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Grokking The Gimp
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5.7 Opacity and Transparency

Transparency is usually not considered to be a blending mode, however, it works just like one! Transparency is just a combination of the Multiply and Addition blending modes; it takes a percentage of the foreground and adds it to the complementary percentage of the background. Thus, if you want the foreground to be 75% opaque (opacity is just the opposite of transparency), you multiply the foreground by 0.75, the background by (1-0.75), and add the two. This relationship can be expressed as

\begin{displaymath}R=\alpha F+(1-\alpha)B
\end{displaymath}

where $\alpha$ represents the opacity. The percentages used in scaling the foreground and background pixels are called complementary because $\alpha + (1-\alpha) = 1$.

By the way, can you guess why the percentage of opacity is represented by the Greek letter alpha in the preceding equation? It is because the opacity of a layer is controlled by its alpha channel. The alpha channel takes values in the range [0,255] where a value of 255 represents 100% opacity, and 0 represents 100% transparency.

Different amounts of transparency/opacity can be obtained by adjusting the value of $\alpha$. For a foreground and background pixel pair, the effect of opacity can be illustrated in the RGB cube. Figure  5.24

  
Figure 5.24: Explanation of Opacity in the RGB Cube
Figure 5.24

shows how combining a percentage of the foreground with a complementary percentage of the background creates a resultant pixel that is somewhere on a line between the two points. The two blue arrows in Figure  5.24 show the locations in the RGB cube of a foreground and background pixel. The line between the two points represents the set of pixels obtained for varying degrees of opacity. The three red arrows show the pixels corresponding to 25, 50, and 75% opacity of the foreground with respect to the background pixel.

Figure  5.25

  
Figure 5.25: An Example Application of Transparency
Figure 5.25

illustrates an example of setting the opacity of a layer. Figure  5.25(a) shows a two-layer image consisting of herons in the upper layer and flowers in the lower. When the Opacity slider for the upper layer is set to 75%, as shown in the Layers dialog in Figure  5.25(b), the result is as shown in Figure  5.25(c).

Transparency/opacity can also be controlled for the GIMP's painting functions. The Opacity slider for these are found in the Brush Selection dialog.




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