WebGL includes a fourth attribute to the RGB model. This attribute is called the alpha channel. The extended model then is known as the RGBA model, where A stands for alpha. The alpha channel contains values in the range from 0.0 to 1.0, just like the other three channels (red, green, and blue). The following diagram shows the RGBA color space. On the horizontal axis you can see the different colors that can be obtained by combining the R, G, and B channels. The vertical axis corresponds to the alpha channel.
The alpha channel carries extra information about the color. This information affects the way the color is rendered on the screen. For instance, in most cases, the alpha value will refer to the amount of opacity that the color contains. A completely opaque color will have an alpha value of 1.0, whereas a completely transparent color will have an alpha value of 0.0. This is the general case, but as we will see later on, there are some considerations that we need...