Blending
RGBA stands for red green blue alpha. It is the three-channel RGB color model supplemented with a fourth alpha channel. Alpha indicates how opaque each pixel is and allows an image to be combined over others using alpha compositing, with transparent areas and anti-aliasing of the edges of opaque regions.
Alpha compositing is the process of combining one image with a background to create the appearance of partial or full transparency.
Alpha blending is the process of combining a translucent foreground color with a background color, thereby producing a new color blended between the two. The degree of the foreground color's translucency may range from completely transparent to completely opaque. If the foreground color is completely transparent, the blended color will be the background color. Conversely, if it is completely opaque, the blended color will be the foreground color. The translucency can range between these extremes, in which case the blended color is computed as a weighted average of the foreground and background colors.
In the formula A represents the pixel's tonal value that lies in the upper layer (or the foreground), d stands for this layer's opacity. B stands for the pixel's tonal value that lies in the lower layer (or the background) and C is the tonal value of the blended pixel.