1991-2006
Silicon Graphics, Inc.
2010-2014
Khronos Group
glBlendFunc
3G

# glBlendFunc

specify pixel arithmetic

## C Specification

                    void glBlendFunc
                    GLenum sfactor
                    GLenum dfactor

## Parameters

`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 following symbolic constants are accepted: `GL_ZERO`, `GL_ONE`, `GL_SRC_COLOR`, `GL_ONE_MINUS_SRC_COLOR`, `GL_DST_COLOR`, `GL_ONE_MINUS_DST_COLOR`, `GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`, `GL_DST_ALPHA`, `GL_ONE_MINUS_DST_ALPHA`. `GL_CONSTANT_COLOR`, `GL_ONE_MINUS_CONSTANT_COLOR`, `GL_CONSTANT_ALPHA`, and `GL_ONE_MINUS_CONSTANT_ALPHA`. 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` defines the operation of blending when it is enabled. `sfactor` specifies which method is used to scale the source color components. `dfactor` specifies which method is used to scale the destination color components. Both parameters must be one of the following symbolic constants: `GL_ZERO`, `GL_ONE`, `GL_SRC_COLOR`, `GL_ONE_MINUS_SRC_COLOR`, `GL_DST_COLOR`, `GL_ONE_MINUS_DST_COLOR`, `GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`, `GL_DST_ALPHA`, `GL_ONE_MINUS_DST_ALPHA`, `GL_CONSTANT_COLOR`, `GL_ONE_MINUS_CONSTANT_COLOR`, `GL_CONSTANT_ALPHA`, `GL_ONE_MINUS_CONSTANT_ALPHA`, `GL_SRC_ALPHA_SATURATE`, 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, 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 color specified by glBlendColor is referred to as R c G c B c A c .

Source and destination scale factors are referred to as s R s G s B s A and d R d G d B d A . The scale factors described in the table, denoted f R f G f B f A , represent either source or destination factors. All scale factors have range 0 1 .

Prior to blending, unsigned normalized fixed-point color components undergo an implied conversion to floating-point using equation 2.1. This conversion must leave the values 0 and 1 invariant. Blending computations are treated as if carried out in floating-point and will be performed with a precision and dynamic range no lower than that used to represent destination components. If the value of `GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING` for the framebuffer attachment corresponding to the destination buffer is `GL_SRGB`, the R, G, and B destination color values (after conversion from fixed-point to floating-point) are considered to be encoded for the sRGB color space and hence must be linearized prior to their use in blending. Each R, G, and B component is converted in the same fashion described for sRGB texture components.

Parameter | f R f G f B f A
--- | ---
`GL_ZERO` | 0 0 0 0
`GL_ONE` | 1 1 1 1
`GL_SRC_COLOR` | R s k R G s k G B s k B A s k A
`GL_ONE_MINUS_SRC_COLOR` | 1 1 1 1 - R s k R G s k G B s k B A s k A
`GL_DST_COLOR` | R d k R G d k G B d k B A d k A
`GL_ONE_MINUS_DST_COLOR` | 1 1 1 1 - R d k R G d k G B d k B A d k A
`GL_SRC_ALPHA` | A s k A A s k A A s k A A s k A
`GL_ONE_MINUS_SRC_ALPHA` | 1 1 1 1 - A s k A A s k A A s k A A s k A
`GL_DST_ALPHA` | A d k A A d k A A d k A A d k A
`GL_ONE_MINUS_DST_ALPHA` | 1 1 1 1 - A d k A A d k A A d k A A d k A
`GL_CONSTANT_COLOR` | R c G c B c A c
`GL_ONE_MINUS_CONSTANT_COLOR` | 1 1 1 1 - R c G c B c A c
`GL_CONSTANT_ALPHA` | A c A c A c A c
`GL_ONE_MINUS_CONSTANT_ALPHA` | 1 1 1 1 - A c A c A c A c
`GL_SRC_ALPHA_SATURATE` | i i i 1

In the table,

i = min &af; A s k A - A d k A

To determine the blended RGBA values of a pixel, the system uses the following equations:

R d = min &af; k R R s &it; s R + R d &it; d R G d = min &af; k G G s &it; s G + G d &it; d G B d = min &af; k B B s &it; s B + B d &it; d B A d = min &af; k A A s &it; s A + A d &it; d A

If the value of `GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING` for the framebuffer attachment corresponding to the destination buffer is `GL_SRGB`, the R, G, and B values after blending are converted into the non-linear sRGB color space by computing where cl is the R, G, or B element and cs is the result (effectively converted into an sRGB color space). If `GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING` is not `GL_SRGB`, then cs = cl: The resulting cs values for R, G, and B, and the unmodified A form a new RGBA color value. If the color buffer is fixed-point, each component is clamped to the range [0; 1] and then converted to a fixed-point value using equation

## Examples

Transparency is best implemented using blend function (`GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`) with primitives sorted from farthest to nearest. Note that this transparency calculation does not require the presence of alpha bitplanes in the frame buffer.

Blend function (`GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`) is also useful for rendering antialiased points and lines in arbitrary order.

## Notes

Incoming (source) alpha is correctly thought of as a material opacity, ranging from 1.0 ( K A ), representing complete opacity, to 0.0 (0), representing complete transparency.

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 glDrawBuffers.)

## Errors

`GL_INVALID_ENUM` is generated if either `sfactor` or `dfactor` is not an accepted value.

## Associated Gets

glGet with argument `GL_BLEND_SRC`

glGet with argument `GL_BLEND_DST`

glIsEnabled with argument `GL_BLEND`

## API Version Support

glBlendFunc

## See Also

glBlendColor, glBlendEquation, glBlendFuncSeparate, glClear, glDrawBuffers, glEnable,

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. Copyright 2010-2014 Khronos Group. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.