GLAPI/glBlendFuncSeparate

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glBlendFuncSeparate
Core in version 4.4
Core since version 1.4

glBlendFuncSeparate: specify pixel arithmetic for RGB and alpha components separately

Function Definition

 void glBlendFunc(GLenum sfactor​, GLenum dfactor​);
 void glBlendFunci(GLuint buf​, GLenum sfactor​, GLenum dfactor​);
buf
For glBlendFunci, specifies the index of the draw buffer for which to set the blend function.
sfactor
Specifies how the red, green, blue, and alpha source blending factors are computed. The initial value is GL_ONE​.
dfactor
Specifies how the red, green, blue, and alpha destination blending factors are computed. The initial value is GL_ZERO​.

Function Definition

 void glBlendFuncSeparate(GLenum srcRGB​, GLenum dstRGB​, GLenum srcAlpha​, GLenum dstAlpha​);
 void glBlendFuncSeparatei(GLuint buf​, GLenum srcRGB​, GLenum dstRGB​, GLenum srcAlpha​, GLenum dstAlpha​);
buf
For glBlendFuncSeparatei, specifies the index of the draw buffer for which to set the blend functions.
srcRGB
Specifies how the red, green, and blue blending factors are computed. The initial value is GL_ONE​.
dstRGB
Specifies how the red, green, and blue destination blending factors are computed. The initial value is GL_ZERO​.
srcAlpha
Specified how the alpha source blending factor is computed. The initial value is GL_ONE​.
dstAlpha
Specified how the alpha destination blending factor is computed. The initial value is GL_ZERO​.

Description

Pixels can be drawn using a function that blends the incoming (source) RGBA values with the RGBA values that are already in the frame buffer (the destination values). Blending is initially disabled. Use glEnable and glDisable with argument GL_BLEND​ to enable and disable blending.

glBlendFunc is equivalent to calling glBlendFuncSeparate with sfactor​ for both the srcRGB​ and srcAlpha​ parameters, and dfactor​ for both the dstRGB​ and dstAlpha​ parameters. glBlendFunci works the same way relative to glBlendFuncSeparatei.

glBlendFuncSeparate defines the operation of blending for all draw buffers when it is enabled. glBlendFuncSeparatei defines the operation of blending for a single draw buffer specified by buf​ when enabled for that draw buffer. srcRGB​ specifies which method is used to scale the source RGB-color components. dstRGB​ specifies which method is used to scale the destination RGB-color components. Likewise, srcAlpha​ specifies which method is used to scale the source alpha color component, and dstAlpha​ specifies which method is used to scale the destination alpha component. The possible methods are described in the following table. Each method defines four scale factors, one each for red, green, blue, and alpha.

In the table and in subsequent equations, first source, second source and destination color components are referred to as (Rs0, Gs0, Bs0, As0), (Rs1, Gs1, Bs1, As1) and (Rd, Gd, Bd, Ad), respectively. The color specified by glBlendColor is referred to as (Rc, Gc, Bc, Ac).

Source and destination scale factors are referred to as (sR, sG, sB, sA) and (dR, dG, dB, dA).

Parameter RGB Factor Alpha Factor
GL_ZERO​ (0, 0, 0) 0
GL_ONE​ (1, 1, 1) 1
GL_SRC_COLOR​ (Rs0, Gs0, Bs0) As0
GL_ONE_MINUS_SRC_COLOR​ (1, 1, 1) - (Rs0, Gs0, Bs0) 1 - As0
GL_DST_COLOR​ (Rd, Gd, Bd) Ad
GL_ONE_MINUS_DST_COLOR​ (1, 1, 1) - (Rd, Gd, Bd) 1 - Ad
GL_SRC_ALPHA​ (As0, As0, As0) As0
GL_ONE_MINUS_SRC_ALPHA​ (1, 1, 1) - (As0, As0, As0) 1 - As0
GL_DST_ALPHA​ (Ad, Ad, Ad) Ad
GL_ONE_MINUS_DST_ALPHA​ (1, 1, 1) - (Ad, Ad, Ad) Ad
GL_CONSTANT_COLOR​ (Rc, Gc, Bc) Ac
GL_ONE_MINUS_CONSTANT_COLOR​ (1, 1, 1) - (Rc, Gc, Bc) 1 - Ac
GL_CONSTANT_ALPHA​ (Ac, Ac, Ac) Ac
GL_ONE_MINUS_CONSTANT_ALPHA​ (1, 1, 1) - (Ac, Ac, Ac) 1 - Ac
GL_SRC_ALPHA_SATURATE​ (i, i, i) 1
GL_SRC1_COLOR​ (Rs1, Gs1, Bs1) As1
GL_ONE_MINUS_SRC_COLOR​ (1, 1, 1) - (Rs1, Gs1, Bs1) 1 - As1
GL_SRC1_ALPHA​ (As1, As1, As1) As1
GL_ONE_MINUS_SRC_ALPHA​ (1, 1, 1) - (As1, As1, As1) 1 - As1

In the table,

i = min(A_{s0}, (1 - A_d))

Despite the apparent precision of the above equations, blending arithmetic is not exactly specified, because blending operates with imprecise integer color values. However, a blend factor that should be equal to 1 is guaranteed not to modify its multiplicand, and a blend factor equal to 0 reduces its multiplicand to 0. For example, when srcRGB​ is GL_SRC_ALPHA​, dstRGB​ is GL_ONE_MINUS_SRC_ALPHA​, and As0 is equal to 1, the equations reduce to simple replacement:

\begin{align}
R_d = R_{s0}\\
G_d = G_{s0}\\
B_d = B_{s0}\\
A_d = A_{s0}\\
\end{align}

Notes

When more than one color buffer is enabled for drawing, the GL performs blending separately for each enabled buffer, using the contents of that buffer for destination color. (See glDrawBuffer.)

When dual source blending is enabled (i.e., one of the blend factors requiring the second color input is used), the maximum number of enabled draw buffers is given by GL_MAX_DUAL_SOURCE_DRAW_BUFFERS​, which may be lower than GL_MAX_DRAW_BUFFERS​.

Errors

GL_INVALID_ENUM​ is generated if either srcRGB​ or dstRGB​ is not an accepted value.

GL_INVALID_VALUE​ is generated by glBlendFuncSeparatei if buf​ is greater than or equal to the value of GL_MAX_DRAW_BUFFERS​.

Associated Gets

glGet with argument GL_BLEND_SRC_RGB​

glGet with argument GL_BLEND_SRC_ALPHA​

glGet with argument GL_BLEND_DST_RGB​

glGet with argument GL_BLEND_DST_ALPHA​

glIsEnabled with argument GL_BLEND​


See Also

glBlendColor, glBlendEquationSeparate, glDrawBuffer, glDrawBuffers

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/.