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tickbox variable: $labelalpha -- label transparency
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default: 128

The "tran" entry (upper left) determines
transparency (alpha) of labels when they are
displayed as a single-color transparent overlay
using the label line "D" button, where 255 is
opaque and 0 is completely transparent
(invisible). It also controls the transparency of
the multiple regions in a displayed MGH
annotation file.

To reset the transparency of all labels and/or
annotations currently displayed to a single
value, enter a new value and do a <Return> in the
"tran" entry (tcl function: set_annot_alpha
<alpha>).

Single label

The current transparency value in "tran" is used
when a single label is initially read in with the
"D" (display) button, analogous to how the "r",
"g", and "b" fields to the right of MESH set the
color of a read-in label.

MGH annotation

When an MGH annotation is read into tksurfer, by
contrast, the transparency of all regions is
initially set to 255 (i.e., non-transparent,
opaque) except for the "Unknown" region, which is
invisible.

Annotation color implementation details

The 4-byte unsigned int vertexwise annotation
colors in a standard MGH *.annot file typically
have their alpha byte set to zero for all regions
(N.B.: alpha=0 conventionally means completely
transparent AKA invisible).

By contrast, csurf/tksurfer uses the alpha byte
to adjust the displayed transparency of
label/annotation regions.  However, before
writing an MGH annotation file, tksurfer clears
the transparency byte to 0 for compatibility.

The C/tcl function called by this entry,
set_annot_alpha, resets the alpha byte of the
vertex color, vertex[i].annot, for all regions to
the requested value. The only exception is region
"Unknown", defined by the color: (r,g,b) =
(0,0,0), whose alpha byte is always set to 0
(invisible).

Currently, tksurfer ignores/overrides
transparency/alpha values in annotation files
(typically 0) when reading them.
