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Grokking The Gimp
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Antialiasing is an important edge treatment for selections. Figure  3.11

Figure 3.11: How Antialiasing Works
Figure 3.11

illustrates the antialiasing concept. Figure  3.11(a) shows an array of pixels that has been partitioned into two regions by a selection edge. However, due to the slope of the selection and the finite area of the pixels, some pixels are on both sides of the selection edge. That is, these pixels are only partially selected. What happens to this set of partially selected pixels is important for the aesthetic presentation of the selection's edge.

For example, let's assume that white represents a selected pixel and black an unselected one. Figure  3.11(b) shows what happens if pixels are included in the selection when more than 50% of the pixel is above the selection edge and unselected otherwise. This seems like a reasonable criterion, however, notice that the selection edge obtained by applying this rule produces a staircase effect on the edge. This staircase effect, known as aliasing,  makes the edge look harsh. We'll see this in a more realistic example in a moment.

Alternatively, Figure  3.11(c) illustrates the concept of antialiasing. Here white represents a pixel which is fully selected, black one that is fully unselected, and gray represents partially selected pixels, where the level of gray indicates the percentage of the pixel that falls inside the selection. Thus, a lighter value of gray indicates a more fully selected pixel and a darker value a less selected one. Assigning gray values to partially selected pixels has the effect of visually smoothing the staircase effect illustrated in Figure  3.11(b), which is why this is called antialiasing.

The way antialiasing is actually implemented is by using the layer's alpha channel.  Alpha channels were introduced in Section  2.2 and a more comprehensive presentation of them is given in Chapters  4 and 5. However, for the purposes of discussing antialiasing it is sufficient to know that the white pixels in Figure  3.11(c) represent pixels that are fully opaque, the black pixels those that are fully transparent, and the gray pixels those that are partially opaque (or transparent).

Figure  3.12

Figure 3.12: A Practical Example of Using Antialiasing
Figure 3.12

illustrates a more realistic example of aliased and antialiased pixels. Figures  3.12(a) and (b) each show a circle created using the Ellipse Select tool. For both, the selections were filled with black using the Bucket Fill  tool. The selection made in part (a) of the figure was made without antialiasing, and that in part (b) with. From these two figures it can immediately be seen that the antialiased circle seems to have a much smoother edge.

Figures  3.12(c) and (d) show zoomed versions of Figures  3.12(a) and (b). The staircase effect can be clearly seen in Figure  3.12(c). Alternatively, in Figure  3.12(d) there are black, edge pixels that are partially transparent and that allow part of the yellow background to show through. Figures  3.12(b) and (d) demonstrate that antialiasing really does improve the aesthetic appearance of the selection edge. Figure  3.12(e) shows that the Antialiasing checkbox in the Tool Options dialog for the Ellipse Select tool is toggled on for Figures  3.12(b) and (d). For all the selection tools, Antialiasing is on by default.

As a final remark, note that the Rectangle Select tool does not have an antialiasing option. This is normal because this selection tool can never produce sloping edges. Consequently, the staircase problem illustrated in Figure  3.11 can never occur, and antialiasing is not needed.

Grokking The Gimp
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