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

[Mallinger, Bernhard]

Thank you for the suggestion, this actually achieves the desired effect!

I'm just wondering if this now reads the data twice?

We're using data from remote locations which can be slow to read, so
reading them twice would be inconvenient.

Best,
Bernhard


On Mon, Nov 28, 2022 at 3:18 PM Timothy Kempisty - NOAA Federal <
timothy.kempisty at noaa.gov> wrote:

> 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
>>
>> _______________________________________________
>> MapServer-users mailing list
>> MapServer-users at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/mapserver-users
>>
>
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