The best way to understand Multiply Layer mode is to know the mathematic formula behind the Multiply Mode. The formula for Multiply Layer Mode are surprisingly simple.
Every pixel consist of 3 channels; Red, Green and Blue. Multiply mode will calculate each channel respectively. For example, if upper layer content 1 pixel with RGB value of (255,0,150) and at the same spot in lower layer content 1 pixel with RGB value of (100,122,33). Multiply mode will first take Red channel, which is 255 at upper layer and 100 at lower layer. Run the Multiply Mode formula
We get (255/255)*100 = 100. Therefore the red value will be 100. Do the same calculation to the remaining channels, we got the final result of (100,0,19). Now you know exactly what is Multiply Mode, you might ask me what we can do with Multiply mode? It is entirely depend on your work, you may not needed it for the less of your life, or you might needed it in everyday work.
For those who are not used to interpret mathematic formula, see the following note for more information:
Instead of immediately showing you how to do a cool layer mode trick that might be rarely use by you, we will explain the true mechanism working behind every layer mode. By fully understanding the fundamental of every layer mode, you will be able to apply layer mode into any problem you might run into when the time come.
Layer mode is about blending the content of two or more layers together to produce intended result. The result of one blending can be both final artistically result or intermediate result that use for further image correction or enchancement.
Layer mode blending the content of layers by calculating the Red, Green and Blue value of every pixels against its lower layer Red, Green and Blue value. Almost all layer mode do calculation on each color channel separately, that is mean upper layer Red value will be only use for calculating against lower layer Red value, and same thing happen to Green and Blue.
While most of the layer mode do calculation on RGB channel, some layer mode do use HSB(Hue, saturation and brightness) for the calculation, e.g. Hue, saturation, color and luminosity mode.