1991-2006
Silicon Graphics, Inc.
glBlendEquation
3G

# 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 &it; s R + R d &it; d R Gr = G s &it; s G + G d &it; d G Br = B s &it; s B + B d &it; d B | Ar = A s &it; s A + A d &it; d A
`GL_FUNC_SUBTRACT` | Rr = R s &it; s R - R d &it; d R Gr = G s &it; s G - G d &it; d G Br = B s &it; s B - B d &it; d B | Ar = A s &it; s A - A d &it; d A
`GL_FUNC_REVERSE_SUBTRACT` | Rr = R d &it; d R - R s &it; s R Gr = G d &it; d G - G s &it; s G Br = B d &it; d B - B s &it; s B | Ar = A d &it; d A - A s &it; s A
`GL_MIN` | Rr = min &af; R s R d Gr = min &af; G s G d Br = min &af; B s B d | Ar = min &af; A s A d
`GL_MAX` | Rr = max &af; R s R d Gr = max &af; G s G d Br = max &af; B s B d | Ar = max &af; 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`

## See Also

glGetString, glBlendColor, glBlendFunc glBlendFuncSeparate

## Copyright

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