GLBLENDEQUATION(3G) GLBLENDEQUATION(3G)
NAME
glBlendEquation - set the 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 equation determines how a new pixel (the ``source'' color) is combined with a pixel already
in the framebuffer (the ``destination'' color).
GL_MIN sets the blend equation so that each component of the result color is the minimum of the cor-responding corresponding
responding components of the source and destination colors.
GL_MAX sets the blend equation so that each component of the result color is the maximum of the cor-responding corresponding
responding components of the source and destination colors.
The remaining blend equations use the source and destination blend factors specified by glBlendFunc.
See glBlendFunc for a description of the various blend factors.
In the equations that follow, source and destination color components are referred to as (Rs, Gs, Bs,
As ) and (Rd, Gd, Bd, Ad ), respectively. The result color is referred to as (Rr, Gr, Br, Ar ). The
source and destination blend factors are denoted (sR, sG, sB, sA ) and (dR, dG, dB, dA ), respec-tively. respectively.
tively. For these equations all color components are understood to have values in the range [0, 1].
GL_FUNC_ADD
sets the blend equation so that the source and destination data are added. Each component of
the source color is multiplied by the corresponding source factor, then each component of the
destination color is multiplied by the corresponding destination factor. The result is the
componentwise sum of the two products, clamped to the range [0, 1].
Gr = min (1, Gs sG + Gd dG )
Ar = min (1, As sA + Ad dA )
GL_FUNC_SUBTRACT
Is like GL_FUNC_ADD except the product of the destination factor and the destination color is
componentwise subtracted from the product of the source factor and the source color. The
result is clamped to the range [0, 1].
Rr = max (0 , Rs sR - Rd dR )
Gr = max (0 , Gs sG - Gd dG )
Br = max (0 , Bs sB - Bd dB )
Ar = max (0 , As sA - Ad dA )
GL_FUNC_REVERSE_SUBTRACT
Is like GL_FUNC_ADD except the product of the source factor and the source color is component-wise componentwise
wise subtracted from the product of the destination factor and the destination color. The
result is clamped to the range [0, 1].
Gr = max (0 , Gd dG - Gs sG )
Ar = max (0 , Ad dA - As sA )
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, the blend equation is set to GL_FUNC_ADD.
NOTES
glBlendEquation is part of the GL_ARB_imaging subset. glBlendEquation is present only if
GL_ARB_imaging is returned when glGetString is called with GL_EXTENSIONS as its argument.
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
SEE ALSO
glGetString(3G), glBlendColor(3G), glBlendFunc(3G)
GLBLENDEQUATION(3G)
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