How to flatten a non-planar face to a targeted plane?

Question

Let's take a non-planar quad here for a simple example, how to only move one vertex on it, to finally make it planar while keeping the other three vertices still?

Answer 1

What @Denis suggested is good, I mean, it can be convenient sometimes with the help of view align. Besides, there are also other ways to do so:

Method 1:

1. Enable Snap, set Snap Element to Vertex.
2. Hold Shift and click on Edge Select button.
3. Use Normal as the Transform coordinate.
4. Use Active Element as the pivot center.
5. First select the vertex you want to move, then the edge that contains any two vertices on the target plane. Then R Y Y, then rotate until it snapped on another vertex on the same plane.

Method 2:

1. Temporarily triangulate the face.
2. Use Active Element as the pivot center.
3. Keep the target triangle selected, which is coplanar with that plane, then S Z Z 0.
4. Select the two triangles, press F to convert them back to quad.

Method 3:

1. Snap the vertex that you want to move to counter vertex.
2. Keep it activated, then G G to slide it along one edge, press C once or holding Alt to disable Clamp.
3. Repeat last step on another edge.

Method 4:

This is sort of similar to Denis's answer, but keeping current point of view unchanged (needed sometimes).

1. Select three vertices, create a new coordinate system based on current selection (or simply press Ctrl Alt Spacebar), and use it as current transform orientation.
2. Use Active Element as the pivot center.
3. First select the vertex you want to move, then select any one of the rest vertices, then S Z Z 0.

Method 5:

This is really a one-stop solution by using a script/addon coded by nirenyang, a friend from local community. After being requested, he made this for us, also posted on BA.

Here is my translated version of his script:

bl_info = {
    "name": "Coplanar by 3 Verts",
    "author": "NirenYang[BlenderCN]",
    "version": (0, 1),
    "blender": (2, 75, 5),
    "location": "3d view - toolbar",
    "description": "Make vertices coplanar using a plane defined by the first/last three selected verts.",
    "warning": "",
    "wiki_url": "",
    "tracker_url": "",
    "category": "mesh",
}


import bpy
import bmesh
from mathutils.geometry import intersect_line_plane, distance_point_to_plane, normal

enum_ref = [( 'first', 'First', 'Defined by the first three selected verts' ),
            ( 'last', 'Last', 'Defined by the last three selected verts' )]
class MESH_OT_3points_flat_trim(bpy.types.Operator):
    """
    Manually pick three vertices to define the reference plane
    """
    bl_idname = 'mesh.3points_flat_trim'
    bl_label = 'Coplanar by 3 Verts'
    bl_options = {'REGISTER', 'UNDO'}

ref_order = bpy.props.EnumProperty(name='Refferece Plane', description='Use the first/last three selected vertices to define the reference plane', items=enum_ref, default="last")
filter_distance = bpy.props.FloatProperty(name='Filter Distance', description='Only affects vertices further than this distance', default=0.0, precision=3, min=0.0)


@classmethod
def poll(cls, context):
    obj = context.active_object
    return (obj and obj.type == 'MESH')

def execute(self, context):
    C = context
    D = bpy.data
    ob = C.active_object

    #if bpy.app.debug != True:
    #    bpy.app.debug = True
    #    if C.active_object.show_extra_indices != True:
    #        C.active_object.show_extra_indices = True

    if ob.mode == 'OBJECT':
        me = C.object.data
        bm = bmesh.new()
        bm.from_mesh(me)
    else:
        obj = C.edit_object
        me = obj.data
        bm = bmesh.from_edit_mesh(me)

    bm.select_history.validate()
    if len(bm.select_history) < 3:
        self.report({'INFO'}, 'Pick three vertices first')
        return {'CANCELLED'}

    points3Index = []
    points3 = []
    _ordering = bm.select_history if self.ref_order=="first" else list(bm.select_history)[::-1]
    for i in _ordering:
        if len(points3) >= 3:
            break
        elif isinstance(i, bmesh.types.BMVert):
            points3.append(i.co)
            points3Index.append(i.index)
    print(points3Index)
    if len(points3) < 3:
        self.report({'INFO'}, 'at least three vertices are needed to be selected')
        return {'CANCELLED'}


    points3Normal = normal(*points3)
    for v in bm.verts:
        if v.select and v.index not in points3Index:
            _move = True
            if self.filter_distance > 0.0:
                _move = abs(distance_point_to_plane(v.co, points3[0], points3Normal)) < self.filter_distance
            if _move == True:
                v.co = intersect_line_plane(v.co, v.co+points3Normal, points3[0], points3Normal)

    if ob.mode == 'OBJECT':
        bm.to_mesh(me)
        bm.free()
    else:
        bmesh.update_edit_mesh(me, True)

    return {'FINISHED'}

def menu_func_MESH_OT_3points_flat_trim(self, context):
    self.layout.operator(MESH_OT_3points_flat_trim.bl_idname,
                        text=MESH_OT_3points_flat_trim.bl_label)

def register():   
    bpy.utils.register_class(MESH_OT_3points_flat_trim) 
    bpy.types.VIEW3D_PT_tools_meshedit.append(menu_func_MESH_OT_3points_flat_trim)

def unregister():
    bpy.types.VIEW3D_PT_tools_meshedit.remove(menu_func_MESH_OT_3points_flat_trim)
    bpy.utils.unregister_class(MESH_OT_3points_flat_trim)

if __name__ == "__main__":
    register()

Answer 2

Blender now has a tool to make faces flat.

From the 3D view header menu:
Mesh -> Cleanup -> Make Planar Faces

This iteratively flattens faces, you can choose how much to move the vertices each iteration too.

Answer 3

You can do it manually in few steps.

1. Create a face of 3 vertices that you want to align to.
2. Hit Shift+7 to align the view to the normal of that face while the face is selected in face selection mode.
3. Hit Shift+S shortcut and select Cursor to Selected.
4. Change Pivot Point to Cursor and Transform Orientation to View.
5. Select the rest of the vertices, hit S button to scale, Z button twice to limit scale on the Z axis of the view and input 0.

Answer 4

If anyone's still wondering, this can easily be done in version 2.8 (maybe possible in previous versions too).

After selecting the face you want to flatten to a plane, use the scale gizmo and begin scaling the face. You'll notice a 'resize' popup appears near the bottom-left side of the screen.

This Popup indicates changes you make to the three axes using the scale gizmo. All you have to do is set the value of the axis you want to flatten your plane in to 0.

This will flatten your plane along X,Y or Z depending on what you choose. You can change the orientation of the plane further by using the rotate gizmo.

If you're transitioning from Maya, this method would be ideal for you.