Binary FX: Photoshop Tech Vault 3.0

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Edge Mask

How To:

Mask the major high frequency edges found in an image.

Edge masks are useful in a number of situations - including sharpening key image content without processing low frequency areas such as sky or skin, and in colour artifact or luminance grain/noise reduction while protecting high contrast edge detail.

Creating an Edge Mask:

1. An edge mask is often created from monotone image levels, such as generated by a Grayscale Image Mask or from individual or averaged/combined channel data using the Apply Image, Calculations or Channel Mixer commands. In addition, the A and B channels from Lab mode may also be of use when combined with tonal based channel content.

2. To reduce the amount of noise and minor high frequency detail detected, a small median or Gaussian blur filtering step is often applied prior to the edge finding operation. This presumes a scan or digital capture, computer generated art or lineart is often noise free, therefore it is usually not necessary to perform this pre-processing step.

3. There are two common filters used for edge detection when creating an edge mask from common photographic content - the simple Find Edges filter or the more powerful Glowing Edges filter, which provides three control variables. It will often depend on the image content on which filter will produce the best results, also note that FE produces a positive mask image (hiding edges) and GE produces a negative image (revealing edges). Simply invert the tonal levels of the filtered result if the chosen filter produces the opposite masking result than intended.

4. Post edge detection processing is often performed using combinations of Gaussian blur and tonal correction adjustments, to clean up edge and non edge areas into content suitable for use in a layer mask or as a selection. Manual toning or painting in white or black may also be performed.

Select Edges Plug-In:

Reindeer Graphics offer a free plug-in, Select Edges, which provides five edge finding operators that result in a selection (the plug is accessed under the Select menu, rather than the Filter menu).

Tip:

The Threshold command can help to expand or contract the width of a duplicated blurred edge mask. A minor blur is often applied to soften the final mask edges. The Apply Image command is used to combine the duplicate version with the original using suitable blending modes (lighten/darken, screen/multiply or add/subtract).
When using an edge mask for sharpening, one can reduce the solid black mask via a simple level or curve shadow endpoint adjustment, to a darker shade of gray to apply minor sharpening to low frequency image areas that are not major high frequency edges but still require subtle sharpening.

Further Information:

The common Adobe edge detection filters mentioned above are often ideal for natural photographic images, although they may be too coarse for detecting fine features as found in computer generated graphics rasterized from page layout or illustration software or in high resolution lineart scans. If the task is detecting noise free, fine edge content - then I recommend the use of the "Detection Sampler: CG Lineart" action found in the "Custom Convolution Kernels.atn" Photoshop action set. This action uses custom convolution based on the Roberts Cross feature detection kernel to isolate fine edge detail with more precision than the native Adobe filters. Custom Filter info & Custom Filter action

Related Links:

How To: Create a Grayscale Image Mask

Custom Convolution: Custom Filter info & Custom Filter action

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