1

What I am looking for is a way to create a path then move the endpoint on one side of the path and paint the area covered by the movement of the path.

So for example, if I have a straight path then I can paint a semicircle by grabbing one end of the path and move 180 degrees around the other endpoint.

Also I want to be able to sweep with a color in a lower opacity so if I move 450 degrees I will have the overlapped region have more saturation.

1

Different way:

  • Render your path on a transparent layer with Edit>Stroke line
  • Rotate the layer to make the path more or less vertitcal
  • Shrink the layer around it: Layer>Crop to content
  • Copy the layer to the clipboard (Select>All, Edit>Copy)
  • Start the brush tool:
    • Set the brush to the clipboard brush (1st in the list, top-left corner in the palette)
    • Set the Spacing to something small
    • Set the dynamics to Track direction
  • Paint

But this isn't very accurate when done by hand, from time to time there are quantum orientation changes that produce gaps, even if you use the "smooth line" options. Things are a bit better if you stroke a path (don't forget to tick Emulate brush dynamics), but you can't have sharp bends:

enter image description here

2

There's no such brush. Something resembling can be done filling a selection with color. You can make the selection as follows:

1) Draw the sweeping path in its start position, duplicate it

NOTE: make every path visible as soon as you have drawn it

2) Edit the duplicate to present the final sweep position

3) Draw new paths which present the routes of the endpoints

4) Merge visible paths

5) Edit the result to properly closed path. You have to connect point pairs by left clicking on them twice holding Ctrl key at the same time. After connecting points you can move them to exactly the same place. Before inserting the line segments also the node handles to outwards must be shortened to zero. Otherwise the result is unpredictable.

6) Convert path to selection

7) Fill the selection. You can paint, use the paint bucket or fill path. The color is inserted to the layer which is active in the layers panel.

Explore carefully the content of the Paths panel and the functions available with right click.

ADD: You seem to want shapes which have well defined edges - paths. In vector graphics programs they are the main subject. Get Inkscape if you haven't one. It's free.

  • 5) Edit the result to properly closed path; have snap to active path =ON to make it possible: You may want to expand on that... "snap to active path" is just a positioning help. AFAIK there is no built-in way in Gimp to splice path fragments. You can at best connect strokes with straight segments. You can splice strokes that have close enough end points using the join strokes function of the ofn-path-edit script. – xenoid Apr 5 at 12:32
  • Hmm I don't think this will work for what I am looking for. For example, I want to be able to sweep with a color in a lower opacity so if I move 450 degrees in my circle example, I will have the overlapped region with more saturation. Kind of like rotating a squeegee. – kkawabat Apr 5 at 17:00
  • @user287001 Can't make that work, even when the anchors are so close that there is no visible gap at 256x zoom. – xenoid Apr 5 at 20:19
  • @xenoid Actually you are right,it's a lucky accident if it works, generally there's something left out of the selection.The working way is to add a line segment with Ctrl+Left Clicks between the nodes and move the the nodes to the same place. I must fix the answer. – user287001 Apr 5 at 22:41
  • Another way is to use the script I mentioned in my first comment. Much faster :) – xenoid Apr 6 at 6:53
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  • Rotate the path with the path tool (initial in blue, rotated in red):

enter image description here

  • Zoom in (25600%) to the center of rotation, and shift slightly (1/10th of a pixel, see ruler for scale) the extremity of at least one of the two paths:

enter image description here

  • Merge the two paths (Merge visible paths in the Paths list right-click menu), and re-connect them with a small segment:

enter image description here

  • Create a circle selection:

enter image description here

  • Extend the path by adding segments around and outside the circle selection:

enter image description here

  • Right click the path in the Path list, click Intersect with Selection, then Bucket-fill the selection:

enter image description here

  • Hmm, I am looking for something that allows me to paint continuously, like painting with a squeegee. For example, I want to be able to sweep with a color in a lower opacity so if I move 450 degrees in my circle example, I will have the overlapped region with more saturation. – kkawabat Apr 5 at 17:02

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