target_scaling.py

bl_info = {
    "name": "Target Scaling",
    "author": "Vladimir Elistratov <vladimir.elistratov@gmail.com>",
    "version": (1, 0, 0),
    "blender": (2, 7, 8),
    "location": "View 3D > Edit Mode > Tool Shelf",
    "description": "Scale your model to the correct target size",
    "warning": "",
    "wiki_url": "https://github.com/vvoovv/blender-tools/wiki/Target-Scaling",
    "tracker_url": "https://github.com/vvoovv/blender-tools/issues",
    "support": "COMMUNITY",
    "category": "3D View",
}

import bpy, bmesh

def getSelectedEdgeLength(context):
    # getting active edge via bmesh and its select_history
    obj = context.active_object
    bm = bmesh.from_edit_mesh(obj.data)
    edge = bm.select_history.active
    l = -1
    if isinstance(edge, bmesh.types.BMEdge):
        # calculating edge length in the world space using obj.matrix_world
        l = obj.matrix_world * edge.verts[1].co - obj.matrix_world * edge.verts[0].co
        l = round(l.length, 5)
    return l

class TargetScalingPanel(bpy.types.Panel):
    bl_space_type = "VIEW_3D"
    bl_region_type = "TOOLS"
    bl_context = "mesh_edit"
    bl_label = "Target Scaling"

    def draw(self, context):
        layout = self.layout
        
        layout.row().operator("edit.select_target_edge")
        row = layout.row()
        if _.target_length <= 0:
            row.enabled = False
        row.operator("edit.do_target_scaling")

class SelectTargetEdge(bpy.types.Operator):
    bl_idname = "edit.select_target_edge"
    bl_label = "Select a target edge"
    bl_description = "Select a target edge"

    def execute(self, context):
        # getting active edge via bmesh and its select_history
        l = getSelectedEdgeLength(context)
        if l > 0:
            _.target_length = l
            self.report({"INFO"}, "The target edge length is {}".format(l))
        else:
            self.report({"ERROR"}, "Select a single target edge!")
        return {"FINISHED"}

class DoTargetScaling(bpy.types.Operator):
    bl_idname = "edit.do_target_scaling"    
    bl_label = "Perform mesh scaling"
    bl_description = "Perform whole mesh scaling, so the selected edge will be equal to the target one."
    bl_options = {"UNDO"}

    def execute(self, context):
        l = getSelectedEdgeLength(context)
        if l > 0:
            scale = _.target_length/l
            
            # do scaling
            bpy.ops.mesh.select_all(action="SELECT")
            bpy.ops.transform.resize(value=(scale, scale, scale))
            
            # deselect everything
            bpy.ops.mesh.select_all(action="DESELECT")
        else:
            self.report({"ERROR"}, "Select a single edge for target scaling!")
        return {"FINISHED"}

class _:
    """An auxiliary class to store plugin data"""
    target_length = -1


def register():
    bpy.utils.register_module(__name__)

def unregister():
    bpy.utils.unregister_module(__name__)