[gdal-dev] Automatically creating an Alpha mask for a RGB raster to extract the backgound? (magic wand style)

[Idan Miara]

Hi,

I'm looking for an automated/semi-automated way to remove the plain color
background of some (jpeg compressed) Orthophoto RGB rasters without
NoDataValue or an Alpha band.

Usually I get an image of some specific region with the background as a
plain color (usually it's plain white or plain black but it could be also
almost white or almost black or some other color, or a composition of some
patches that each one is a plain color).

The problem begins when when I want to make a mosaic. It could be hard to
remove those plain regions which should be transparent - setting a single
NoDataValue won't do the trick for most cases and it will also remove some
data pixels.
It gets even worse if the outside was not black and then I use gdal-warp to
transform the CRS and I get extra black filling on the outside.

A simple rule to classify the background pixels - a large cluster of pixels
with exactly (or almost exactly) the same value - a human can tell the
background instantly.

Is there an easy way to create an alpha binary mask band to classify the
background?
Afterwards, is there a tool to crop the image so I'll get a minimal frame
of background?

Kind regards,
Idan
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