Lowpass filtering can lessen the severity of tone change by reducing high-frequency detail in an image in areas where pixel intensities change rapidly.
The result is a slightly blurred image.
AutoCAD Raster Design toolset uses the following lowpass filters:
- Lowpass Filter #1 averages differences between the target pixel value and its side to side and top and bottom neighbors. Diagonally adjacent pixels are ignored.
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Lowpass Filter #2 evenly weighs all pixels in the kernel including the target.
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Lowpass Filter #3 evenly weighs all pixels surrounding the target while providing extra weight to the target pixel. This tends to reduce the blurring effect to the image.
- Lowpass Filter #4 heavily weighs the target pixel while weighing adjacent side to side and top and bottom pixels more heavily than diagonally connected pixels. Heavy weighting on the target pixel tends to reduce blurring effects and less weighting on the diagonal pixels acknowledges that their centers are further away than side adjacent pixels.
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Blurring evenly weighs all pixels in the kernel including the target. This filter, while similar to Lowpass Filter #2, gives slightly brighter results.
On a low resolution scan, you can also use the sharpening filters to make differences in shading more distinct and enhance edges in the image. For more information on sharpening filters, see
Highpass Filters and
Edge Enhancements Filters.
AutoCAD Raster Design toolset uses Single Image Optimization to speed up the editing commands.