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Alpha Transparency

  • Category: Gimp
  • Published: Monday, 03 July 2017 16:31
  • Written by Super User
  • Hits: 484


Most people are familiar with the concept that Red, Green and Blue make a full colour image. In computer graphics there is a fourth channel, called alpha. What is it and how does it work?


 When a new image is created by Gimp the image has the three colour channels (R,G and B). There are default foreground and background colours which can be seen and changed using the last item in the Gimp Toolbox (found in the Windows menu). If your content does not use all of the available pixels then, by default, they will display the background colour If we go to “Layers, dockable dialogues, channels” we can see and manage the channels in use. As expected there are Red, Green and Blue channels.


If we then create a duplicate layer (using “Layer, duplicate layer”) another property – Alpha - is added. In fact we could also have added an Alpha channel to the original layer.  What does it do?


Most activities in computer graphics involve building several layers, each containing some form of image. At the end of our edit we usually need to collapse all of the layers into an image on only one layer. This process is referred to as compositing.

 sums on blackboard

Now, suppose our foreground colour as yellow and our background colour is blue. Set the first layer to the colour blue and the image will appear blue.


Create the duplicate layer but set its colour to yellow and the image will change from Blue to yellow.


If we change the Alpha channel (transparency) to 50% the the image will now appear green. How has the Alpha channel achieved this when each pixel can only have one value of R, G and B. The answer is that the graphics program adds the values of R,G and B provided by the image’s two layers. The pixel displays the colour defined by the sum of the two layers.


Naturally, things are not quite that simple and there are many ways to produce an R,G and B value for each pixel (multiplication, subtraction etc) and these are available for you experiment within Gimp.


If you want more detail then a good starting point is:- https://en.wikipedia.org/wiki/Alpha_compositing

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