W.I.P. Start of paint-tool plugin UX work.

Should be able to paint pixels now if the tools is active, and the model loaded is with UV-coords (that rules out our current impl. of 3MF at the moment -- use OBJ instead), and you position the model outside of the build-plate so the paint-shadr that is temporarily replacing the 'disabled' one is showing.

Will need a lot of extra features and optimizations still!

part of CURA-12543

plugins/PaintTool/PaintTool.py

@@ -0,0 +1,135 @@
+# Copyright (c) 2025 UltiMaker
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from typing import cast, Optional
+
+import numpy
+from PyQt6.QtCore import Qt
+
+from UM.Application import Application
+from UM.Event import Event, MouseEvent, KeyEvent
+from UM.Tool import Tool
+from cura.PickingPass import PickingPass
+
+
+class PaintTool(Tool):
+    """Provides the tool to paint meshes.
+    """
+
+    def __init__(self) -> None:
+        super().__init__()
+
+        self._shortcut_key = Qt.Key.Key_P
+
+        """
+        # CURA-5966 Make sure to render whenever objects get selected/deselected.
+        Selection.selectionChanged.connect(self.propertyChanged)
+        """
+
+    @staticmethod
+    def _get_intersect_ratio_via_pt(a, pt, b, c):
+        # compute the intersection of (param) A - pt with (param) B - (param) C
+
+        # compute unit vectors of directions of lines A and B
+        udir_a = a - pt
+        udir_a /= numpy.linalg.norm(udir_a)
+        udir_b = b - c
+        udir_b /= numpy.linalg.norm(udir_b)
+
+        # find unit direction vector for line C, which is perpendicular to lines A and B
+        udir_res = numpy.cross(udir_b, udir_a)
+        udir_res /= numpy.linalg.norm(udir_res)
+
+        # solve system of equations
+        rhs = b - a
+        lhs = numpy.array([udir_a, -udir_b, udir_res]).T
+        solved = numpy.linalg.solve(lhs, rhs)
+
+        # get the ratio
+        intersect = ((a + solved[0] * udir_a) + (b + solved[1] * udir_b)) * 0.5
+        return numpy.linalg.norm(pt - intersect) / numpy.linalg.norm(a - intersect)
+
+    def event(self, event: Event) -> bool:
+        """Handle mouse and keyboard events.
+
+        :param event: The event to handle.
+        :return: Whether this event has been caught by this tool (True) or should
+        be passed on (False).
+        """
+        super().event(event)
+
+        # Make sure the displayed values are updated if the bounding box of the selected mesh(es) changes
+        if event.type == Event.ToolActivateEvent:
+            return False
+
+        if event.type == Event.ToolDeactivateEvent:
+            return False
+
+        if event.type == Event.KeyPressEvent and cast(KeyEvent, event).key == KeyEvent.ShiftKey:
+            return False
+
+        if event.type == Event.MousePressEvent and self._controller.getToolsEnabled():
+            if MouseEvent.LeftButton not in cast(MouseEvent, event).buttons:
+                return False
+            if not self._selection_pass:
+                return False
+
+            camera = self._controller.getScene().getActiveCamera()
+            if not camera:
+                return False
+
+            evt = cast(MouseEvent, event)
+
+            ppass = PickingPass(self._selection_pass._width, self._selection_pass._height)
+            ppass.render()
+            pt = ppass.getPickedPosition(evt.x, evt.y).getData()
+
+            self._selection_pass._renderObjectsMode()  # TODO: <- Fix this!
+
+            node_id = self._selection_pass.getIdAtPosition(evt.x, evt.y)
+            if node_id is None:
+                return False
+            node = Application.getInstance().getController().getScene().findObject(node_id)
+            if node is None:
+                return False
+
+            self._selection_pass._renderFacesMode()  # TODO: <- Fix this!
+
+            face_id = self._selection_pass.getFaceIdAtPosition(evt.x, evt.y)
+            if face_id < 0:
+                return False
+
+            meshdata = node.getMeshDataTransformed()  # TODO: <- don't forget to optimize, if the mesh hasn't changed (transforms) then it should be reused!
+            if not meshdata:
+                return False
+
+            va, vb, vc = meshdata.getFaceNodes(face_id)
+            ta, tb, tc = node.getMeshData().getFaceUvCoords(face_id)
+
+            # 'Weight' of each vertex that would produce point pt, so we can generate the texture coordinates from the uv ones of the vertices.
+            # See (also) https://mathworld.wolfram.com/BarycentricCoordinates.html
+            wa = PaintTool._get_intersect_ratio_via_pt(va, pt, vb, vc)
+            wb = PaintTool._get_intersect_ratio_via_pt(vb, pt, vc, va)
+            wc = PaintTool._get_intersect_ratio_via_pt(vc, pt, va, vb)
+            wt = wa + wb + wc
+            wa /= wt
+            wb /= wt
+            wc /= wt
+            texcoords = wa * ta + wb * tb + wc * tc
+
+            solidview = Application.getInstance().getController().getActiveView()
+            if solidview.getPluginId() != "SolidView":
+                return False
+
+            solidview.setUvPixel(texcoords[0], texcoords[1], [255, 128, 0, 255])
+
+            return True
+
+        if event.type == Event.MouseMoveEvent:
+            evt = cast(MouseEvent, event)
+            return False #True
+
+        if event.type == Event.MouseReleaseEvent:
+            return False #True
+
+        return False

plugins/PaintTool/PaintTool.qml

@@ -0,0 +1,14 @@
+// Copyright (c) 2025 UltiMaker
+// Cura is released under the terms of the LGPLv3 or higher.
+
+import QtQuick 2.2
+
+import UM 1.7 as UM
+
+Item
+{
+    id: base
+    width: childrenRect.width
+    height: childrenRect.height
+    UM.I18nCatalog { id: catalog; name: "cura"}
+}

plugins/PaintTool/__init__.py

@@ -0,0 +1,21 @@
+# Copyright (c) 2025 UltiMaker
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from . import PaintTool
+
+from UM.i18n import i18nCatalog
+i18n_catalog = i18nCatalog("cura")
+
+def getMetaData():
+    return {
+        "tool": {
+            "name": i18n_catalog.i18nc("@action:button", "Paint"),
+            "description": i18n_catalog.i18nc("@info:tooltip", "Paint Model"),
+            "icon": "Visual",
+            "tool_panel": "PaintTool.qml",
+            "weight": 0
+        }
+    }
+
+def register(app):
+    return { "tool": PaintTool.PaintTool() }

plugins/PaintTool/plugin.json

@@ -0,0 +1,8 @@
+{
+    "name": "Paint Tools",
+    "author": "UltiMaker",
+    "version": "1.0.0",
+    "description": "Provides the paint tools.",
+    "api": 8,
+    "i18n-catalog": "cura"
+}