Depth, blending & cullingGL 1.4

glBlendEquation

Chooses how the two blended terms are combined: added (default), subtracted, or the minimum or maximum.

Signature

void glBlendEquation(GLenum mode);

How it works

glBlendFunc picks the two factors; this picks the operation between the two products. With S = source × sfactor and D = destination × dfactor:

GL_FUNC_ADD: S + D GL_FUNC_SUBTRACT: S − D GL_FUNC_REVERSE_SUBTRACT: D − S GL_MIN / GL_MAX: the smaller or larger of source and destination, channel by channel (the factors are ignored).

Results are clamped to [0, 1] in a normal 8-bit framebuffer.

Parameters

modeGLenum

The operation.

Common values

GL_FUNC_ADD: Add (default): transparency, additive glow.
GL_FUNC_REVERSE_SUBTRACT: Stored minus new: darken what is behind (shadows painted on top, ink).
GL_FUNC_SUBTRACT: New minus stored.
GL_MIN / GL_MAX: Keep the darker or brighter value per channel.

Tips & pitfalls

The equation stays set: return to GL_FUNC_ADD after a special pass, or later transparent objects blend strangely.
glBlendEquationSeparate takes one mode for colour and one for alpha.

Errors

When the call is invalid it does nothing and sets an error, which glGetError returns:

GL_INVALID_ENUM: mode is not one of the five operations.

Example

example.cpp

1glBlendFunc(GL_ONE, GL_ONE);
2glBlendEquation(GL_FUNC_REVERSE_SUBTRACT); // darken
3drawDarkDecal();
4glBlendEquation(GL_FUNC_ADD);              // back to normal

Official Khronos documentation