This filter is found in
This filter halftones the image using a clustered-dot dither. Halftoning
is the process of rendering an image with multiple levels of grey or
colour (i.e. a continuous tone image) on a device with fewer tones;
often a bi-level device such as a printer or typesetter.
The basic premise is to trade off resolution for greater apparent tone
depth (this is known as spatial dithering).
There are many approaches to this, the simplest of which is to throw
away the low-order bits of tone information; this is what the posterize
filter does. Unfortunately, the results don't look too good. However, no
spatial resolution is lost.
This filter uses a clustered-dot ordered dither, which reduces the
resolution of the image by converting cells into spots which grow or
shrink according to the intensity that cell needs to represent.
Imagine a grid super-imposed on the original image. The image is
divided into cells by the grid - each cell will ultimately hold a single
spot made up of multiple output pixels in order to approximate the
darkness of the original image in that cell.
Obviously, a large cell size results in a heavy loss in resolution! The
spots in the cells typically start off as circles, and grow to be
diamond shaped. This change in shape is controlled by a spot
function. By using different spot functions, the evolution
in the shape of the spots as the cell goes from fully black to fully
white may be controlled.
All your setting changes will appear in the Preview without
affecting the image until you click on OK.
Note that the preview displays the whole image, even if the
final result will concern a selection. Don't keep
Do Preview checked if your computer is too
This group controls the cell size, either by setting the
input and output resolutions, or directly.
Input SPI: Resolution of the
original input image, in Samples Per Inch (SPI). This
is automatically initialised to the input image's
Output LPI: Desired output
resolution, in Lines Per Inch (LPI).
Cell Size: Resulting cell size, in
pixels. Most often you will want to set this directly.
Separate To RGB, CMYK, Intensity
Separate To RGB, CMYK, Intensity:
Select which colorspace you
wish to operate in. In RGB mode, no
colorspace conversion is performed. In
CMYK, the image is first
internally converted to CMYK, then each colour channel is
separately halftoned, before finally being
recombined back to an RGB image.
In Intensity mode, the
image is internally converted to grayscale,
halftoned, then the result used as the alpha channel
for the input image. This is good for special
effects, but requires a little experimentation to
achieve best results. Hint: try CMYK if you don't know
which to go for initially.
Black Pullout (%)
Black Pullout (%) When doing
RGB->CMYK conversion, how much K (black) should be used?
Lock Channels: Make channel
modifications apply to all channels.
Factory Defaults: Restore the
default settings which should give pleasing results.
Angle: Cell grid angle for this
Spot Function: Spot function to
be used for this angle (see preview in blue cell-boxes).
Proper halftoning does not need antialiasing: the aim is to
reduce the colour depth after all! However, since this
plugin is mainly for special effects, the results are
displayed on screen rather than by a black/white printer.
So it is often useful to apply a little anti-aliasing to
simulate ink smearing on paper. If you do want to print the
resulting image then set the antialising to 1 (ie, off).
Oversample: Number of subpixels to
sample to produce each output pixel. Set to 1 to disable
this feature. Warning: large numbers here will lead to
very long filter runtimes!
More explanations, illustrative diagrams, a gallery and a
tutorial are to be found on the plug-in author site:
You are strongly encouraged to visit it, since halftoning
is a large and complex area.