Parameters

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 $\left({\mathit{R}}_{\mathit{s}},{\mathit{G}}_{\mathit{s}},{\mathit{B}}_{\mathit{s}},{\mathit{A}}_{\mathit{s}}\right)$, and $\left({\mathit{R}}_{\mathit{d}},{\mathit{G}}_{\mathit{d}},{\mathit{B}}_{\mathit{d}},{\mathit{A}}_{\mathit{d}}\right)$, respectively. The color specified by glBlendColor is referred to as $\left({\mathit{R}}_{\mathit{c}},{\mathit{G}}_{\mathit{c}},{\mathit{B}}_{\mathit{c}},{\mathit{A}}_{\mathit{c}}\right)$.

Source and destination scale factors are referred to as $\left({\mathit{s}}_{\mathit{R}},{\mathit{s}}_{\mathit{G}},{\mathit{s}}_{\mathit{B}},{\mathit{s}}_{\mathit{A}}\right)$ and $\left({\mathit{d}}_{\mathit{R}},{\mathit{d}}_{\mathit{G}},{\mathit{d}}_{\mathit{B}},{\mathit{d}}_{\mathit{A}}\right)$. The scale factors described in the table, denoted $\left({\mathit{f}}_{\mathit{R}},{\mathit{f}}_{\mathit{G}},{\mathit{f}}_{\mathit{B}},{\mathit{f}}_{\mathit{A}}\right)$, represent either source or destination factors. All scale factors have range $\left[0,1\right]$.

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.

In the table,

$\mathit{i}=\frac{\mathit{min}\left({\mathit{A}}_{\mathit{s}},{\mathit{k}}_{\mathit{A}}-{\mathit{A}}_{\mathit{d}}\right)}{{\mathit{k}}_{\mathit{A}}}$

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

${\mathit{R}}_{\mathit{d}}=\mathit{min}\left({\mathit{k}}_{\mathit{R}},{\mathit{R}}_{\mathit{s}}{\mathit{s}}_{\mathit{R}}+{\mathit{R}}_{\mathit{d}}{\mathit{d}}_{\mathit{R}}\right)$ ${\mathit{G}}_{\mathit{d}}=\mathit{min}\left({\mathit{k}}_{\mathit{G}},{\mathit{G}}_{\mathit{s}}{\mathit{s}}_{\mathit{G}}+{\mathit{G}}_{\mathit{d}}{\mathit{d}}_{\mathit{G}}\right)$ ${\mathit{B}}_{\mathit{d}}=\mathit{min}\left({\mathit{k}}_{\mathit{B}},{\mathit{B}}_{\mathit{s}}{\mathit{s}}_{\mathit{B}}+{\mathit{B}}_{\mathit{d}}{\mathit{d}}_{\mathit{B}}\right)$ ${\mathit{A}}_{\mathit{d}}=\mathit{min}\left({\mathit{k}}_{\mathit{A}},{\mathit{A}}_{\mathit{s}}{\mathit{s}}_{\mathit{A}}+{\mathit{A}}_{\mathit{d}}{\mathit{d}}_{\mathit{A}}\right)$

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 (${\mathit{K}}_{\mathit{A}}$), 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

Tutorials

API Version Support

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 https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/.