Running Linux, 5th Edition - Matthias Kalle Dalheimer [170]
Figure 9-17. Original photo
Let's fix that problem using the Levels dialog! Open the dialog for the Levels tool by choosing Levels from the menu. The dialog can be seen in Figure 9-18.
Figure 9-18. Levels dialog
The diagram seen under "Input Levels" is a histogram of the brightness values in the image. The left end of the histogram represents black, and the right end white. We see that that the lower 40% of the histogram is empty—this means that we are wasting useful dynamic range. Below the histogram are three triangular sliders. The black and the white ones are for setting the darkest and brightest point in the image, and the gray one is for adjusting how values are distributed within the two other ones. We can move the black point up as shown in Figure 9-19 to remove the haziness of the image. The result is shown in Figure 9-20.
Contrast enhancement can be done either with the Brightness-Contrast tool or with the Curves tool. The former is quite basic consisting of two sliders, one for brightness and one for contrast; the latter allows much more control. Figure 9-21 shows an original image and two modified versions with different curves applied. The middle image has the contrast-enhancing curve shown in Figure 9-22 applied, and the right image has the contrast-decreasing curve shown in Figure 9-23 applied. The curves describe a mapping from pixel values onto itself. A straight line at a 45-degree slope is the identity mapping; anything else will modify the image. Best results are obtained if you only deviate a little bit from the 45-degree straight line.
Figure 9-19. Levels dialog
Figure 9-20. Level adjusted
Figure 9-21. Curve adjusted
Colors can be changed with several tools, such as the Color Balance and Hue-Saturation tools. The Levels and Curves tools can also be set to operate on individual color channels to achieve various effects. But there is also another tool available: the Channel Mixer. Unlike the other tools this is located in the Filters/Colors/Channel Mixer context menu. The Channel Mixer can be used to create a weighted mix of each color channel (red, green, and blue) for each of the output channels. It is particularly useful for converting color images to monochrome, often giving better results than simply desaturating the image. Figure 9-24 shows the Channel Mixer, and Figure 9-25 shows two monochrome versions of the same color image. The upper one is simply desaturated, and the lower one is based only on the blue channel and seems to emphasize the bird rather than the background. When judging how to convert a color image to monochrome, it can be helpful to examine each color component individually. See the paragraph about channels for more about this.
Layers and channels. The most convenient way to access layers and channels is through the combined layers, channels, paths, and undo history window. It can be accessed by right-clicking in the image's windows and selecting the Dialogs → Create New Dock → Layers, Channels & Paths menu item. Layers and channels allow you to view and manipulate different aspects of your images in a structured way.
Figure 9-22. Contrast-enhancing curve
Channels
An image is made up from one or more channel(s). True color images have three color channels, one for each of the red, green, and blue components. Index-colored and grayscale images have only one color channel. All types can have an optional alpha channel that describes the opacity of the image (white is completely opaque; black is completely transparent). By toggling the eye button for each channel, you can selectively view only a subset of the channels in an image. Channels can be selected or deselected for manipulation. For normal operation, all color channels are selected, but if you only want to paint into the red channel, for example, deselect the other channels. All drawing operations will then only affect the red channel. This can, for example, be used to remove the red flash