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glBlendEquation

Name

glBlendEquation — specify the equation used for both the RGB blend equation and the Alpha blend equation

C Specification

void glBlendEquation( GLenum mode);
 
void glBlendEquationi( GLuint buf,
  GLenum mode);
 

Parameters

buf

for glBlendEquationi, specifies the index of the draw buffer for which to set the blend equation.

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 equations determine how a new pixel (the ''source'' color) is combined with a pixel already in the framebuffer (the ''destination'' color). This function sets both the RGB blend equation and the alpha blend equation to a single equation. glBlendEquationi specifies the blend equation for a single draw buffer whereas glBlendEquation sets the blend equation for all draw buffers.

Some of these equations use the source and destination blend factors specified by either glBlendFunc or glBlendFuncSeparate. See glBlendFunc or glBlendFuncSeparate for a description of the various blend factors.

In the equations that follow, 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 result color is referred to as R r G r B r A r . The source and destination blend factors are denoted s R s G s B s A and d R d G d B d A , respectively. For these equations all color components are understood to have values in the range 0 1 .

Mode RGB Components Alpha Component
GL_FUNC_ADD Rr = R s ⁢ s R + R d ⁢ d R Gr = G s ⁢ s G + G d ⁢ d G Br = B s ⁢ s B + B d ⁢ d B Ar = A s ⁢ s A + A d ⁢ d A
GL_FUNC_SUBTRACT Rr = R s ⁢ s R - R d ⁢ d R Gr = G s ⁢ s G - G d ⁢ d G Br = B s ⁢ s B - B d ⁢ d B Ar = A s ⁢ s A - A d ⁢ d A
GL_FUNC_REVERSE_SUBTRACT Rr = R d ⁢ d R - R s ⁢ s R Gr = G d ⁢ d G - G s ⁢ s G Br = B d ⁢ d B - B s ⁢ s B Ar = A d ⁢ d A - A s ⁢ s A
GL_MIN Rr = min ⁡ R s R d Gr = min ⁡ G s G d Br = min ⁡ B s B d Ar = min ⁡ A s A d
GL_MAX Rr = max ⁡ R s R d Gr = max ⁡ G s G d Br = max ⁡ B s B d Ar = max ⁡ A s A d

For the advanced blending equations, the source and destination color components are premultiplied by the alpha component prior to blending. The base source and destination colours are referred to as R' s G' s B' s and R' d G' d' B' d' , respectively, and are calculated as follows: R' s G' s B' s = 0 0 0 , if A s == 0, otherwise R' s G' s B' s = R s A s G s A s B s A s .

All of the advanced blending equations perform the following equations:

R = f ⁡ R' s R' d * p0 ⁡ A s A d + R' s * p1 ⁡ A s A d + R' d * p2 ⁡ A s A d
G = f ⁡ G' s G' d * p0 ⁡ A s A d + G' s * p1 ⁡ A s A d + G' d * p2 ⁡ A s A d
B = f ⁡ B' s B' d * p0 ⁡ A s A d + B' s * p1 ⁡ A s A d + B' d * p2 ⁡ A s A d
A = p0 ⁡ A s A d + p1 ⁡ A s A d + p2 ⁡ A s A d

Where the weighting functions p0, p1, and p2 are defined as follows:

p0 ⁡ A s A d = A s * A d
p1 ⁡ A s A d = A s * 1 - A d
p2 ⁡ A s A d = A d * 1 - A s

Advanced Mode Blend Coefficients: f ⁡ C s C d Conditions
GL_MULTIPLY C s * C d n/a
GL_SCREEN C s + C d - C s * C d n/a
GL_OVERLAY 2 * C s * C d

1 - 2 * 1 - C s * 1 - C d

C d <= 0.5

otherwise

GL_DARKEN min C s C d n/a
GL_LIGHTEN max C s C d n/a
GL_COLORDODGE 0

min 1 C d / 1 - C s

1

C d <= 0

C d > 0 and C s < 1

C d > 0 and C s >= 1

GL_COLORBURN 1

1 - min 1 1 - C d / C s

0

C d >= 1

C d < 1 and C s > 0

C d < 1 and C s <= 0

GL_HARDLIGHT 2 * C s * C d

1 - 2 * 1 - C s * 1 - C d

C s <= 0.5

otherwise

GL_SOFTLIGHT C d - 1 - 2 * C s * C d * 1 - C d

C d + 2 * C s - 1 * C d * 16 * C d - 12 * C d + 3

C d + 2 * C s - 1 * sqrt C d - C d

C s <= 0.5

C s > 0.5 and C d <= 0.25

C s > 0.5 and C d > 0.25

GL_DIFFERENCE abs C d - C s n/a
GL_EXCLUSION C s + C d - 2 * C s * C d n/a

For the HSL modes, the following functions are defined:

float minv3 vec3 c { return min(min(c.r, c.g), c.b); }
float maxv3 vec3 c { return max(max(c.r, c.g), c.b); }
float lumv3 vec3 c { return dot(c, vec3(0.30, 0.59, 0.11)); }
float satv3 vec3 c { return maxv3(c) - minv3(c); }
vec3 SetLum vec3 cbase vec3 clum {

float lbase = lumv3(cbase);

float llum = lumv3(clum);

float ldiff = llum - lbase;

vec3 color = cbase + vec3(ldiff);

if (minv3(color) < 0.0 ) {

return llum + ((color-llum)*llum) / (llum-minv3(color));

} else if (maxv3(color) > 1.0 ) {

return llum + ((color-llum)*(1-llum)) / (maxv3(color)-llum);

} else {

return color;

}

}
vec3 SetLumSat vec3 cbase vec3 csat vec3 clum {

float minbase = minv3(cbase);

float sbase = satv3(cbase);

float ssat = satv3(csat);

vec3 color;

if (sbase > 0 ) {

color = (cbase - minbase) * ssat / sbase;

} else {

color = vec3(0.0);

}

return SetLum(color, clum);

}
HSL Mode Blend Coefficients: f ⁡ C s C d
GL_HSL_HUE SetLumSat C s C d C d
GL_HSL_SATURATION SetLumSat C d C s C d
GL_HSL_COLOR SetLum C s C d
GL_HSL_LUMINOSITY SetLum C d C s

The results of all these equations are clamped to the range 0 1 .

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, both the RGB blend equation and the alpha blend equation are set to GL_FUNC_ADD.

Notes

Only the GL_FUNC_ADD, GL_FUNC_SUBTRACT, and GL_FUNC_REVERSE_SUBTRACT equations use the source or destination factors, while the other equations use only the source and destination colors.

Advanced blending equations are supported only when rendering to a single color buffer using fragment color zero. If any non-NONE draw buffer uses an advanced blend equation the error INVALID_OPERATION is generated by glDrawArrays or any other drawing command.

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_VALUE is generated by glBlendEquationi if buf is greater than or equal to the value of GL_MAX_DRAW_BUFFERS.

Associated Gets

glGet with an argument of GL_BLEND_EQUATION_RGB

glGet with an argument of GL_BLEND_EQUATION_ALPHA

API Version Support

OpenGL ES API Version
Function Name 2.0 3.0 3.1 3.2
glBlendEquation ✔ ✔ ✔ ✔
glBlendEquationi - - - ✔
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