v.buffer: implement -c -s flags also for GEOS

          explain corners (caps) with screenshots 
          (merge https://trac.osgeo.org/grass/changeset/65710, https://trac.osgeo.org/grass/changeset/65712, https://trac.osgeo.org/grass/changeset/65715, https://trac.osgeo.org/grass/changeset/65759)


git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@65957 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.buffer/geos.c

@@ -142,12 +142,12 @@ int geos_buffer(struct Map_info *In, struct Map_info *Out,
 		struct spatial_index *si,
 		struct line_cats *Cats,
 		struct buf_contours **arr_bc,
-		int *buffers_count, int *arr_bc_alloc)
+		int *buffers_count, int *arr_bc_alloc, int flat, int no_caps)
 {
     GEOSGeometry *IGeom = NULL;
     GEOSGeometry *OGeom = NULL;
     
-    G_debug(3, "geos_buffer()");
+    G_debug(3, "geos_buffer(): id=%d", id);
 
     if (type == GV_AREA)
 	IGeom = Vect_read_area_geos(In, id);
@@ -158,10 +158,25 @@ int geos_buffer(struct Map_info *In, struct Map_info *Out,
      * A value of 8 gives less than 2% max error in the buffer distance.
      * For a max error of < 1%, use QS = 12.
      * For a max error of < 0.1%, use QS = 18. */
+#ifdef GEOS_3_3
+    if (flat || no_caps) {
+        GEOSBufferParams* geos_params = GEOSBufferParams_create();
+        GEOSBufferParams_setEndCapStyle(geos_params,
+                                        no_caps ? GEOSBUF_CAP_FLAT : GEOSBUF_CAP_SQUARE);
+
+        OGeom = GEOSBufferWithParams(IGeom, geos_params, da);
+        GEOSBufferParams_destroy(geos_params);
+    }
+    else {
+        OGeom = GEOSBuffer(IGeom, da, 12);
+    }
+#else
     OGeom = GEOSBuffer(IGeom, da, 12);
-
-    if (!OGeom)
-	G_warning(_("Buffering failed"));
+#endif
+    
+    if (!OGeom) {
+        G_fatal_error(_("Buffering failed (feature %d)"), id);
+    }
     
     geom2ring(OGeom, Out, Buf, si, Cats, arr_bc, buffers_count, arr_bc_alloc);
 

vector/v.buffer/local_proto.h

@@ -1,3 +1,6 @@
+#if (GEOS_VERSION_MAJOR >= 3 && GEOS_VERSION_MINOR >= 3)
+#define GEOS_3_3
+#endif
 
 struct buf_contours
 {
@@ -19,6 +22,6 @@ int geos_buffer(struct Map_info *, struct Map_info *,
 		struct spatial_index *,
 		struct line_cats *,
 		struct buf_contours **,
-		int *, int *);
+		int *, int *, int, int);
 #endif
 

vector/v.buffer/main.c

@@ -475,6 +475,12 @@ int main(int argc, char *argv[])
 
 #ifdef HAVE_GEOS
     initGEOS(G_message, G_fatal_error);
+
+    /* check required version for -s/-c flag */
+#ifndef GEOS_3_3
+        G_warning(_("Flags -%c/%c ignored by this version, GEOS >= 3.3 is required"),
+                  's', 'c');
+#endif
 #endif
     if(!use_geos && (da < 0. || db < 0.)) {
 	G_warning(_("Negative distances for internal buffers are not supported "
@@ -544,7 +550,8 @@ int main(int argc, char *argv[])
 #ifdef HAVE_GEOS
 	    if (use_geos)
 		geos_buffer(&In, &Out, &Buf, area, GV_AREA, da,
-			    &si, CCats, &arr_bc, &buffers_count, &arr_bc_alloc);
+			    &si, CCats, &arr_bc, &buffers_count, &arr_bc_alloc,
+                            straight_flag->answer, nocaps_flag->answer);
 #endif
 	    if (!use_geos) {
 		Vect_area_buffer2(&In, area, da, db, dalpha,
@@ -668,7 +675,8 @@ int main(int argc, char *argv[])
 #ifdef HAVE_GEOS
 		if (use_geos)
 		    geos_buffer(&In, &Out, &Buf, line, type, da,
-				&si, CCats, &arr_bc, &buffers_count, &arr_bc_alloc);
+				&si, CCats, &arr_bc, &buffers_count, &arr_bc_alloc,
+                                straight_flag->answer, nocaps_flag->answer);
 #endif
 		if (!use_geos) {
 		    Vect_line_buffer2(Points, da, db, dalpha,

vector/v.buffer/v.buffer.html

@@ -21,11 +21,47 @@ areas with category X (see example below).
 Buffers for lines and areas are generated using the algorithms from
 the GEOS library.
 
-<!-- built-in buffer algorithm no longer desired, we use GEOS:
-If GRASS is not compiled with GEOS support or environmental
-variable <tt>GRASS_VECTOR_BUFFER</tt> is defined, then GRASS generates
-buffers using built-in buffering algorithm (which is still buggy for
-some input data).
+<p>
+<i>For advanced users:</i> built-in buffer algorithm no longer
+desired, we use GEOS: If GRASS is not compiled with GEOS support
+or <a href="variables.html">environmental
+variable</a> <tt>GRASS_VECTOR_BUFFER</tt> is defined, then GRASS
+generates buffers using built-in buffering algorithm (which is still
+buggy for some input data).
+
+<h3>Corner settings</h3>
+
+The following corners (also called "cap") exist:
+<ul>
+<li>no cap: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <img src="v_buffer_no_cap.png" valign="middle" alt="line buffer: no cap"></li>
+<li>rounded cap: <img src="v_buffer_rounded_cap.png" valign="middle" alt="line buffer: rounded cap"></li>
+<li>square cap: <img src="v_buffer_square_cap.png" valign="middle" alt="line buffer: square cap"></li>
+</ul>
+
+By default <em>v.buffer</em> creates rounded buffers (blue color on
+figure below): 
+
+<center>
+  <img src="v_buffer_line.png">
+</center>
+
+Straight corners with caps are created by <b>-s</b> flag (red color on
+the figure below), while <b>-c</b> flag doesn't make caps at the ends of
+polylines (green color on the figure below):
+
+<center>
+  <img src="v_buffer_line_s.png">
+  <img src="v_buffer_line_c.png">
+</center>
+
+<!-- Only support by GRASS buffer 
+Flag <b>-s</b> also influences corners around polygons (see red color
+on the figure below):
+
+<center>
+  <img src="v_buffer_area.png">
+  <img src="v_buffer_area_s.png">
+</center>
 -->
 
 <h2>EXAMPLES</h2>