We could have a case where we do not want to interpolate the source and destination fragment colors with scale or add operations. For example, we may want to subtract one from the other. In this case, WebGL provides the gl.blendEquation function. This function receives one parameter that determines the operation on the scaled source and destination fragment colors. For example, gl.blendEquation(gl.FUNC_ADD) is calculated as such:

color = S * sW + D * dW;

And, gl.blendEquation(gl.FUNC_SUBTRACT) corresponds to the following:

color = S * sW - D  * dW;

There is a third option, gl.blendEquation(gl.FUNC_REVERSE_SUBTRACT), that corresponds to the following:

color = D* dw - S * sW;

As expected, you can define the blending equation separately for the RGB channels and for the alpha channel. For that, we use the gl.blendEquationSeparate function.