Working with the Curves Controls

You use the Curves adjustment controls when you want to manually set the tonal values of the shadows, midtones, and highlights in an image using a tonal curve. Unlike the Levels adjustment controls, the Curves controls do not reapportion the luminance values in the image by constraining the white and black points. Instead, you use the Curves controls to precisely remap the position of the midtones relative to the white and black points. Because the human eye’s perception of light is logarithmic rather than incremental, a curve is necessary to distribute the luminance values across all tonal ranges in an image in a way that matches how the eye perceives light in nature.

In addition to adjusting the tonal values of an image for overall luminance, you can also color correct an image by applying the Curves adjustment to the red, green, and blue channels independently. Applying a Curves adjustment to each color channel allows you to control the color tonality in an image.

You can also have Aperture automatically identify tonal ranges in an image using the Black Point, Gray Point, and White Point eyedropper tools in the Curves adjustment controls. When you select a tonal range using one of the eyedropper tools, Aperture plots the corresponding curve over the Curves histogram.

You can also brush the Curves adjustment on selected parts of an image. For more information, see Making Brushed Adjustments.

Important: The Curves adjustment controls are only available for images using Aperture 3 image processing. For more information about reprocessing your images, see Reprocessing Images from Previous Versions of Aperture.

An Overview of the Curves Adjustment Controls

Before applying a Curves adjustment, it’s a good idea to get acquainted with the Curves adjustment controls.

Figure. Controls in the Curves area of the Adjustments inspector.
  • Auto Curves Combined button: Click this button to apply a tonal curve adjustment when you want to correct contrast based on total luminance, without affecting the color cast of the image.
  • Auto Curves Separate button: Click this button to apply a tonal curve adjustment when you want to correct contrast as well as remove color casts in the image.
  • Black Point eyedropper tool: Use this eyedropper to set the black point in the image.
  • Gray Point eyedropper tool: Use this eyedropper to set the gray point in the image.
  • White Point eyedropper tool: Use this eyedropper to set the white point in the image.
  • Add Point button: Click this button to use the Loupe to sample pixels in the image and then add a point to the tonal curve for each color channel as well as the RGB tonal curve.
  • In and Out fields: Enter the In and Out point values for a selected point on the tonal curve. The In point represents the horizontal axis (x-axis), and the Out point represents the vertical axis (y-axis).

Working with the Auto Curves Buttons

You use the Auto Curves buttons to analyze the image and apply a tonal curve adjustment to the image based on that analysis. This is a good way to make a quick correction to an image before fine-tuning the Curves adjustment with the manual adjustment controls.

Using the Auto Curves Combined Button to Adjust Combined Luminance

When you want to quickly adjust the tonal curve of an image based on total luminance values—red, green, and blue channels combined—you click the Auto Curves Combined button. Red, green, and blue channels are adjusted by the same amount based on the total luminance. You use the Auto Curves Combined button for luminance when you want to correct the contrast of the image without modifying the color cast of the image.

Figure. Image before and after an Auto Curves Combined adjustment.
To automatically adjust the tonal curve of an image based on total luminance
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

  3. In the Curves area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, click the Auto Curves Combined button.

    Figure. Auto Curves Combined button in the Curves area of the Adjustments inspector.

The tonal curve of the image is adjusted. To fine-tune the tonal curve adjustment, see Adjusting the Tonal Curve of an Image Using the Curves Controls.

Using the Auto Curves Separate Button to Adjust Color Channels Individually

When you want to automatically adjust the tonal curve of an image based on individual evaluations of the red, green, and blue channels, you use the Auto Curves Separate button. Red, green, and blue channels are adjusted based on the evaluation of each channel. You use the Auto Curves Separate button when you want to correct the color cast in the image in addition to correcting the contrast.

Figure. Image before and after an Auto Curves Separate adjustment.
To automatically adjust the tonal curve of an image based on an evaluation of each color channel
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

  3. In the Curves area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, click the Auto Curves Separate button.

    Figure. Auto Curves Separate button in the Curves area of the Adjustments inspector.

The tonal curve of the image is automatically adjusted for each color channel. To fine-tune the tonal curve adjustment, see Adjusting the Tonal Curve of an Image Using the Curves Controls.

Adjusting the Tonal Curve of an Image Using the Curves Controls

You use the tonal curve in the Curves adjustment controls to adjust the overall tonality of an image. There are two types of tonal curve adjustments: RGB and Luminance. When you want to specify a tonal curve adjustment that modifies both the contrast and the tint in the image, you use the RGB tonal curve. When you want to adjust the overall tonality of an image without affecting its color cast, you use the luminance tonal curve.

As you perform a tonal curve adjustment, a histogram is displayed behind the tonal curve and is updated as you manipulate the tonal curve. You can specify the tonal range of the histogram to focus the graph on the part of the tonal curve you plan to work with. For example, if you plan to manipulate the contrast in the shadow areas of the image, you can limit the tonal range of the tonal curve and histogram to display pure black to 50 percent gray only. You can also extend the range of the histogram and tonal curve beyond pure white to capture highlight detail that would be clipped otherwise.

You can also adjust the tonal curve of each color channel independently. For more information, see Using the Curves Controls for Color Correction.

Figure. Image before and after a Curves adjustment.
To adjust the tonal curve of an image based on luminance
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

    Figure. Channel pop-up menu and Curves Action pop-up menu in the Curves area of the Adjustments inspector.
  3. Do one of the following:

    • Choose RGB from the Channel pop-up menu.

      Choosing the RGB tonal curve allows you to manipulate the tonal curve of the red, green, and blue color channels combined. This is the proper choice when you want to modify the tint of the image in addition to the contrast.

    • Choose Luminance from the Grayscale section of the Curves Action pop-up menu, then choose Luminance from the Channel pop-up menu.

      Choosing the luminance tonal curve allows you to manipulate the luminance of the combined red, green, and blue color channels without affecting the image’s tint.

  4. Choose the tonal range to display from the Range pop-up menu.

    For more information about the options in the Range pop-up menu, see An Overview of the Curves Adjustment Controls.

  5. Choose the type of tonal curve adjustment to apply to the image from the Type section of the Curves Action pop-up menu.

    Choosing Linear—the default option—applies the Curves adjustment to the image evenly from pure black to pure white and beyond. Choosing Gamma-Corrected applies the Curves adjustment logarithmically to match the way the human eye perceives light, adding additional weight to the shadows.

  6. Drag the Black Point and White Point sliders to where they touch the outside of the histogram graph, constraining the image to its new black and white points.

    Figure. Black Point and White Point sliders in the Curves area of the Adjustments inspector.
  7. Set a point along the curve by doing one of the following:

    • Click a place on the curve where you want to add a point.

      Figure. New point added to the tonal curve in the Curves area of the Adjustments inspector.
    • Click the Add Point button and use the eyedropper tool to sample a portion of the image.

      Note: You can change the Loupe’s color value sample size by choosing an option from the Adjustment Action pop-up menu. For more information, see Sampling Color Values.

      Aperture places a point on the curve in the exact tonal range of the sampled area of the image.

  8. Adjust the shape of the curve by doing one of the following:

    • Drag the point on the curve until that tonal area of the image looks correct.

      For example, dragging a point down in the shadow area of the curve darkens the shadows in the image, and dragging a point up in the shadow area lightens the shadows in the image.

    • Click a point on the curve and enter new In point and Out point values in the In and Out fields.

      The In point values represent the horizontal axis of the graph, and the Out point values represent the vertical axis of the graph.

    Points on the curve that are not selected remain anchored, allowing you to independently adjust each tonal area.

    Figure. An S-curve in the Curves area of the Adjustments inspector.
  9. Repeat steps 7 and 8 for each tonal area in the image that you want to correct.

The tonal values in the image are adjusted, giving the image the appropriate amount of contrast.

Using Eyedropper Tools to Set Black, Gray, and White Points

You use the Black Point, Gray Point, and White Point eyedropper tools in the Curves adjustment controls when you want to have Aperture selectively modify the tonal values of the shadows, midtones, and highlights in an image automatically. Using an eyedropper tool, you sample the pixels of the tonal value in the image you intend to correct, and Aperture places a point on the tonal curve and automatically adjusts the image so that the pixels that fall in the same tonal range as the sampled pixels are set to either black, gray, or white. For example, you use the Black Point eyedropper tool to select the specific area of the image that should be black. Aperture samples the pixels within the selection, places a point on the curve, and then adjusts the curve to make the selected pixels—and all other pixels in the image of the same tonal value or darker—black.

In some difficult cases, you can use the Black Point, Gray Point, and White Point eyedropper tools in combination with points that you manually place on the tonal curve to neutralize a tint or create the specific contrast you want in the image. For more information about manually adjusting the tonal curve, see Adjusting the Tonal Curve of an Image Using the Curves Controls.

You use the Black Point eyedropper to automatically set the black point in the image.

Figure. Image before and after a Black Point Curves adjustment.
To set the black point in an image
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

  3. Select the Black Point eyedropper tool to activate the Loupe.

    Figure. Black Point eyedropper tool in the Curves area of the Adjustments inspector.

    The pointer changes to an eyedropper, and the Loupe appears, showing a magnified view of the target area. By default, the Loupe is set to magnify the image to 100 percent (full size). If necessary, you can increase the magnification of the Loupe by choosing a larger magnification value from the Loupe pop-up menu. For more information, see Viewing Images with the Loupe.

  4. Position the eyedropper over the darkest pixels in the image, then click.

    Figure. Loupe showing a magnified view of the darkest pixels in the image.

The sampled pixels are used to set the black point in the image.

You use the Gray Point eyedropper to automatically set the gray point in the image.

Figure. Image before and after a Gray Point Curves adjustment.
To set the gray point in an image
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

  3. Select the Gray Point eyedropper tool to activate the Loupe.

    Figure. Gray Point eyedropper tool in the Curves area of the Adjustments inspector.

    The pointer changes to an eyedropper, and the Loupe appears, showing a magnified view of the target area. By default, the Loupe is set to magnify the image to 100 percent (full size). If necessary, you can increase the magnification of the Loupe by choosing a larger magnification value from the Loupe pop-up menu. For more information, see Viewing Images with the Loupe.

  4. Position the eyedropper over a midrange color that is as close as possible to medium gray, then click.

    Figure. Loupe showing a magnified view of medium gray color pixels in the image.

The sampled pixels are used to set the gray point in the image.

You use the White Point eyedropper to automatically set the white point in the image.

Figure. Image before and after a White Point Curves adjustment.
To set the white point in an image
  1. Select an image.

  2. If the Curves controls are not shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Adjustments pop-up menu.

  3. Select the White Point eyedropper tool to activate the Loupe.

    Figure. White Point eyedropper tool in the Curves area of the Adjustments inspector.

    The pointer changes to an eyedropper, and the Loupe appears, showing a magnified view of the target area. By default, the Loupe is set to magnify the image to 100 percent (full size). If necessary, you can increase the magnification of the Loupe by choosing a larger magnification value from the Loupe pop-up menu. For more information, see Viewing Images with the Loupe.

  4. Position the eyedropper over the lightest pixels in the image, then click.

    Figure. Loupe showing a magnified view of the lightest pixels in the image.

The sampled pixels are used to set the white point in the image.

Using the Curves Controls for Color Correction

One of the most powerful ways to remove a color cast from an image or accentuate a desirable color cast is to adjust the tonal curves of the red, green, and blue color channels independently. It’s important to understand that you’re adjusting the colors of the image within the RGB spectrum; therefore, you reduce yellow when you increase blue, you reduce magenta when you increase green, and so on.

To adjust the tonal curve of a single color channel in an image
  1. In the Curves area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose a single color channel from the Channel pop-up menu.

    Figure. Color Channel pop-up menu in the Curves area of the Adjustments inspector.
  2. Place points on the curve, and adjust them as necessary, to remove or accentuate a color cast in the image.

    For more information about placing points on the tonal curve, see Adjusting the Tonal Curve of an Image Using the Curves Controls.

Adjusting the Tonal Curve of the Red Color Channel

You adjust the tonal curve of the red color channel when you want to remove red and cyan color casts in the image.

Figure. Image before and after a red tonal curve adjustment.
Red adjustment
Effect
Move tonal curve down in the shadows
Adds cyan to the shadows, removing red.
Move tonal curve up in the shadows
Adds red to the shadows, removing cyan.
Move tonal curve down in the midtones
Adds cyan to the midtones, removing red.
Move tonal curve up in the midtones
Adds red to the midtones, removing cyan.
Move tonal curve down in the highlights
Adds cyan to the highlights, removing red.
Move tonal curve up in the highlights
Adds red to the highlights, removing cyan.

Adjusting the Tonal Curve of the Green Color Channel

You adjust the tonal curve of the green color channel when you want to remove green and magenta color casts in the image.

Figure. Image before and after a green tonal curve adjustment.
Green adjustment
Effect
Move tonal curve down in the shadows
Adds magenta to the shadows, removing green.
Move tonal curve up in the shadows
Adds green to the shadows, removing magenta.
Move tonal curve down in the midtones
Adds magenta to the midtones, removing green.
Move tonal curve up in the midtones
Adds green to the midtones, removing magenta.
Move tonal curve down in the highlights
Adds magenta to the highlights, removing green.
Move tonal curve up in the highlights
Adds green to the highlights, removing magenta.

Adjusting the Tonal Curve of the Blue Color Channel

You adjust the tonal curve of the blue color channel when you want to remove blue and yellow color casts in the image.

Figure. Image before and after a blue tonal curve adjustment.
Blue adjustment
Effect
Move tonal curve down in the shadows
Adds yellow to the shadows, removing blue.
Move tonal curve up in the shadows
Adds blue to the shadows, removing yellow.
Move tonal curve down in the midtones
Adds yellow to the midtones, removing blue.
Move tonal curve up in the midtones
Adds blue to the midtones, removing yellow.
Move tonal curve down in the highlights
Adds yellow to the highlights, removing blue.
Move tonal curve up in the highlights
Adds blue to the highlights, removing yellow.