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Grokking The Gimp
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6.2.7.2 The Brightness-Contrast Tool

Figure  6.26(a)

  
Figure 6.26: Comparing Brightness-Contrast with the Curves Tool
Figure 6.26

displays the dialog for the Brightness-Contrast tool. The way this tool functions can be fully simulated using the Curves tool. Figure  6.26(b) illustrates the Value channel of the Curves tool, showing that the curve has been displaced upward. Moving the value curve up maps the input value domain to an output range that is uniformly brighter. This modification to the value curve exactly simulates the action of the Brightness-Contrast tool when the Brightness slider in the dialog is moved to the right. Moving the Brightness slider to the left corresponds to displacing the value curve downward.

Thus, brightening or darkening an image using the Brightness slider has the effect of diminishing tonal range because the result maps the input value domain to a smaller output range. From the discussion in Section  6.1, this is clearly a disadvantage. The Curves tool, on the other hand, can be used to increase or decrease brightness without loosing tonal range. To brighten an image place a control point on the Value curve and displace the point upwards. This brightens the image without losing tonal range. To darken the image drag the point downwards.

Figure  6.26(c) also shows the Value channel of the Curves tool. Here, the curve has been rotated counter-clockwise around the center of the input-output dialog. This has the effect of increasing contrast in the midtones of the image. However, it simultaneously eliminates detail in the shadow and highlight ranges. This action is exactly what the Brightness-Contrast tool does when the Contrast slider of the dialog is moved to the right. Moving the slider to the left corresponds to rotating the curve in the clockwise direction.

The conclusion is that the Curves tool performs much better than the Brightness-Contrast tool.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire