[Qgis-user] Raster Calculator Expression to determine color

[Nick Papadonis]

> On Jul 30, 2015, at 9:29 PM, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> On 31 Jul 2015 11:07 am, "Nick Papadonis" <npapadonis at gmail.com <mailto:npapadonis at gmail.com>> wrote:
> >
> > HI Folks,
> >
> > I’m trying to use the following expression to pick up the red colored routes on a map in which I used the info pointer tool to get the band values for the color.
> >
> > (“m at 1" < 238 AND “m at 1" > 213 AND “m at 2" < 123 AND “m at 2" > 98 AND “m at 3" < 125 AND “m at 3" < 99) * 1
> 
> Should that last check be "m at 3" > 99?
> 
> 

I corrected above to:
(“m at 1" < 238 AND “m at 1" > 213 AND “m at 2" < 123 AND “m at 2" > 98 AND “m at 3" < 125 AND “m at 3” > 99) * 1

with the same results which is black output tiff.

I then tried individual expressions for each band as separate steps (sanity check):
(“m at 1" < 238 AND “m at 1" > 213) * 1
(“m at 2" < 123 AND “m at 2" > 98) * 1
(“m at 3" < 125 AND “m at 3” > 99) * 1

These all produce a masked image, however masked incorrectly because I’m evaluating only a single band in the Raster Calculator.

When I combine evaluation of bands as above or below to pick pixels in color range:

 ((“m at 1" < 238 AND “m at 1" > 210) AND (“m at 2" < 123 AND “m at 2" > 94) AND (“m at 3" < 130 AND “m at 3" > 98)) * 1

I get a black image with no matches.  I expected at least one pixel identified using this expression because I evaluate a pixel at 224, 108, 115 and there are many other pixels in above range.

Does anyone know if my expression needs correction or the Raster Calculator can only evaluate a single band in the expression?

Thanks again,
Nick

> Nyall
> 
> >
> > Unfortunately, when this expression is used the resulting map is black with no other pixels for the detected color.
> >
> > If I break down this expression to a simple “greater than” line for single band, then things work.
> >
> > Does anyone know where this expression needs to be corrected?
> >
> > Thanks again,
> > Nick
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