OpenGL blending; what exactly do Source Color and Destination Color refer to, with examples

Question

I'm confused by exactly what this means:

When doing blending for a particular buffer, the color from the fragment output is called the source color. The color currently in the buffer is called the destination color.

(from the OpenGL wiki)

I understand what the blending equation itself is, but I do not understand quite the distinction between source color and destination color.

Can anyone provide an example or more specific definition?

Answer 1

To keep it short and simple:

• Source color: This is the color you're currently using. For example, when you use glColor4f(...) you set the source color of the operation.
• Destination color: This is the color of the fragment (pixel if you prefer to think it this way, though it's not necessarily the same) in a certain coordinate in the video buffer.

Usually, the reason why one uses:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Is because you want to use the alpha value you just supplied. You use it to multiply your current color, and then use (1 - alpha) and use it to multiply the buffer's current color in that coordinate.

Therefore, if you paint a quad with glColor4f(1.0f, 1.0f, 1.0f, 0.6f) and the buffer is filled with glColor4f(1.0f, 0.0f, 0.0f, 1.0f), the final operation will be:

(1.0f, 1.0f, 1.0f) * ALPHA + (1.0f, 0.0f, 0.0f) * (1 - ALPHA)
(0.6f, 0.6f, 0.6f) + (0.4f, 0.0f, 0.0f)

So the final color is (1.0f, 0.6f, 0.6f)