The Very High Radius High Pass Contrast .8bf Plugin
Version 1.112, updated 12/12/2015
High-pass contrast enhancement requires two steps: the creation of a high-pass filtered image, and the blending of this image with the original. The Very High Radius High Pass Contrast filter gives you full control over the entire process.
High-pass filtration consists of first applying a Gaussian blur to a copy of the image, inverting it, then blending it with the original at 50% opacity. For contrast enhancement, the blur radius is large compared to that used for sharpening, often an appreciable fraction of the overall image dimensions. Unfortunately, the blur algorithms used in most software are necessarily approximations to a true mathematical Gaussian blur, and tend to depart from the ideal at large radii. This limits the usefulness of the technique when editing large images, such as those with dimensions of 4000 pixels or more. Even when high radii are available, the final result is often very different from the preview, and the intended effect may be impossible to achieve.
Like the Unsharp Mask filter, the Very High Radius High Pass Contrast filter takes advantage of the fact that since the blurred image contains no fine detail, it can be greatly resized with little effect on the final result. In use, a copy of the original image is reduced in size, comparable to the image preview itself, and a similarly-scaled blur applied. The blurred image is then enlarged to the size of the original image, and used as the unsharp mask. The resulting image is very similar to the preview, even if many times its size.
The High-Pass Step:
• The Radius setting is a percent of the image diagonal. The apparent and scaled radii in pixels are also displayed. Except at the highest contrast settings, halos are not nearly as prominent as they are with unsharp masking.
• Intensity is the blending opacity of the inverted Gaussian-blurred image with the original, ×2 (the normal opacity, 50%, is 100% intensity). High intensities result in an equalization effect, best seen at low contrast.
• Linear Light is the blend mode often used by software applications for creating the high-pass image; the contrast is higher.
• Color controls the saturation of the high-pass image. The results are sometimes paradoxical depending on the other settings.
The Blend Step:
• The mode used to blend the high-pass image with the original has the greatest effect on the overall contrast.
• Amount controls the opacity of the blended image. Values less than 100% attenuate the effect; those greater than 100% amplify it.
• Dark/Light Mixer works as follows: the central position (zero) displays the processed image; the far left is like Darken blend mode, and the far right is like Lighten.
• Use the masks to protect blown shadows and highlights. Try starting with Tonal Width and Radius 0 and Amount 100. Raise the Tonal Width and Radius to create an appropriate mask, then reduce Amount to 0 and gradually raise it again to achieve the desired effect. Amounts greater than 100 only affect blurred areas of the mask.
• The white histogram (background) is the original image; the black one (foreground) is the processed image.
A few presets are included to illustrate the effect of various settings:
Works with 8- and 16-bit RGB images.
Download the zipped file, then copy VHRHighPassContrast.8bf into your plugins folder. It will appear under “RC Filters.”
VHRHighPassContrast_x64.zip version 1.112, 294 KB, 64-bit applications. It works for me in Photoshop CS5 and PhotoLine 19.5, Windows 7.
VHRHighPassContrast.zip version 1.112, 290 KB, 32-bit applications, Windows 32 or 64 bit. It works for me in IrfanView 4, Windows 7. | source code
VHRHighPassContrast_XP.zip version 1.112.1, 181 KB, “retro” 32-bit version compatible with Windows XP.
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© 2015 by Russell Cottrell; released under the
GNU General Public License.