Sometimes when you draw a box, you may wish to pretty up the corners a bit, for example by making them rounded or just cutting them off. This might be for many reasons, perhaps you are making a box to be a nice frame around your map legend, or you are making some kind of map marker. And of course, this technique doesn’t just work with boxes, but with every case where you have two lines joining in some kind of corner.
If you have been seeing some of the recent live mapping videos, you may have noticed Ralf using this command to produce simple village symbols.
We have two sets of commands that are very helpful here, the Chamfer and Fillet commands. If you right click the Trim 
button you’ll find two versions of each of these commands. The Fillet command is for making a rounded corner, while the Chamber command makes an angled corner. The difference between the regular and Trim version of the commands is that the regular version will add the line/curve but will not trim away the extra part of the line, while the Trim version also trims the lines to the new corner shape. You normally want to use the Trim versions of these commands, but feel free to experiment.
Let us take these commands for a run to make a nice box with trimmed corners, for example to use as decoration around your map legend. I am just going to make a solid-colored frame here, but of course, you can use a bitmap fill for the box instead, and you can produce both a background and frame with the same corners by simply making the background first, then making a copy of it, changing the color, and setting line width to a value higher than zero.
I start by using the Box 
tool to draw a background behind my legend.
Now, I deliberately used the box tool here. This is because we cannot trim a box directly, we need to turn it into individual lines first. But we do often find ourself starting with a box (and wanting to end up with one, not individual lines, when we are done) so I deliberately forced this step here by starting with the box tool. If you have drawn it as individual lines, you can skip this step.
Use the Explode 
command on the box you just drew. This will reduce it from a box to the four individual lines the box is made up of. Note that this WILL make the fill disappear, as it isn’t a box anymore, so there is no actual area to fill. This is completely normal and expected, and we will have our fill back in the end when we are done.
Now, we have four lines meeting in four points, our corners. We need to use the command once for each corner. When you run either of the Chamfer or Fillet commands, keep an eye on the command line, it will ask you for three things. The first two prompts are for the first and second entities. This will be the two lines whose corner you want to fix. The third prompt is the radius/length of the the corner, for this prompt you need to type in a value from the keyboard and hit enter. The actual length you’ll need depends on the size of the box and your personal preferences. Since the box is likely to be sized depending on the overall size of the map, it is hard to give a good guideline, but for a legend like this, try with 10 first (this value is in map units, so in a dungeon map, 10 means 10 feet, in an overland map, it means 10 miles). If you aren’t satisfied, just hit undo and do the command again with a different value.
Here I’ve used Fillet & Trim on the top left and bottom right corner, and Chamfer & Trim on the other two.
One little troubleshooting tip here. When trimming the lines to the corners, CC3+ will always keep the part of the line that contains the point you clicked on it to select it, so if you click too close to the corner when selecting the lines, you risk CC3+ keeping the part you wanted to cut away, and cutting away the rest of the line you wished to keep, as shown in the bottom right corner of the screenshot. To avoid this, simply make sure you select the lines by clicking somewhere on it in good distance away from the corner.
The final step is to turn those lines back into a single polygon. Because the way we made this ensures that all the lines meet up perfectly, this is quite easy. Just right click the explode button and pick the Line to Path option, then select all the lines. There should be 8 lines now, the four we started with, but also one extra line for each corner (the lines for the rounded corners are actually arcs, which in some cases requires special handling, but the Line to Path will handle this for us). Make sure you do not select anything else other than these 8 lines, like the text, as the extra entities will prevent Line to Path from combining the polygon properly. Once you executes the command, the lines will turn into a path, but CC3+ will notice that since the path is a closed path that starts and ends in the same place, it will also automatically turn it into a polygon for us, and we should now have our filled polygon back, but with nicer corners. (Note that if you have a path that isn’t properly closed, you can always finish off with the Path to Poly command to force it into a polygon, but that shouldn’t be required in out case here).
And just to add a frame, I made a copy of my new polygon, made sure the copy was placed above the original poly, then changed the color of the copy, and the line width to 1. By making it a non-zero width, I get a nice hollow shape. Since the frame is a separate entity, it can be put on a different sheet and give effects if you so desire, like a bevel.
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