significantly reduced sparse matrix footprint:

http://code.opencv.org/issues/2206,
http://code.opencv.org/issues/2924

modules/core/src/matrix.cpp

@@ -4714,8 +4714,8 @@ SparseMat::Hdr::Hdr( int _dims, const int* _sizes, int _type )
     refcount = 1;
 
     dims = _dims;
-    valueOffset = (int)alignSize(sizeof(SparseMat::Node) +
-        sizeof(int)*std::max(dims - CV_MAX_DIM, 0), CV_ELEM_SIZE1(_type));
+    valueOffset = (int)alignSize(sizeof(SparseMat::Node) - MAX_DIM*sizeof(int) +
+                                 dims*sizeof(int), CV_ELEM_SIZE1(_type));
     nodeSize = alignSize(valueOffset +
         CV_ELEM_SIZE(_type), (int)sizeof(size_t));
 
@@ -5114,7 +5114,8 @@ uchar* SparseMat::newNode(const int* idx, size_t hashval)
     if( !hdr->freeList )
     {
         size_t i, nsz = hdr->nodeSize, psize = hdr->pool.size(),
-            newpsize = std::max(psize*2, 8*nsz);
+            newpsize = std::max(psize*3/2, 8*nsz);
+        newpsize = (newpsize/nsz)*nsz;
         hdr->pool.resize(newpsize);
         uchar* pool = &hdr->pool[0];
         hdr->freeList = std::max(psize, nsz);

modules/core/test/test_mat.cpp

@@ -1248,3 +1248,27 @@ TEST(Core_SVD, orthogonality)
         ASSERT_LT(norm(mat_U, Mat::eye(2, 2, type), NORM_INF), 1e-5);
     }
 }
+
+
+TEST(Core_SparseMat, footprint)
+{
+    int n = 1000000;
+    int sz[] = { n, n };
+    SparseMat m(2, sz, CV_64F);
+
+    int nodeSize0 = (int)m.hdr->nodeSize;
+    double dataSize0 = ((double)m.hdr->pool.size() + (double)m.hdr->hashtab.size()*sizeof(size_t))*1e-6;
+    printf("before: node size=%d bytes, data size=%.0f Mbytes\n", nodeSize0, dataSize0);
+
+    for (int i = 0; i < n; i++)
+    {
+        m.ref<double>(i, i) = 1;
+    }
+
+    double dataSize1 = ((double)m.hdr->pool.size() + (double)m.hdr->hashtab.size()*sizeof(size_t))*1e-6;
+    double threshold = (n*nodeSize0*1.6 + n*2.*sizeof(size_t))*1e-6;
+    printf("after: data size=%.0f Mbytes, threshold=%.0f MBytes\n", dataSize1, threshold);
+
+    ASSERT_LE((int)m.hdr->nodeSize, 32);
+    ASSERT_LE(dataSize1, threshold);
+}