With the royal blessing of guitarGeek, I commit the sharp/flat mesh

editmode selection tools. The documentation can (currently) be
found here:

http://mediawiki.blender.org/index.php/Requests/SharpFlatSelect

source/blender/blenlib/BLI_editVert.h

@@ -66,6 +66,7 @@ typedef struct HashEdge {
 typedef struct EditEdge
 {
 	struct EditEdge *next, *prev;
+	/* Note: vn is for general purpose temporary storage */
 	struct EditVert *v1, *v2, *vn;
 	short f1, f2;	/* short, f1 is (ab)used in subdiv */
 	unsigned char f, h, dir, seam;

source/blender/include/BIF_editmesh.h

@@ -134,6 +134,8 @@ extern void editmesh_align_view_to_selected(struct View3D *v3d, int axis);
 
 	/* Selection */
 extern void select_non_manifold(void);
+extern void select_sharp_edges(void);
+extern void select_linked_flat_faces(void);
 extern void select_faces_by_numverts(int numverts);
 extern void select_more(void);
 extern void select_less(void);

source/blender/src/editmesh_mods.c

@@ -1506,6 +1506,264 @@ void select_faces_by_numverts(int numverts)
 		BIF_undo_push("Select non-Triangles/Quads");
 }
 
+void select_sharp_edges(void)
+{
+	/* Find edges that have exactly two neighboring faces,
+	 * check the angle between those faces, and if angle is
+	 * small enough, select the edge
+	 */
+	EditMesh *em = G.editMesh;
+	EditEdge *eed;
+	EditFace *efa;
+	EditFace **efa1;
+	EditFace **efa2;
+	long edgecount = 0, i, *vnptr;
+	static short sharpness = 135;
+	float fsharpness;
+
+	if(G.scene->selectmode==SCE_SELECT_FACE) {
+		error("Doesn't work in face selection mode");
+		return;
+	}
+
+	if(button(&sharpness,0, 180,"Max Angle:")==0) return;
+	/* if faces are at angle 'sharpness', then the face normals
+	 * are at angle 180.0 - 'sharpness' (convert to radians too)
+	 */
+	fsharpness = ((180.0 - sharpness) * M_PI) / 180.0;
+
+	i=0;
+	/* count edges, (ab)use vn to be a long */
+	eed= em->edges.first;
+	while(eed) {
+		edgecount++;
+		vnptr = (long *) &eed->vn;
+		*vnptr = i;
+		eed= eed->next;
+		++i;
+	}
+
+	/* for each edge, we want a pointer to two adjacent faces */
+	efa1 = MEM_callocN(edgecount*sizeof(EditFace *), 
+					   "pairs of edit face pointers");
+	efa2 = MEM_callocN(edgecount*sizeof(EditFace *), 
+					   "pairs of edit face pointers");
+
+#define face_table_edge { \
+		i = *vnptr; \
+		if (i != -1) { \
+			if (efa1[i]) { \
+				if (efa2[i]) { \
+					*vnptr = -1; /* bad, edge has more than two neighbors */ \
+				} \
+				else { \
+					efa2[i] = efa; \
+				} \
+			} \
+			else { \
+				efa1[i] = efa; \
+			} \
+		} \
+	}
+
+	/* find the adjacent faces of each edge, we want only two */
+	efa= em->faces.first;
+	while(efa) {
+		vnptr = (long *) &efa->e1->vn;
+		face_table_edge;
+		vnptr = (long *) &efa->e2->vn;
+		face_table_edge;
+		vnptr = (long *) &efa->e3->vn;
+		face_table_edge;
+		if (efa->e4) {
+			vnptr = (long *) &efa->e4->vn;
+			face_table_edge;
+		}
+		efa= efa->next;
+	}
+
+#undef face_table_edge
+
+	eed= em->edges.first;
+	while(eed) {
+	vnptr = (long *) &eed->vn;
+		i = *vnptr;
+		if (i != -1) { 
+			/* edge has two or less neighboring faces */
+			if ( (efa1[i]) && (efa2[i]) ) { 
+				/* edge has exactly two neighboring faces, check angle */
+				float angle;
+				angle = saacos(efa1[i]->n[0]*efa2[i]->n[0] +
+							   efa1[i]->n[1]*efa2[i]->n[1] +
+							   efa1[i]->n[2]*efa2[i]->n[2]);
+				if (fabs(angle) >= fsharpness)
+					EM_select_edge(eed, 1);
+			}
+		}
+
+		eed= eed->next;
+	}
+
+	MEM_freeN(efa1);
+	MEM_freeN(efa2);
+
+	countall();
+	addqueue(curarea->win,  REDRAW, 0);
+	BIF_undo_push("Select Sharp Edges");
+}
+
+void select_linked_flat_faces(void)
+{
+	/* Find faces that are linked to selected faces that are 
+	 * relatively flat (angle between faces is higher than
+	 * specified angle)
+	 */
+	EditMesh *em = G.editMesh;
+	EditEdge *eed;
+	EditFace *efa;
+	EditFace **efa1;
+	EditFace **efa2;
+	long edgecount = 0, i, *vnptr, faceselcount=0, faceselcountold=0;
+	static short sharpness = 135;
+	float fsharpness;
+
+	if(G.scene->selectmode!=SCE_SELECT_FACE) {
+		error("Only works in face selection mode");
+		return;
+	}
+
+	if(button(&sharpness,0, 180,"Min Angle:")==0) return;
+	/* if faces are at angle 'sharpness', then the face normals
+	 * are at angle 180.0 - 'sharpness' (convert to radians too)
+	 */
+	fsharpness = ((180.0 - sharpness) * M_PI) / 180.0;
+
+	i=0;
+	/* count edges, (ab)use vn to be a long */
+	eed= em->edges.first;
+	while(eed) {
+		edgecount++;
+		vnptr = (long *) &eed->vn;
+		*vnptr = i;
+		eed= eed->next;
+		++i;
+	}
+
+	/* for each edge, we want a pointer to two adjacent faces */
+	efa1 = MEM_callocN(edgecount*sizeof(EditFace *), 
+					   "pairs of edit face pointers");
+	efa2 = MEM_callocN(edgecount*sizeof(EditFace *), 
+					   "pairs of edit face pointers");
+
+#define face_table_edge { \
+		i = *vnptr; \
+		if (i != -1) { \
+			if (efa1[i]) { \
+				if (efa2[i]) { \
+					*vnptr = -1; /* bad, edge has more than two neighbors */ \
+				} \
+				else { \
+					efa2[i] = efa; \
+				} \
+			} \
+			else { \
+				efa1[i] = efa; \
+			} \
+		} \
+	}
+
+	/* find the adjacent faces of each edge, we want only two */
+	efa= em->faces.first;
+	while(efa) {
+		vnptr = (long *) &efa->e1->vn;
+		face_table_edge;
+		vnptr = (long *) &efa->e2->vn;
+		face_table_edge;
+		vnptr = (long *) &efa->e3->vn;
+		face_table_edge;
+		if (efa->e4) {
+			vnptr = (long *) &efa->e4->vn;
+			face_table_edge;
+		}
+
+		/* while were at it, count the selected faces */
+		if (efa->f & SELECT) ++faceselcount;
+
+		efa= efa->next;
+	}
+
+#undef face_table_edge
+
+	eed= em->edges.first;
+	while(eed) {
+		vnptr = (long *) &eed->vn;
+		i = *vnptr;
+		if (i != -1) { 
+			/* edge has two or less neighboring faces */
+			if ( (efa1[i]) && (efa2[i]) ) { 
+				/* edge has exactly two neighboring faces, check angle */
+				float angle;
+				angle = saacos(efa1[i]->n[0]*efa2[i]->n[0] +
+							   efa1[i]->n[1]*efa2[i]->n[1] +
+							   efa1[i]->n[2]*efa2[i]->n[2]);
+				/* flag sharp edges */
+				if (fabs(angle) >= fsharpness)
+					*vnptr = -1;
+			}
+			else {
+				/* less than two neighbors */
+				*vnptr = -1;
+			}
+		}
+
+		eed= eed->next;
+	}
+
+#define select_flat_neighbor { \
+				i = *vnptr; \
+				if (i!=-1) { \
+					if (! (efa1[i]->f & SELECT) ) { \
+						EM_select_face(efa1[i], 1); \
+						++faceselcount; \
+					} \
+					if (! (efa2[i]->f & SELECT) ) { \
+						EM_select_face(efa2[i], 1); \
+						++faceselcount; \
+					} \
+				} \
+	}
+
+	while (faceselcount != faceselcountold) {
+		faceselcountold = faceselcount;
+
+		efa= em->faces.first;
+		while(efa) {
+			if (efa->f & SELECT) {
+				vnptr = (long *) &efa->e1->vn;
+				select_flat_neighbor;
+				vnptr = (long *) &efa->e2->vn;
+				select_flat_neighbor;
+				vnptr = (long *) &efa->e3->vn;
+				select_flat_neighbor;
+				if (efa->e4) {
+					vnptr = (long *) &efa->e4->vn;
+					select_flat_neighbor;
+				}
+			}
+			efa= efa->next;
+		}
+	}
+
+#undef select_flat_neighbor
+
+	MEM_freeN(efa1);
+	MEM_freeN(efa2);
+
+	countall();
+	addqueue(curarea->win,  REDRAW, 0);
+	BIF_undo_push("Select Linked Flat Faces");
+}
+
 void select_non_manifold(void)
 {
 	EditMesh *em = G.editMesh;

source/blender/src/header_view3d.c

@@ -859,6 +859,12 @@ void do_view3d_select_meshmenu(void *arg, int event)
 		case 13: /* select non-triangles/quads */
 			select_faces_by_numverts(5);
 			break;
+		case 14: /* select less */
+			select_sharp_edges();
+			break;
+		case 15: /* select less */
+			select_linked_flat_faces();
+			break;
 
 	}
 	allqueue(REDRAWVIEW3D, 0);
@@ -887,6 +893,12 @@ static uiBlock *view3d_select_meshmenu(void *arg_unused)
 	uiDefIconTextBut(block, BUTM, 1, ICON_BLANK1, 
 					 "Non-Manifold|Ctrl Alt Shift M", 
 					 0, yco-=20, menuwidth, 19, NULL, 0.0, 0.0, 1, 9, "");
+	uiDefIconTextBut(block, BUTM, 1, ICON_BLANK1, 
+					 "Sharp Edges|Ctrl Alt Shift S", 
+					 0, yco-=20, menuwidth, 19, NULL, 0.0, 0.0, 1, 14, "");
+	uiDefIconTextBut(block, BUTM, 1, ICON_BLANK1, 
+					 "Linked flat faces|Ctrl Alt Shift F", 
+					 0, yco-=20, menuwidth, 19, NULL, 0.0, 0.0, 1, 15, "");
 	
 	uiDefBut(block, SEPR, 0, "", 0, yco-=6, 
 			 menuwidth, 6, NULL, 0.0, 0.0, 0, 0, "");

source/blender/src/space.c

@@ -1245,6 +1245,11 @@ static void winqreadview3dspace(ScrArea *sa, void *spacedata, BWinEvent *evt)
 							edge_flip();
 						else if (G.qual==0)
 							addedgeface_mesh();
+						else if ( G.qual == 
+							 (LR_SHIFTKEY | LR_ALTKEY | LR_CTRLKEY) ) {
+							select_linked_flat_faces();
+						}
+
 					}
 					else if ELEM(G.obedit->type, OB_CURVE, OB_SURF) addsegment_nurb();
 				}
@@ -1632,7 +1637,9 @@ static void winqreadview3dspace(ScrArea *sa, void *spacedata, BWinEvent *evt)
 						initTransform(TFM_TOSPHERE, CTX_NONE);
 						Transform();
 					}
-					
+					if ( G.qual == (LR_SHIFTKEY | LR_ALTKEY | LR_CTRLKEY) ) {
+						if(G.obedit->type==OB_MESH) select_sharp_edges();
+					}
 				}
 				else if(G.qual==LR_ALTKEY) {
 					if(G.f & G_WEIGHTPAINT)

source/blender/src/toolbox.c

@@ -1700,6 +1700,8 @@ static TBitem tb_mesh_select[]= {
 {	0, "SEPR",                          0, NULL},
 {	0, "Random...",			            5, NULL},
 {	0, "Non-Manifold|Shift Ctrl Alt M", 9, NULL},
+{	0, "Sharp Edges|Shift Ctrl Alt S", 14, NULL},
+{	0, "Linked Flat Faces|Shift Ctrl Alt F", 15, NULL},
 {	0, "Triangles|Shift Ctrl Alt 3",    11, NULL},
 {	0, "Quads|Shift Ctrl Alt 4",        12, NULL},
 {	0, "Non-Triangles/Quads|Shift Ctrl Alt 5", 13, NULL},