How do you make pixels which are not visible from a point be made transparent and the pixels which are visible opaque so that I can overlay the raster over a map to see clearly where you can and cannot see a wind farm from?
From the openWind Forum website, it is mentioned here:
"...set the invalid value to -9999. What this does is make sure that areas outside of the raster are painted as transparent due to being interpreted as NoData values. This is in the Interpretation Tab of the layer properties."
I cannot confirm if this will work as I am not experienced with using openWind but I am guessing that if you change the Do not display values option to equal 0 (provided your raster has visible values set to 1; non-visible to 0), then this might work.
Joseph has provided a 'how to' for OpenWind. However, if you are like me, you may prefer to do the cartography in QGIS. In which case go to the Transparency tab of the raster layer properties. Set the transparency for the whole layer to about 50%. Then click the little green plus button on the RHS of the dialog box to add a new custom transparency option. Set the 'from' value to a large negative value (-99999 usually does the trick). Next set the 'to' value to zero and accept the default 100% transparency for for pixel range.
NoData will be treated as transparent anyway and the above operation takes care of the zeros.
A good tip (and the usual convention) is to set the background map to gray-scale and use a suitable pseudo colour scheme for the visible turbine count. You can set your backdrop map to gray-scale easily by going to it's layer properties and on the Style tab, move the Saturation slider as far left as it will go (-100).