Create a polygonal padstack: expanding the copper polygon to create the mask

[auxym]

I am trying to draw the center pad for a SOT-89 land pattern (https://www.nxp.com/docs/en/package-information/SOT89.pdf), which is a polygon that looks like two rectangles smashed together. I have drawn the copper layer, but is there any way to automatically generate the mask layer by expanding the copper polygon by (eg.) 0.1 mm? Do I need a parameter program for this, or is there a drawing tool to do it?

[auxym]

Also, this page (https://horizon-eda.readthedocs.io/en/latest/cursed-footprints.html) mentions the “Convert to Pad” tool, but I can't find this anywhere.

[carrotIndustries]

Do I need a parameter program for this, or is there a drawing tool to do it?

Parameter program it is. Select the polygon and click the "Polygon expand" button in the parameter program editor: https://horizon-eda.readthedocs.io/en/latest/parameter-programs.html#expand-polygon

“Convert to Pad” tool, but I can't find this anywhere.

Draw a copper polygon in the package, "Convert to Pad" should be available from the context menu.

[auxym]

Ah! I was wondering what this button (expand polygon) did, it seemed to do nothing when I clicked on it. For what it's worth, here are detailed instructions on what I had to do:

1. Copy the copper polygon and change the copy to the mask layer
2. In the polygon property, assign a Parameter class (eg. "mask").
3. Select a single edge of the mask polygon. If I selected the full polygon, the "Expand polygon" button did nothing for some reason. Is this a bug?
4. Open the Parameters window and add the "Solder mask expansion" parameter.
5. Click the "Insert parameter into program" button next to the parameter
6. Finally, click Expand polygon (should add an expand-polygon ... line in the program), Apply, and it should work.