Add Polygon Select to the Lasso tool

[Acyone]

Description

When using the lasso tool, crtl-clicking or shift-clicking a point would draw the selection in a straight line from the last selected point to the next.

This would :

• allow selection of polygonal shapes with perfect edges
• make selecting precise shapes easier
• speed up rough selections

Additional context

Discussed in #610

^{Originally posted by enderpuff December 15, 2023}
Pretty self explanatory, but polygon selects are very very useful for editing images, for stuff like isolating objects or removing backgrounds, etc...

[cameronwhite]

Doing this as part of the lasso tool sounds reasonable to me 👍

There could also be an option in the toolbar to switch between the two modes, as an alternative idea to the ctrl/shift hotkeys

[Mtothexmax]

Here is the untested source code made with an LLM

using System;
using System.Collections.Generic;
using Cairo;
using ClipperLib;
using Pinta.Core;

namespace Pinta.Tools;

public sealed class LassoSelectTool : BaseTool
{
    private readonly IWorkspaceService workspace;

    private bool is_drawing = false;
    private CombineMode combine_mode;
    private SelectionHistoryItem? hist;
    private List<IntPoint> polygon_points = new List<IntPoint>();

    public LassoSelectTool(IServiceProvider services) : base(services)
    {
        workspace = services.GetService<IWorkspaceService>();
    }

    public override string Name => Translations.GetString("Lasso Select");
    public override string Icon => Pinta.Resources.Icons.ToolSelectLasso;
    public override string StatusBarText => Translations.GetString("Click to set points for a polygonal selection area. Click near the starting point to close the selection.");
    public override Gdk.Key ShortcutKey => new(Gdk.Constants.KEY_S);
    public override Gdk.Cursor DefaultCursor => Gdk.Cursor.NewFromTexture(Resources.GetIcon("Cursor.LassoSelect.png"), 9, 18, null);
    public override int Priority => 17;

    protected override void OnBuildToolBar(Gtk.Box tb)
    {
        base.OnBuildToolBar(tb);
        workspace.SelectionHandler.BuildToolbar(tb, Settings);
    }

    protected override void OnMouseDown(Document document, ToolMouseEventArgs e)
    {
        PointD pd = document.ClampToImageSize(e.PointDouble);
        IntPoint ip = new IntPoint((long)Math.Round(pd.X), (long)Math.Round(pd.Y));

        if (!is_drawing)
        {
            hist = new SelectionHistoryItem(workspace, Icon, Name);
            hist.TakeSnapshot();
            combine_mode = workspace.SelectionHandler.DetermineCombineMode(e);
            polygon_points.Clear();
            polygon_points.Add(ip);
            is_drawing = true;
            document.PreviousSelection = document.Selection.Clone();
        }
        else
        {
            polygon_points.Add(ip);
            if (polygon_points.Count >= 3 && SquaredDistance(polygon_points[polygon_points.Count - 1], polygon_points[0]) < 25)
            {
                polygon_points.RemoveAt(polygon_points.Count - 1);
                document.Selection.SelectionPolygons.Clear();
                document.Selection.SelectionPolygons.Add(polygon_points);
                SelectionModeHandler.PerformSelectionMode(document, combine_mode, document.Selection.SelectionPolygons);
                if (hist != null)
                {
                    document.History.PushNewItem(hist);
                    hist = null;
                }
                is_drawing = false;
                polygon_points.Clear();
            }
        }
    }

    protected override void OnMouseMove(Document document, ToolMouseEventArgs e)
    {
        if (!is_drawing)
            return;

        PointD pd = document.ClampToImageSize(e.PointDouble);
        IntPoint ip = new IntPoint((long)Math.Round(pd.X), (long)Math.Round(pd.Y));
        List<IntPoint> temp_polygon = new List<IntPoint>(polygon_points);
        temp_polygon.Add(ip);

        document.Selection.SelectionPolygons.Clear();
        document.Selection.SelectionPolygons.Add(temp_polygon);
        SelectionModeHandler.PerformSelectionMode(document, combine_mode, document.Selection.SelectionPolygons);
        document.Workspace.Invalidate();
    }

    protected override void OnMouseUp(Document document, ToolMouseEventArgs e)
    {
        // No action needed for polygon lasso
    }

    private static long SquaredDistance(IntPoint a, IntPoint b)
    {
        long dx = a.X - b.X;
        long dy = a.Y - b.Y;
        return dx * dx + dy * dy;
    }
}

Can someone please try it?

Replace the LassoSelectTool for a quick test but create both as tools and call this one PolygonLassoSelectTool