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

[Mallinger, Bernhard]

Thank you for the suggestion, I have tried it now.
Since the values below the minimum scale is mapped to 0, i tried using the
following instruction:

PROCESSING "LUT=0:1"

This actually maps the values below the minimum to 1, but also the no data
values. Do you know if there's a way where I can distinguish between those
two using LUT?

Best,
Bernhard


On Mon, Nov 28, 2022 at 3:26 PM Rahkonen Jukka <
jukka.rahkonen at maanmittauslaitos.fi> wrote:

> Hi,
>
>
>
> Have you tried the LUT processing option
> https://mapserver.org/input/raster.html?
>
>
>
> -Jukka Rahkonen-
>
>
>
> *Lähettäjä:* MapServer-users <mapserver-users-bounces at lists.osgeo.org> *Puolesta
> *Mallinger, Bernhard
> *Lähetetty:* maanantai 28. marraskuuta 2022 15.35
> *Vastaanottaja:* mapserver-users at lists.osgeo.org
> *Aihe:* [MapServer-users] Distinguish between pixels outside of SCALE
> range and nodata pixels
>
>
>
> 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
> <https://eur06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FMapServer%2FMapServer%2Fblob%2Fmain%2Fmapdrawgdal.c%23L1555&data=05%7C01%7Cjukka.rahkonen%40maanmittauslaitos.fi%7Cc759129ba15f4a44e03d08dad1456ef8%7Cc4f8a63255804a1c92371d5a571b71fa%7C0%7C0%7C638052393387117846%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=t6zAXKjbvlaZx5FiDt3lPMGBui7IshCIXHLiBdxXSBk%3D&reserved=0>
>
>
>
> 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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