fglBlendEquationEXT(3G) OpenGL Reference fglBlendEquationEXT(3G)NAMEfglBlendEquationEXT - set the blend equation
FORTRAN SPECIFICATION
SUBROUTINE fglBlendEquationEXT( INTEGER*4 mode )
PARAMETERS
mode specifies how source and destination colors are combined. It must
be GL_FUNC_ADD_EXT, GL_FUNC_SUBTRACT_EXT,
GL_FUNC_REVERSE_SUBTRACT_EXT, GL_MIN_EXT, GL_MAX_EXT,
GL_ALPHA_MIN_SGIX, GL_ALPHA_MAX_SGIX, or GL_LOGIC_OP.
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_EXT
sets the blend equation so that each component of the result color
is the minimum of the corresponding components of the source and
destination colors.
GL_MAX_EXT
sets the blend equation so that each component of the result color
is the maximum of the corresponding components of the source and
destination colors.
GL_ALPHA_MIN_SGIX
sets the blend equation so that the result color is either the
source or destination color, according to whether the alpha
component of the source color is less than, or greater than or equal
to, the alpha component of the destination color, respectively.
GL_ALPHA_MAX_SGIX
sets the blend equation so that the result color is either the
source or destination color, according to whether the alpha
component of the source color is greater than, or less than or equal
to, the alpha component of the destination color, respectively.
GL_LOGIC_OP
sets the blend equation so that each component of the result color
is a bitwise logical combination of the corresponding components of
the source and destination colors. Use the fglLogicOp command to
specify the logical operator.
The remaining blend equations use the source and destination blend
factors specified by fglBlendFunc. See fglBlendFunc for a description of
the various blend factors.
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fglBlendEquationEXT(3G) OpenGL Reference fglBlendEquationEXT(3G)
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), respectively. For
these equations all color components are understood to have values
between 0.0 and 1.0, inclusive.
GL_FUNC_ADD_EXT
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].
Rr = min(1, Rs sR + Rd dR)
Gr = min(1, Gs sG + Gd dG)
Br = min(1, Bs sB + Bd dB)
Ar = min(1, As sA + Ad dA)
GL_FUNC_SUBTRACT_EXT
Is like GL_FUNC_ADD_EXT 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_EXT
Is like GL_FUNC_ADD_EXT except the product of the source factor and
the source color is componentwise subtracted from the product of the
destination factor and the destination color. The result is clamped
to the range [0,1].
Rr = max(0, Rd dR - Rs sR)
Gr = max(0, Gd dG - Gs sG)
Br = max(0, Bd dB - Bs sB)
Ar = max(0, Ad dA - As sA)
The GL_MIN_EXT and GL_MAX_EXT equations are useful for applications that
analyze image data (image thresholding against a constant color, for
example). The GL_LOGIC_OP equation is most commonly used with GL_XOR for
reversible drawing. The GL_FUNC_ADD_EXT equation is useful for anti-
aliasing and transparency, among other things.
Initially the blend equation is set to GL_FUNC_ADD_EXT.
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fglBlendEquationEXT(3G) OpenGL Reference fglBlendEquationEXT(3G)NOTES
The GL_MIN_EXT, GL_MAX_EXT, GL_ALPHA_MIN_SGIX, GL_ALPHA_MAX_SGIX and
GL_LOGIC_OP 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_EXT,
GL_FUNC_SUBTRACT_EXT, GL_FUNC_REVERSE_SUBTRACT_EXT, GL_MAX_EXT,
GL_MIN_EXT, or GL_LOGIC_OP.
GL_INVALID_OPERATION is generated if fglBlendEquationEXT is executed
between the execution of fglBegin and the corresponding execution of
fglEnd.
ASSOCIATED GETS
fglGet with an argument of GL_BLEND_EQUATION_EXT.
fglGetString with an argument of GL_EXTENSIONS.
This feature is not part of standard GL. The extensions string of
fglGetString will contain the name blend_minmax if GL_MIN_EXT and
GL_MAX_EXT are supported. It will contain the name blend_logic_op
if GL_LOGIC_OP is supported in RGB mode. It will contain the name
blend_subtract if GL_FUNC_SUBTRACT_EXT and
GL_FUNC_REVERSE_SUBTRACT_EXT are supported.
MACHINE DEPENDENCIES
RealityEngine, RealityEngine2, and VTX systems do not support the use of
blend factor GL_SRC_ALPHA_SATURATE with blend equation
GL_FUNC_REVERSE_SUBTRACT_EXT; this combination produces incorrect
results.
The SGIX_blend_alpha_minmax extension is supported only on Octane2 VPro
systems.
SEE ALSO
fglGetString, fglBlendColorEXT, fglBlendFunc.
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