[mapserver-dev] Legend images and scale factor

[Steve Lime]

I've always preferred the constant sizes in legends, simpler for my users
that way but I can see your use case. Something like LEGENDSCALEDENOM could
be useful. Are you proposing something be added to the legendObj? Could
maye just use SYMBOLSCALEDENOM there?

On Fri, Sep 24, 2021 at 3:19 PM Tamas Szekeres <szekerest at gmail.com> wrote:

> Hi All,
>
> Looks like we’ve been struggling for years with how the legend images
> should be drawn properly, especially when the symbol sizes are changing
> along with the scale, for example when the SIZEUNITS is set to METERS in
> the layer definition. One of the recent commit
> <https://github.com/MapServer/MapServer/pull/6359/commits/3a49bab2da26fef4e7d28fffb2b754897abfbf6c>
> related to this forcibly sets the scale factor to 1.0 when the size units
> is not PIXELS. But I don't think this is a reasonable solution in all cases.
>
> 1. It overrides the implementation which attempts to set the scale factor
> in the mapscript drawLegendIcon
> <https://github.com/MapServer/MapServer/blob/1c14821bc4c6fc10e966fd05230ad71c05817c7d/mapscript/swiginc/class.i#L165>
> .
> 2. It requires to define the size/widths in pixel sizes in the CLASS/STYLE
> definitions, so I cannot set a symbol size to 100m for example or a line
> width to 20m because it would cause giant symbol sizes in the legend images
> according to the current implementation.
>
> So what folks would think how should we set the scale factor in legend
> properly? At first glance I might think the symbol sizes in the legends
> should look exactly like the sizes on the map to avoid confusion. So if I
> set the symbol size to 10m to draw a polygon outline and it is 1 pixels on
> the map, I would draw the polygon outline with 1 pixel width on the legend,
> too.
>
> However it also seems to be a valid use case to display the symbols at
> constans sizes for example to utilize the SYMBOLSCALEDENOM setting or
> specify something different, like a new LEGENDSCALEDENOM whatever.
>
> In any case, we would definitely want to specify the scale factor either
> explicitly or implicitly from the mapscript API, but that is not the case
> anymore in the mapserver 7.6.4 or above.
>
>
> Best regards,
>
> Tamas
>
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