29th Sep 2005, sjh
This method is designed to increase image detail in both shadows and highlights without affecting the overall brightness of the image. It is related to localised contrast enhancement and the unsharp mask, but avoids sharpening edges. Instead the aim is to accentuate the more subtle variations in the image. It is useful in high contrast scenes where an overall levels or curves adjustment would leave some areas washed-out.
Before → After
The basic technique is to perform an unsharp mask, but during the blur stage, to use a selective blur that avoids altering sharp edges. The steps are:
- Create a selectively blurred copy of the image
- Subtract this blurred copy from the original image to get the difference image
- Add the difference image back on to the original image to enhance all of the details that were lost in the blurred image.
As the high-contrast edges aren't blurred in the blur stage, then they are not enhanced when the difference image is added on. That is the primary change to the standard local contrast enhancement. Primarily, it allows the subtle variations to be enhanced much more, without the high contrast edges causing halos on the image.
2) Procedure: Photoshop
Create a Blurred Image
- In the Layers Dialog, Select Background Layer
- Right-click and select Duplicate Layer. Name the new layer "Blur"
- In the image window select Filters->Blur->Smart Blur Blur. Choose a radius of 30, and threshold of 50.
This is the most critical step, and the one with the most flexibility. It is the details that are removed at this step which are ultimately going to be enhanced in the final image. The radius controls the maximum size of detail to be affected, and the threshold is the maximum edge contrast that should be affected.
Calculate the DifferenceThe aim here is to create a new channel that represents the numerical difference between the original layer and the blurred one. Calculations are used to calculate the difference, and an empty white layer is also created to mask the channel against. Note the offset of 128 is used so that the result is placed in the centre of the dynamic range in 8-bit mode.
- Create a new layer, and call it "White"
- Fill the new layer with white fill.
- Turn off the White layer visibility.
- Select Image->Calculations
- Set parameters to find the subtraction between the Blur and Background layers. For some reason it is the top one that is subtracted off, so make that one the Blur layer. Choose the grey channel for each to work on the image luminosity.
- Choose an offset of 128 for the result of the subtraction.
- Select OK to create the channel, and the image will become gray
Increasing the EffectTo make the effect more agressive, more gain can be added by using a curves adjustment on the grain layer. This is equivalent to adjusting the "Amount" in an unsharp mask. By default, without doing a curves adjustment, it is like an amount of 1.0. It doesn't hurt to put too much gain in here, as it can later be toned back by adjusting the opacity of the layer.
- Select Image->Adjustments->Curves
- Make a steep curve passing through the centre.
- Later the opacity can be used to adjust the effect level.
Add on the Difference
- Select Image->Calculations
- Set one source to be the Background layer and Gray Channel.
- Set the other source to be the White layer with the new channel created in the last Calculations step.
- Choose the Add blending mode, and an offset of -128.
- Select OK to create the new channel, and the image will become a greyscale version of the enhanced image
Creating a LayerCurrently the result is still in a channel, so here we put it in a layer and blend it against the original image.
- Choose Select->All
- Click on the white layer in the layers dialog.
- Edit->Paste will create the new layer
- Change the layer mode to luminosity
- turn off the Blur layer visibility
- Adjust the opacity of the new luminosity layer
Adjust SaturationThe added detail will generally benefit from greater saturation in the image. These steps are the ordinary method of adjusting saturation.
- Select Layer->New Adjustment Layer->Hue/Saturation
- Adjust the saturation to suit the image. In this case, +20