At this point, we draw four graphical layers:
The first three layers are simply drawn occluding each other. Only the fourth layer has control of its transparency (color a slider), so we can hide and show it back smoothly.
To control the visibility of the first three layers, let's create a video mixer. It will be a new GUI group, Mixer, consisting of three sliders, adjusting layers' transparencies. Additionally, we will draw the layers using a special mixing mode called additive blending. It's a mixing mode where drawing colors at each pixel are summed up to obtain the resultant picture.
Additive blending models ...