AJR 2003; 181:383-386
© American Roentgen Ray Society
Photoshop for Radiologists |
Color Correction and Automating Repetitive Tasks
George A. Taylor1
1 Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA
02115.
Received November 20, 2002;
accepted after revision November 27, 2002.
Address correspondence to G. A. Taylor.
Introduction
The topics covered so far in this series of articles have included methods
for resizing an image, methods for removing unwanted information and
imperfections such as dust and scratches, and tools for restoring image
sharpness
[1–4].
In this, the final installment of this series, I will cover some methods for
color correction and work-flow management.
Color Correction
Many digitally acquired color images will have a strong and visually
distracting color cast. This is often due to illumination of the subject by
fluorescent or incandescent lights that emit different wavelengths of light.
The simplest method for color correction is one that we have discussed
already: namely, the "Auto Levels" (Image 
Adjustments Auto Levels). In a color image, it spreads red,
blue, and green channels evenly across the existing histogram of color
densities. The example shows an intraoperative digital image of an inflamed
appendix. The original image (Fig.
1A) has an overall greenish cast, and the specimen is overexposed.
After application of "Auto Levels"
(Fig. 1B), the specimen is well
exposed and the greenish cast is reduced.
Often the use of "Auto Levels" alone allows an acceptable
approximation of the true color of the image. However, additional color
correction may be necessary, as seen in
Figure 1B. This may be
accomplished by selecting the "Color Balance" adjustment. Begin by
accessing the "Color Balance" dialog box under "Image"
on the Photoshop marquee (Image Adjustments Color
Balance), as shown in Figure
1C. The dialog box consists of slider bars for cyan/red,
magenta/green, and yellow/blue color levels, and numeric representations of
these levels. At baseline, the slider bar indicators are set at a color level
of 0 representing a color setting that is "unshifted" from the
primary red, green, or blue colors. Movement of the slider bar to the left or
right results in a negative or positive value for each color pair
(representing a shift away from or toward each of the primary colors,
respectively). Changes in color balance may be made for shadow, midtone, or
highlight regions of any image. Changes are made by trial and error until an
acceptable color balance is achieved. In
Figure 1C, only the midtone
color balance was modified. The resulting color-balanced image depicts the
operating room drapes in a more neutral gray, and the surgical specimen is
well exposed.
A new method not previously available has been released in the newest
version of Photoshop (version 7.0, Adobe Systems, Mountain View, CA). It is
called "Auto Color" and is an easy, one-step process that more
easily corrects color for any given light source. This adjustment may also be
accessed under "Image" in the Photoshop marquee (Image
Adjustments Auto Color). The resulting image
(Fig. 1D) has good color
balance and exposure.
Blending Modes
Another common problem often encountered with graphics copied from printed
illustrations is a loss of original color intensity and contrast. One simple
method of restoration involves duplicating the image layer and using the
"Multiply" blending mode. Begin by opening the
"Layers" window (Window Show Layers) and duplicate
the background layer by dragging the background layer to the bent page icon at
the bottom of the "Layers" window
(Fig. 2A). Change the blending
mode of the duplicate layer from "Normal" to
"Multiply" to intensify the colors of the graphic without
affecting the color balance (Fig.
2B). If the image is too dark, decrease the opacity of the
duplicate layer until you reach the desired level of effect. Finally, merge
(or "flatten") the image by clicking on the arrowhead on top of
the marquee of the "Layers" window and choosing "Flatten
Image."
View larger version (83K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 2A. —Blending modes in Photoshop (Adobe Systems, Mountain View, CA).
Original image (A) of graph illustrates relationship of intravascular
volume loss and cardiovascular response in children; note light color of
background. Use of "Multiply" blending mode: Open
"Layers" window (Window Show Layers) and duplicate
background layer by dragging it to bent page icon at bottom of window
(arrow, A) as shown in left upper corner. Blending mode in
"Layers" window is changed from "Normal" to
"Multiply." Resulting blended image is shown on right. Note darker
background and improved contrast.
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View larger version (68K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 2B. —Blending modes in Photoshop (Adobe Systems, Mountain View, CA).
Original image (A) of graph illustrates relationship of intravascular
volume loss and cardiovascular response in children; note light color of
background. Use of "Multiply" blending mode: Open
"Layers" window (Window Show Layers) and duplicate
background layer by dragging it to bent page icon at bottom of window
(arrow, A) as shown in left upper corner. Blending mode in
"Layers" window is changed from "Normal" to
"Multiply." Resulting blended image is shown on right. Note darker
background and improved contrast.
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Automating Repetitive Tasks
At this point, many of you may be thinking "How am I possibly going
to find the time to do all these manipulations to all of the images I want to
use in my next electronic talk?" The answer lies in automating as many
of the recurring tasks as possible. Automating can be accomplished by using
macro commands (called "Actions" in Photoshop) and applying them
to groups of slides. This will allow you to perform the same tasks on many
images automatically.
Begin by accessing the "Actions" palette (Window
Show Actions) (Fig. 3A). Click
on the arrowhead located on the right edge of the marquee to create a
"New Action." The "New Action" dialog box will appear
asking you to name the new action. Call it by a descriptive name, such as
"gray scale," "resize," or "JPEG" (Joint
Photographic Experts Group) so that you will know what this particular action
does and to separate it from other actions you might record in the future.
Once the new action is named, click "Record" and perform series of
actions to be repeated (Fig.
3B). When finished, click the stop-record button. This will store
actions you just performed as a single macro function. To invoke the actions,
click File Automate Batch
(Fig. 3C). This opens a dialog
box that asks you to select the actions and the folder on which to perform the
actions. It also asks that you select a folder in which to save. If you are
going to use these images only for an electronic presentation, choose
"Close and Save." Doing so will permanently alter the images and
cannot be redone. The safer option is to select a different destination folder
(Fig. 3D). This will leave you
with your unretouched original set of images and a copy of presentation-ready
images stored in a separate folder.
View larger version (76K):
[in this window]
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Fig. 3A. —Creating actions in Photoshop (Adobe Systems, Mountain View, CA).
"Actions" palette is selected by clicking "Window"
menu bar (Window Show Actions), as shown in upper left corner.
"Actions" dialogue window is shown in upper right. Choose
"New Action" by clicking on arrowhead at top of box (white
arrow). "New Action" dialogue box will appear. Type name of
new action in box titled "Name." Leave set as "Default
Actions." Click "Record" button (black arrow) to
record set of actions.
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View larger version (89K):
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[in a new window]
[as a PowerPoint slide]
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Fig. 3B. —Creating actions in Photoshop (Adobe Systems, Mountain View, CA).
Perform actions you want to record on any open image. Recording function is
active when recording button is highlighted (white arrow). Once all
actions you wish to record are recorded, click black square
"stop-record" button (black arrow). (Fig.
3A, 3B,
3C,
3D continues on next
page)
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View larger version (110K):
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[in a new window]
[as a PowerPoint slide]
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Fig. 3D. —Creating actions in Photoshop (Adobe Systems, Mountain View, CA).
Choose folder on which to perform actions by clicking "Choose"
under "Source" (long black arrow). Under
"Destination" (short black arrow), choose "Save and
Close" option. Click OK button in right upper corner of dialog box. Set
of selected actions will be performed on every image in source folder and
saved automatically.
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Conclusion
This is the fifth and final installment of the Photoshop for Radiologists
series. If you have followed along, you will have a number of tools at your
disposal to improve your images before electronic presentation. If you are a
ripper and saver of articles, keep the series as a set and refer to them as
needed until you master the tools. One final caution to reemphasize: These
techniques are not intended to alter image content but merely to optimize the
images for a more effective electronic presentation. Happy Photoshopping!
References
- Taylor GA. Initial steps in image preparation.
AJR 2002;179:1411
–1413[Free Full Text]
- Taylor GA. Improving image contrast. AJR2003; 180:329
–331[Free Full Text]
- Taylor GA. Removal of dust and scratches from electronic image
files. AJR2003; 180:1533
–1536[Free Full Text]
- Taylor GA. Sharpening the image. AJR2003; 181:43
–45[Free Full Text]
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Introduction
Color Correction
