SUBTRACT`	$Rr = R_{s} s_{R} - R_{d} d_{R}$ $Gr = G_{s} s_{G} - G_{d} d_{G}$ $Br = B_{s} s_{B} - B_{d} d_{B}$	$Ar = A_{s} s_{A} - A_{d} d_{A}$
`GL_FUNC_REVERSE_SUBTRACT`	$Rr = R_{d} d_{R} - R_{s} s_{R}$ $Gr = G_{d} d_{G} - G_{s} s_{G}$ $Br = B_{d} d_{B} - B_{s} s_{B}$	$Ar = A_{d} d_{A} - A_{s} s_{A}$
`GL_MIN`	$Rr = \min (R_{s}, R_{d})$ $Gr = \min (G_{s}, G_{d})$ $Br = \min (B_{s}, B_{d})$	$Ar = \min (A_{s}, A_{d})$
`GL_MAX`	$Rr = \max (R_{s}, R_{d})$ $Gr = \max (G_{s}, G_{d})$ $Br = \max (B_{s}, B_{d})$	$Ar = \max (A_{s}, A_{d})$

The results of these equations are clamped to the range $[0, 1]$ .

The GL_MIN and GL_MAX equations are useful for applications that analyze image data (image thresholding against a constant color, for example). The GL_FUNC_ADD equation is useful for antialiasing and transparency, among other things.

Initially, both the RGB blend equation and the alpha blend equation are set to GL_FUNC_ADD.

Notes

The GL_MIN, and GL_MAX equations do not use the source or destination factors, only the source and destination colors.

Errors

GL_INVALID_ENUM is generated if mode is not one of GL_FUNC_ADD, GL_FUNC_SUBTRACT, GL_FUNC_REVERSE_SUBTRACT, GL_MAX, or GL_MIN.

GL_INVALID_OPERATION is generated if glBlendEquation is executed between the execution of glBegin and the corresponding execution of glEnd.

Associated Gets

glGet with an argument of GL_BLEND_EQUATION_RGB

glGet with an argument of GL_BLEND_EQUATION_ALPHA

Copyright

Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/.

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