Pel Unsharper Mask

Unsharper Mask is an advanced sharpening tool for Digital Fusion. Designed for professionals by professionals, the Pel Unsharper Mask allows fine-grade control over sharpening your images, while limiting the effect on grain or noise.

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The Pel Unsharper Mask plugin has its controls in three tabs. The Settings tab contains the settings used most often. The Tweaks tab contains the powerful fine tuning controls, letting you adjust the white and black halos independently. Finally, the Performance tab contains settings linked to improving the speed of your workflow.

Settings Tab:

Amount:
Controls the overall amount of sharpening. 0 is the minimum 500 is the maximum.

Radius:
Controls the width of the sharpening "halos" (the black and white edges that the USM technique adds to edges to increase their apparent sharpness). Normally the radius will be between 1.0 and 3.0, unless you are doing an overall contrast boost using a wide radius with a low amount. (There will be more notes about this technique soon.)

Thresh Low and Thresh High:
These threshold values control the edge detection. Their primary use is to reduce oversharpening of grain and noise. You may think of an "edge" as an area of local contrast around the current pixel. The thresholds reduce sharpening applied when this local contrast is below a certain level. The Thresh Low sets the level below which pixels will not be sharpened at all. The Thresh High sets the level of local contrast above which pixels will be sharpened fully. Pixels with local contrast values between the Thresh Low and Thresh High will be partially sharpened in a smooth gradient between the thresholds. Typically you want to keep a little space between these two thresholds in order to avoid "threshold artifacts," which are usually seen as odd, overly-sharp stray pixels or clumps of pixels. You may set the thresholds as high as 1.0 as you see fit. Experiment with setting the Thresh Low as close to zero as possible, and Thresh High a little higher up. Though when both are at zero there will be no chance of threshold artifacts, this rarely produces acceptable results unless you have a VERY clean source image.

Note: We have set up the default range of the thresholds to be within what we would consider a normal range. Although the thresholds may be set to a value up to 1.0 you must enter these values numerically in the text boxes. If you drag either of the thresholds after setting them outside the default range they will "snap" back to the default range. This is a limitation of the control provided by DF. We may add a workaround for this in the future. For now, just enter the values numerically in these cases.

Threshold Chan:
This popup controls what the plugin looks at to determine if an edge is indeed an edge. The default is to evaluate RGB separately. Though you may find yourself using Luminance some of the time, especially if you see odd colored pixels with large amount settings. This setting has a very subtle effect. On many images you will not see a difference. The setting of Alpha uses the alpha channel to determine where edges are. The clip must have an alpha for this to show up. It may be useful for stylized effects.

Smooth Non-Sharpened:
This is an interesting "sharpening" technique. It works by actually blurring the areas that are NOT considered edges (based on the threshold settings). When used subtly, softening the rest of the image can sometimes make the sharp parts appear even sharper. For this subtle effect the
setting will usually be less than 2.0 or even less than 1.0. Using higher values will produce more stylized effects. The setting is actually the radius of the blur.

Tweaks Tab:

White Halo Scale / Black Halo Scale:
An Unsharp Mask filter works by adding black and white "halos" to what it perceives as edges in the image. This increase in contrast along the edges increase the apparent contrast. The Halo Scale controls allow you to independently control the white and black halos. Sometimes, when you sharpen an image the white halo may seem overly bright, or the black halo overly dark. Use these controls to fine-tune the effect. Scaling the white halos way down while leaving the black at 1.0 can give an effect similar to a Harrison Black Dot filter. These can be set up to 10.0 if needed, though typically you will use them to trim one or the other with a value less than one.

White Halo Clip / Black Halo Clip:
Where the Halo Scale controls tune the value of the halos for every pixel they effect, the Halo Clip controls control only the maximum amount of change for each pixel. The Halo Clip controls evaluate the sharpening effect and then determine if the pixel would change more than the amount indicated by the Clip. If the pixel would change more than the set amount the amount of change is "clipped" to the setting used. So if a white halo would cause a pixel to go from a value of 0.5 to 0.8, and you have the White Halo Clip set to 0.2 the pixel will end up with a value of 0.7 because the difference between the original pixel and the processed pixel is clipped to a maximum of 0.2. You will probably tend to use the Halo Scale controls more often. But using the Clip controls can prevent strong edges from being oversharpened when you are trying to sharpen softer edges in the scene. The Halo Clip settings will often be close to zero when in use.

 

Performance Tab:

LowQ/AutoProxy:
This popup controls how the plugin behave when Fusion has HiQ turned OFF, and during autoproxy. It is meant as a time saver. You can choose Full Quality, which means the output will be identical to a HiQ render. You may choose Reduced Quality (the default), which means that the blur used to calculate the Unsharp Mask will be a "Soften," and the blur used for the Smooth Non-Sharpened effect will be a "Box" blur. Reduced Quality (No Smoothing) is the same as Reduced Quality, but Smooth Non-Sharpened is disabled. And finally, Pass-thru passes thru the original image unchanged. If you have a hi-res image you may want to set this to pass-thru once you finalize your settings. The sharpen will then be disabled until you enable HiQ mode (like for final rendering). This is a real time-saver!

HQ USM Blur:
This is the Blur Mode used to calculate the Unsharp Mask in HiQ mode. This defaults to Gaussian, which should give the best result. However, you may find you can get away with Soften a lot of the time, at a significant increase in speed. When using floating point images you may have to set this to Bartlett to avoid a bug in DF4.x that causes "ringing" with Gaussian blurs.

HQ Smooth Blur:
This is the Blur Mode used to apply the smoothing for the Smooth Non-Sharpened feature. It defaults to Soften, which usually looks fine at a low radius.

 

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