I have found a way to get a very precise rotation using the following steps. They assume that you want to get two adjacent vertices in the polygon to have exactly the same 'x' or exactly the same 'y'. You'll need to convert your polygon to a path in order to do this, but the end result should be as close to exactly what you want as you're going to get without special code to do this type of snapping for you.
1. Get the polygon's centre exactly on one of the two adjacent verticesThe centre is stored in the XML using attributes "inkscape:transform-center-x" and "inkscape:transform-center-y". These are relative to the centre of gravity of the polygon. In order to set them, then, you need to find out the coordinates of the vertex relative to the centre of gravity. The easiest way I found to achieve that is to put the polygon's centre of gravity at the origin (0, 0), which I did this way (there might be an easier way?):
- Select the freehand lines tool (F6).
- Click anywhere in the canvas, and then hold down Ctrl while clicking below the line to make a perfectly vertical line.
- Switch to the selection tool (F1) and select the line as an object.
- In the toolbar, manually enter an X coordinate of 0, snapping the line to Y axis.
- Select your polygon, then shift-click the line.
- In the Align and Distribute panel, make sure that "Relative to" is set to "Last Selected", then click on "Center on vertical axis" in the first row.
- Select the guide line and delete it. Next, we'll create a similar guide line for the horizontal axis.
[*} Click once, and then use Ctrl while clicking to the right of the line to create a perfectly horizontal line. - Select it with the selection tool, and set its Y coordinate in the toolbar to 0, snapping it to the X axis.
- Select your polygon, then shift-click the line.
- In the Align and Distribute panel, with "Relative to" still set to "Last Selected", click on "Center on horizontal axis" in the second row.
Now the polygon's centre is at exactly (0, 0). This means that the absolute coordinates of its vertices are also relative to its centre of mass. So, now we move the centre to *exactly* coincide with one of the two vertices you want to flatten out.
- Using the selection tool (F1), click on the object twice to put it into rotate mode. Then, drag the centre anywhere. This will create the attributes for specifying the transform centre so you don't have to input them yourself.
- Open the XML editor (Ctrl-Shift-X). It should immediately have the <svg:path> element selected.
- Switch to the nodes tool (F2) and select one of the ends of the segment you want to be flat.
- Select & copy the value of the X coordinate in the toolbar.
- In the XML editor, click on the row "inkscape:transform-center-x" in the right upper pane. In the right lower pane, replace the value with the number you copied from the toolbar. Press Ctrl-Enter to commit the change.
- Select & copy the value of the Y coordinate in the toolbar.
- In the XML editor, click on the row "inkscape:transform-center-y" in the right upper pane. In the right lower pane, replace the value with the number you copied from the toolbar. Press Ctrl-Enter to commit the change.
You should now see the little '+' icon for the transform centre exactly coinciding with the vertex you selected.
2. Create a guide & rotate the polygonAt this point, rotating the polygon will preserve the position of the first vertex, the one that now coincides with the transform centre. To actually get the rotation you want, you now want to apply rotation so as to move the other vertex to the same x or y coordinate as the first. Start by creating a guide:
- Create a perfectly horizontal or vertical line using the free-form lines tool (F6), by clicking, and then holding down Ctrl before clicking a second time.
- Switch to the nodes tool (F2) and select the first vertex in your polygon, the one from the previous steps. If your guide is horizontal, select & copy the Y coordinate. If your guide is vertical, select & copy the X coordinate.
- Switch to the selection tool (F1) and select your guide again. Paste the copied coordinate into the corresponding coordinate on the toolbar for your line. It should now pass straight through the point of rotation of the polygon!
You're now all set for the final step: the actual rotation of the polygon. After completing the previous steps, the polygon will rotate precisely around the first point, and you can rotate it until the second point is also exactly on the guide:
- With the selection tool (F1), click on your polygon twice to get it into rotate/skew mode and rotate it until the second point is "pretty close" to the guide.
- Zoom in extremely close on that second point, as far as you can while keeping both the point and the guide on the screen.
- The keyboard shortcut Alt-[ or Alt-] rotates "by pixels", which means that the further zoomed in you are, the finer the rotation is. Rotate the polygon until you can't see any difference between the location of your second vertex and the guide.
- If you can zoom in further, do so and repeat the previous step. Repeat as needed.
- When you're done, delete the guide and admire your handy-work.
Hope that's helpful to someone out there.
It's a bit of leg work, but it does get the job done!
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). Pressing Ctrl while drawing, the rotation snaps to multiple of 15°, but it never look "horizontal". Same thing when rotating after draw.
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