/usr/share/doc/gimp-gap/plug-in-bluebox.txt is in gimp-gap 2.6.0+dfsg-4ubuntu1.
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Blubox Filter
Start from Menu:
<Image>/Video/Bluebox
The bluebox filter makes the selected keycolor transparent.
The bluebox filter operates on a single layer that must be of RGB(A) type.
(INDEXED and GRAY images are not supported)
Keycolor:
This colorbutton shows the wanted keycolor. Pixels that are similar
to this color can be set more or less transparent by the bluebox filter.
You can define the detection of similar colors by Threshold(s) in
4 different ways depending on threshold mode:
Threshold Mode:
HSV ..... has 3 separate thresholds for hue, saturation and value
and operates in the HSV colormodel.
RGB ..... has 3 separate thresholds for red, green and blue channels
and operates in the RGB colormodel
VALUE ... has only one single threshold for the maximum difference
of red, green, and blue channels and operates in the RGB colormodel.
ALL ..... has 6 separate thresholds for HSV and RGB
Thresholds
All thresholds have the range from 0.0 to 1.0
where 0.0 has no tolerance the compared channel (requires exact match)
and 1.0 allows the maximum tolerance (all values will match)
Alpha Tolerance:
Alpha tolerance value 0.0 upto 1.0
where 0.0 makes hard pixel selection by color threshold(s)
greater values allow more or less variable transparency (alpha channel values)
for the selected pixels within the threshold(s)
depending on their difference to the keycolor.
Pixels with color equal or near the keycolor appear more transparent
than pixels with more color differnce.
Pixels with color differences bigger than the threshold(s)
are not affected by the "Alpha Tolerance" setting.
Source Alpha:
Select only pixels with alpha channel >= source_alpha
where 0.0 is full transparent, 1.0 is full opaque.
Normally this is set to 0.2 to protect the alpha_channel
of pixels that are already transparent against changes
by the bluebox filter.
Target Alpha:
Control the minimum alpha value for the selected pixels.
where 0.0 sets a full selected pixel (== matching with keycolor)
to full transparent,
0.5 sets a full selected pixel to half transparent
and 1.0 to full opaque.
Feather Edges:
Enable smoothing the selection using the feather radius.
Feather Radius:
Feather radius in pixels, makes the selection smooth.
(this is done internally by applying a blur filter on the
selection that was built by keycolor, threshold(s) alpha tolerance,
shrink/grow and target alpha settings)
Shrink/Grow:
Grow or shrink the selection in pixels,
where negative values are for shrinking the selection
(that was built by keycolor, threshold(s) alpha tolerance,
and target alpha settings)
The bluebox filter creates an image for preview of the effect.
This image contains an (optinal scaled) copy of the original layer.
Automatic Preview:
When checked perform automatical update of the preview image
on all changes of the parameter settings.
Preview Button:
Create or update the preview image.
Previewsize:
Set size of the preview image in percent of the original.
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