High Pass Contrast Enhancement with Background Mask

This tutorial explains how to use a high pass filter to enhance contrast. A background mask is create and used with the High Pass Filter tool to enhance constrast of a nebula with reduce effect on the background and stars.

Similar steps can be used to apply any stretch, contrast, deconvolution, or color adjustment filter with blend mode, opacity, and mask.

Step 1) Initial Image

Initial NGC 6992 3nm Ha image after brightness stretch. Digital Development or Arcsin can be used to perform the initial brightness increase.

Step 2) Apply High Pass Filter

Use the High Pass Filter button on the left vertical toolbar to open the High Pass Filter tool. Adjust pixel radius until NGC 6992 appears in the gray high pass image.

Open the Process Histiory window and with High Pass as the current command pointed to by >> in the Operation list set blend mode to Soft Light. Soft Light blends the gray high pass image with the initial image.

Adjust pixel radius on the high pass filter to fine tune contrast enhancement. A background mask could be used as shown in the next steps to reduce contrast enhancement of stars and dark background.

Step 3) Create Background Mask

Minimize the Process History and High Pass windows. Use the Area Mask button on the left vertical toolbar to create an area mask. Adjust the blur window size (radius) as needed to isolate important detail.

Use the Stretch | Histogram Contrast Stretch tool to brighten the lighter mask areas.

Use the Save As then Copy Mask toolbar button on the left vertical toolbar to name and save the background mask. The mask is now ready to be assigned to a filter using the Process History window.

Step 4) Assign Background Mask to High Pass

Restore or open the Process History and High Pass tools.

With High Pass as the current command pointed to by >> in the Process History Operation list press the Mask button. The Mask Surface window is displayed and set to the default all white control point mask. Right click on the white mask area and select Paste. Since the last mask saved was the background mask it is assigned to the High Pass tool. Note that you can also right click on the mask area and select Copy to copy a mask to a different filter of the same image or different image.

Change pixel radius on the High Pass tool and modify the mask as needed to fine tune the contrast enhancement.

NGC 6992 before high pass contrast enhancement.

NGC 6992 with high pass contrast enhancement applied to entire image.

NGC 6992 mask controlled high pass contrast enhancement. Reduced effect on stars and background with softer contrast increase to nebula.

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