Correcting tonality will improve the quality of almost any image. GIMP offers several options to do this.
The Levels options (tonality correction) can be found via Tools > Color Tools > Levels or Colors > Levels.
The most striking effect in the Levels dialog is the Input Levels curve, which is referred to as the color histogram of the image. Initially, it is created from the image's RGB color channel (Channel: Value).
The curve shows how the color lightness values are distributed in the image. In the recently modified miami.tif image, you can see that the curve starts a short distance from the left margin and ends before the right margin. Roughly speaking, this means that the image does not possess "real" black values (shadows). It also tells you that the image has only a small amount of "real" white values (highlights).
Underneath the histogram are the numerical values for the image's lightness (Output Levels). There is also a black-to-white gradient bar, which corresponds to these values. You can move the triangles under the gradient to change the lightness of the image. This tool works similarly to the settings in the Brightness-Contrast dialog. However, you'll probably find that the handling of the Brightness-Contrast dialog is more comfortable.
The Levels window.
Directly underneath the histogram curve you find another gradient bar. Just below, you see black, gray, and white triangles that correspond to the shadows, midtones, and highlights in the image. These are initially positioned at the margins of the histogram window and in its center.
By moving these triangles from the border into the area of the histogram's curve, you can adjust the brightness values of the image toward the target values. If you move the black triangle to the right, just under the histogram, the dark colors in the image become darker. Accordingly, the bright colors become brighter when you adjust the white triangle. You can correct the brightness of the midtones by moving the gray triangle, which increases the color gamut and contrast of the image.
Make sure the Preview option is checked in the Levels dialog so you can see the effect of your changes as you edit.
When you are satisfied with the result, click OK to confirm your changes.
USING GIMP: CORRECTING AND TOUCHING UP YOUR IMAGES
Comparing the image, before and after
Comparing the image, before and after
At the top of the Levels dialog is a Preset drop-down menu. Since GIMP 2.6, you can find this in almost all program windows. You can use this menu to name and save your settings.
Click the Channel: Value drop-down menu (top left in Figure 2.46) to edit each of the red, green, and blue color channels individually. This is important when working with color cast images (more about this later).
To the right of the Channel: Value menu, you'll see two additional buttons. If you hover your cursor over these buttons, you'll see that the left button is called Linear and the right button is called Logarithmic. Depending on the button you select, the representation of the histogram curve will change. The logarithmic method is more data intensive, thus also more exact. The histogram you see when you select the linear method is more significant, so this is the method that was chosen for this exercise. For most images the logarithmic histogram is smooth and quite flat.
You also find buttons represented by black, gray, and white eyedropper icons. If you select the black eyedropper and click on an area of the image that should be set in pure black, the program will recalculate the lightness values. The same holds true for the white eyedropper. Using the black and white eyedroppers may be sufficient to obtain a good tonality correction. The additional gray eyedropper can be used to set the midtones of the image. This can be helpful when working with colorcast images because it tells the program what hue you want to assign to a gray shade (e.g., a shadow on a white surface).
If you wish, you can open the image with the settings that you just made for further editing in the Curve function (gradation curves) by selecting the Edit these Settings as Curves button.
The Reset button allows you to discard your settings without closing the window.
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