Chapter 20, Color Correction
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Inverting an Image |
Chapter 20, Color Correction |
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Setting Exposure, Contrast, Pivot Point, and Lift of an Image
The Photo Lab tool lets you simulate camera exposure and photo development changes in images. You can change the exposure to brighten or darken an image in incremental steps, providing perceptually relative uniform changes in luminance. Photo development adjustments can produce images with different color distribution.
With the Photo Lab tool, you can set the exposure, gamma, pivot, and lift of each color channel independently and in a variety of units, such as F-stops or printer lights for exposure.
The input image is first subjected to a gain adjustment, then to a gamma correction around a pivot point, and finally to a lift. Each step is optional.
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Note: Many of the Photo Lab tool's fields are animatable attributes--see Setting Keys Manually and Validating and Applying the Expression String.
Exposure -- Exposure can be expressed in terms of F-stops, printer lights, or gain. As with cameras, increasing the exposure by one F-stop results in an image with the luminance value that is double that of the original.
Printer light units work very much like F-stops, but it usually takes six printer light units (default setting) to get the same effect that you get if you set exposure by one F-stop. If you are trying to simulate a calibrated printing device for which you have accurate measurements, Photo Lab allows you to specify how many printer lights there are in one F-stop. Increasing the exposure by one F-stop also increases the gain by a factor of two.
Contrast and Pivot -- Gamma correction is applied about a pivot value. In other words, pixel values equal to the pivot value are left unchanged. This is useful when you want to use gamma correction to change the contrast of an image, but do not want to affect a particular luminance level. The amount of gamma correction can be expressed in terms of contrast or in terms of the usual gamma exponent. Increasing the gamma exponent decreases contrast.
Lift -- The lift is simply a uniform offset added to all the pixel values (different offsets for different color components). Lift is usually applied as the last step of the process and can be used to control the overall brightness of the image.
Note: This tool simulates the physical workings of camera exposures and printing devices as long as the media is encoded in a linear color space. All channels are linked by default.
Use the following procedures to set values for exposure, contrast, pivot point and lift. Note that menus are available for selecting exposure and contrast units.
From the Tools tab, drag a Photo Lab tool from the Color Correction folder to the dependency graph in the Schematic view.
Select exposure and contrast units.
Set Printer Lights per F-stop by dragging the field to the right or to the left.
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Do one of the following:
Drag any of the red (R), green (G), or blue (B) channel fields to the right to increase or to the left to decrease levels uniformly on all three channels. Alternatively you can also drag the overall luminance (L) field to achieve the same result.
Enter a value into any channel's field and press Enter.
Select a single channel that you want to modify by deselecting Lock, then drag the channel field to the right to increase, or to the left to decrease, the values of the channel. You may also enter a value into any channel's field and press Enter.
The other two channel fields and the overall luminance field update to reflect the change.
The trackball allows you to modify values with a more freestyle, intuitive approach. The trackball emulates a typical 360-degree color wheel that can be dynamically changed by dragging the center of the color wheel. Consequently, the effect on the red, blue, and green channels is predictable.
For example, dragging the trackball towards the red portion of the color wheel increases the value of the red channel, but decreases the values of both the blue and green channels, and adds blue and green to the shadows and midtones of the image.
Modifications made using the trackball are cumulative; each movement of the trackball is added to the previous one. The trackball changes color to reflect the degree of change.
Drag the trackball to modify levels in the image.
Note: To reset the trackball, channel, and luminance levels, select the Reset button below the trackball or the Reset button in Tool Options area on the right side of the UI (the latter resets everything).
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