Image Enhancement Using the Photoshop Select Color Range Tool

Introduction

The following is a step-by-step procedure for enhancing astronomical CCD images using the Photoshop "Select Color Range" tool and by applying the basic concept that areas with high signal-to-noise ratio can be specifically selected, sharpened and enhanced without introducing processing artifacts to the remainder of the image.  The two images directly below demonstrate how portions of an image can be selectively enhanced.

The top image was processed using basic curves and levels adjustment in Photoshop.   The red areas of interest in the center of M 27 are somewhat washed out and are not distinct. 

The second image illustrates how selectively choosing an area of interest in an image and then applying sharpening and color adjustments to those areas can improve the overall quality of the image.

Before

After

The following procedure illustrates the basic concepts of selective sharpening and color enhancement.  The specific details will change from image to image or object to object.  However, the basic concepts can be applied to a wide range of objects and situations.

The Procedure

 

Step 0:    The starting point.

Start with a well processed image with good detail and minimal noise.

 

Step 1:    Selection of the area to be enhanced.

a.    Click on Select/Color Range to bring up the tool menu.

b.    Click on a moderately bright red area 1/4 of the way toward the center of the nebula with the eyedropper.

c.    Set the "Fuzziness" to 165.  You will see the selected area in white in the preview box.

d.    Click "OK". 

e.    Click on Select/Feather. 

f.    Select a feather radius of two pixels.

g.    Click "OK".  You will now see the selected area outlined in a dashed line similar to the screen shot immediately below.

 

 

Step 2:    Initial sharpening.

a.    Click on Filter/Sharpen/Unsharp Mask to bring up the the Unsharp Mask tool.

b.    Set the Amount to 50% the Radius to 1.6  pixels and the Threshold to 0.  [See the screen shot below] This is a fairly moderate setting and will result in moderate sharpening without making the image look too unnatural.  Note that the sharpening only occurs inside the the dashed lines.  This is the advantage of the Select Color Range Tool.  A specific area can be selected for processing without causing unwanted artifacts in the rest of the image.   You can also experiment with the "Amount" and "Radius" settings to sharpen more or less according to your tastes.

c.    Click on OK to perform the sharpening and to close the tool.

 

Step 3.    Adjust the Color

We will also want to adjust the color of the selected area to enhance the red portions which we just sharpened.  We can do this by use of the "Curves" tool.  You can see that the red in the selected area is washed out.  If we are going to be sharpening up this area, then we will want to want to make the color more visually pleasing.  We can do this by first dimming the brightness slightly by pulling down the RGB curve and then enhancing the red by slightly raising the red curve as follows:

a.    Click on Image/Adjustments/Curves to bring up the "Curves" tool.  [See screen shot above.]

b.    Slightly lower the RGB curve from the center of the curve as shown below.  This will have the effect of slightly dimming the entire selected area. [See screen shot below.]

c.    Select the red channel using the color channel drop down menu in the Curves tool.   Slightly raise the red curve from the center of the curve in the same amount that you lowered the RGB channel.  Here again, you can use the curves tool more or less than I have illustrated to your own tastes.

d.    Click OK to make the change to the image.

e.    Click on Select/Deselect to examine the results of Steps 1 through 3. 

 

There is now a noticeable improvement in the fine detail in the center of the nebula in the image above.   However, the brightest teal-colored areas are somewhat washed out and distracting to the eye.  We can slightly dim this area as well by use of the Select Color Range tool in the manner as described above.

 

Step 4:  Color Adjustment

a.    Click on Select/Color Range

b.    Click on the brighter teal area at the center of the image to select the color.

c.    Set the fuzziness slider to 162.

d.     Click OK.

e.     Click on Select/Feather.

f.     Set the feather radius to 2 pixels.

g.    Click OK.

You will now see that the area that is surrounded by the dashed lines is the brightest teal colored area in the center of the nebula.  The red areas we were just working on are not selected and will not be affected

h.    Click on Image/Adjustments/Curves to bring up the curves tool.

i.     To lower the brightness of the teal colored area we now can pull down the center of the RBG curve slightly as illustrated above.  Click on OK in the curves tool when you are satisfied with the color adjustment.

j.    Click on Select/Deselect to deselect the area and view the effect of this last step.  The result can be seen below.  The teal colored are is dimmed slightly and less distracting.

 

 

By looking at the image above I can see that additional brightening of the red portion of the nebula will  saturate the bright red areas at the top and bottom of the nebula.  By using the same procedure outlined above we can also dim the bright red areas at the top and bottom of the nebula.

 

Step 5:  Color Adjustment

a.    Click on Select/Color Range

b.    Click on the bright red at the top or bottom of the image to select the color.

c.    Set the fuzziness slider to 130.

d.     Click Ok.

e.     Click on Select/Feather.

f.     Set the feather radius to 2 pixels.

g.    Click OK.

You will now see that the area that is surrounded by the dashed lines is the brightest red colored area at the top and the bottom of the nebula. The dimmer red areas and the teal we were just working on are not selected and will not be affected

h.    Click on Image/Adjustments/Curves to bring up the curves tool.

i.     To lower the brightness of the red colored area we now can pull down the center of the RBG curve slightly as illustrated above.  Click on OK in the curves tool when you are satisfied with the color adjustment.

j.    Click on Select/Deselect to deselect the area and view the effect of this last step. 

Step 6:  Additional enhancement of color and sharpening.

I now want to further brighten and sharpen the the red areas of interest.  This can be done using the same selection, sharpening and color enhancement techniques we have used to get to this point.

a.    Click on Select/Color Range

b.    Click on an area of medium brightness red near the center of the nebula to select the color.

c.    Set the fuzziness slider to 165.

d.     Click Ok.

e.     Click on Select/Feather.

f.     Set the feather radius to 2 pixels.

g.    Click OK.

You will now see that the area that is surrounded by the dashed lines is again the red areas that we were working on earlier.

h.    Click on Image/Adjustments/Curves to bring up the curves tool.

i.     To increase the color of the red area we now can pull up the center of the red curve slightly as illustrated above.  Click on OK in the curves tool when you are satisfied with the color adjustment.

Step 7:  Further sharpening of the area of interest.

We can do some more sharpening of our area of interest while we have it selected.  The procedure is the is the same as in Step 2 above.

a.    Click on Filter/Sharpen/Unsharp Mask to bring up the the Unsharp Mask tool.

b.    Set the Amount to 50% the Radius to 1.6  pixels and the Threshold to 0. 

c.    Click on OK to perform the sharpening and to close the tool.

Step 8:  In this step I just brightened up the entire image with the Curves tool as illustrated above.

Step 9:  As the final step I slightly dimmed the brightest teal area again by using the Select Color Range tool and the Curves tool as discussed above.

The screen shot below is the final product.  The amount of fuzziness, the amount the curves are adjusted, the  the precise settings of the unsharp mask tool, etc. will vary from person to person and object to object depending on individual tastes.  However, this tutorial demonstrates the basic concepts.

Good luck with with your own processing and experimentation and let me know how you like this page.

Last modified on 04/23/2006