[MapServer-users] Distinguish between pixels outside of SCALE range and nodata pixels

[Timothy Kempisty - NOAA Federal]

Try adding a separate background layer using the same data, with CLASS
statements only for your out of bounds pixel values.  Simplified a bit here:

LAYER
    # -- Your existing layer
    NAME my_actual_data
*    GROUP mydata*
    DATA same_data
    PROCESSING "SCALE_1=10.0,20.0"

    <... yadda yadda ...>
END

LAYER
# -- New layer adds black/white colors where pixel values exceed scale
bounds
   NAME background
*   GROUP mydata*
   DATA same_data
      CLASS
          EXPRESSION ([pixel] < 10)
              <...black...>
      END
      CLASS
          EXPRESSION ([pixel] > 20)
              <...white...>
      END
END

Display both layers together by requesting the "mydata" group name, just
like you would request any other layer.  It will render all layers with the
same group name.

-Tim

On Mon, Nov 28, 2022 at 8:35 AM Mallinger, Bernhard <
bernhard.mallinger at eox.at> wrote:

> Hello,
>
> We have maps where we want the color scale go from e.g. 10 to 20, so we
> are using this directive:
>
> PROCESSING "SCALE_1=10.0,20.0"
>
> This works great by itself, however it also means that pixels with values
> of e.g. 5 are rendered as transparent, which is the same as the nodata
> pixels. So in the final image, you can't tell if data is present but the
> values are too low, or if there just isn't any data.
>
> So the behaviour we would like is that values below 10 are rendered as
> black and values over 20 are rendered as white.
>
> Is this somehow possible with mapscript?
>
>
> We tried to have a look in the source code, and there is this line, which
> effectively assigns 0 to pixels below the minimum:
> https://github.com/MapServer/MapServer/blob/main/mapdrawgdal.c#L1555
>
> It turns out that this small code change actually produces the desired
> behavior:
>
>        fScaledValue = (float) ((pafRawData[i]-dfScaleMin)*dfScaleRatio);        if( fScaledValue < 0.0 )-        pabyBuffer[i] = 0;+        if (pafRawData[i] > 0) {+            pabyBuffer[i] = 1;+        } else {+            pabyBuffer[i] = 0;+        }
>
> (I.e. if the original value was greater than 0, then assign 1 to this pixel such that it will be black and not transparent.)
>
> If this behavior can't yet be configured via mapscript, we could work on a pull request to implement this behavior, which would then be activated via a new configuration option.
> Do you have any thoughts on this? Does this make sense to you as a feature?
>
> Best regards,
> Bernhard
>
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>
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