Photoshop Curves
Anchor points

 

Anchor Points

The purpose of anchor points is to allow us to adjust tone by reshaping the curve.  By placing anchors on the curve and then moving the anchors, we change the relationship between the input tones and the output tones.  We can also use anchors to lock part of the curve so when we make a change in one part of the curve the rest of the curve does not change.
 

Adobe Photoshop automatically includes anchors for the black and white points.  In addition, we can add up to 14 additional anchor points of our choosing for a total of 16 points.

Figure 1 shows an example of reshaping the curve using two anchor points.  One anchor point was added at tone 64 and another anchor was added to tone 192.  Tone 64 is the tone of the dark gray chip on the left and tone 192 is the tone of the light gray chip on the right.  For this particular example, the tone of the dark gray chip was raised to 94 and the tone of the light gray chip was lowered to 162.  This should lighten the dark gray chip and darken the light gray chip.  Mouse over Figure 1 to see the result.  As the red arrows show, the tones between the dark and light gray chips became closer.  Therefore, the contrast between the two has decreased.
 

Adding Anchor Points

To add an anchor to a curve, click on the curve.  If the anchor is not exactly where needed, use the Left/Right arrow keys to change the Input tone value and use the Up/Down arrow keys to change the Output tone value in increments of two.  If the Shift key is pressed while using the arrow keys, tone changes by increments of fifteen.  If we click in either the Input or Output boxes first, the arrow keys increment by one and Shift + arrow keys increment by ten.

 

Adding an Anchor Based on Image Tone

We can add an anchor to the composite channel's curve or to an individual channel's curve based on the image tone itself.  First, change the Channel drop down box on the Curves dialog box to the desired channel.  Inside the image, Ctrl - click-hold (Command - click-hold) the left mouse button and drag over the image.  When the desired location is found, release the left mouse button.  This will place an anchor on the selected channel's curve.  Even though it is not necessary to continue to press the Ctrl (Command) key while dragging, it is necessary for the Ctrl (Command) key to be held down while first clicking.

Or, Ctrl - click (Command - click) the area of the image.  This is the same as the preceding paragraph, but it removes the option of dragging over the image.
 

Deleting Anchor Points

To delete an anchor, click and hold the anchor and drag it out of the grid.  As we drag, the curve will reshape itself.  But once we drag the anchor out of the grid, the curve will snap back to the appropriate shape.

Or, activate the anchor and press the Delete key.
 

Navigating Anchors

With the Curves dialog box open, hold down the Ctrl (Command) key and press the Tab key.  Photoshop will activate an anchor.  An anchor is activated when its circle becomes solid black.  As we continue to Ctrl + Tab (Command + Tab), Photoshop will deactivate the current anchor and activate the next anchor.  Photoshop will cycle through the anchors from black to white.  If we Shift + Ctrl + Tab (Shift + Command + Tab), Photoshop will cycle through the anchors from white to black.
 

Using Anchors to Lock the Curve

Both Levels and Curves are non-linear adjustments.  This means the closer a pixel is to the change the more it will be affected and the further away it is the less it will be affected.  When we use Levels we cannot isolate the nearby pixels from the change.  We can with Curves.

In Figure 2, an anchor was placed on tone 192, which is the tone of the light gray chip on the left.  The Output value for this anchor was changed to 128, which is a mid tone.  The middle chip originally had a tone of 128, but when we made the adjustment, the entire curve was reshaped.  In order to isolate this change from the rest of the curve, additional anchor points were placed on the curve and not changed.  By not changing these additional points, they anchored the curve in place.  Mouse over Figure 2 to see the additional anchor points.

Did you notice that the middle and right chips are now the same tone?  This is because we were able to change the right chip's tone to 128 and keep the middle chip's tone 128 by using additional anchors to isolate the change.
 

 

Table 1.  Curve Dialog Box Shortcuts

To...	Then...
Add an anchor point	Click on the curve.  Use the arrow keys or the Input box to fine tune its position.
Add an anchor to the specified channel for a specific area of the image	Change the Channel drop down box on the Curves dialog box to the desired channel.  Inside the image, Ctrl - click-hold  (Command - click-hold ) the left mouse button and drag over the image.  When the desired location is found, release the left mouse button.  Even though the Ctrl (Command) key does not have to be pressed down the entire time, it does have to be pressed before click-holding the left mouse button and the left mouse button has to be pressed while dragging over the image. Or, Ctrl - click (Command - click) the area of the image.  Similar as above, but removes the option of dragging over the image.
Add an anchor to each individual color channel, but not to the composite channel, for a specific area of the image	Ctrl + Shift - click (Command + Shift - click) the area of the image
Select one or more anchors	Shift - click the anchor(s)
Cycle through the anchors	Ctrl + Tab (Command + Tab) to cycle through the anchors.  The active anchor will be solid black.
Deselect all anchors	Ctrl + D (Command + D)
Change the Input value of the active anchor	Left/Right arrow keys. Or, click in the Input box and use the Up/Down arrow keys.
Change the Output value of the active anchor	Up/Down arrow keys. Or, click in the Output box and use the Up/Down arrow keys.
Delete an anchor point	Ctrl - click (Command - click) the anchor. Or, activate the anchor and press the Delete key. Or, click and drag the anchor off the grid.
Remove all anchors and adjustments	Alt - click (Option - click) the Cancel button.  The black and white point anchors will not be removed, but they will be repositioned to 0 and 255 respectfully.
See where part of the image falls on the curve	Click and hold the mouse button over the image area.  A clear ball will appear on the curve showing the location of that area on the curve.  If the mouse is moved around the image while holding down the left mouse button, the ball will move along the curve showing the location of the area the mouse is currently over.
Draw straight line with pencil tool	Once the pencil tool is active, click in the grid, move the pencil and then Shift - click to complete the line.  After drawing several lines, the lines can be connected by clicking the Smooth button.