Blending in OpenGL

OpenGL has a wide array of different blend functions that it will do for you with the change of a state. 

However getting your scene to blend nicely can be tricky, especially when you have multiple passes you want to composite together and having transparent objects just makes blending even trickier.

The OpenGL wiki has a fantastic page about all of the different blending options in OpenGL so I won’t spend a lot of time restating what that very descriptive wiki page already states. 

The basic idea of blending is: if you are drawing an object on top of another, how do the overlapping fragments behave.

 If you have an opaque object with depth testing enabled, the fragments that belong to the object that is closer to the camera will be drawn with full opacity. Simple enough but if you have a transparent object in front of an object, you need to take the alpha channel of the fragment into considering. Fortunately there are built in OpenGL blend functions that do this for you. However it is up to you to draw the objects in the correct order (transparent objects are drawn last).

Here is a demonstration of the light accumulation pass in A Case of the Mondays using  a couple of different blending modes.

Above is a screenshot of the light accumulation using the following blending states: 

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Here is the same screenshot using different blending options:

glBlendEquation(GL_FUNC_ADD);

glBlendFunc(GL_ONE, GL_ONE);

Notice that the two images look very different and all I did was change a few OpenGL states. The first image is a lot darker than the second. In the game we use the second option simply because the light has more of a presence and therefore influence in the scene.