diff --git a/TODO b/TODO
index dc6ed5fd..12072e97 100644
--- a/TODO
+++ b/TODO
@@ -3,6 +3,11 @@
 	nip2: im_prepare.c:324: im_prepare_to: 
 		Assertion `clipped.left == r->left' failed.
 
+  also, im_spcor a rectangular image one pixel smaller with itself and the 
+  peak comes at (eg. 134x134) rather than in the centre of the image
+
+  suspicious!
+
 - doing im_create_fmask() and friends
 
 - how about im_invalidate_area()? we currently repaint the whole window on
diff --git a/libvips/convolution/im_spcor.c b/libvips/convolution/im_spcor.c
index 252bc458..b7c552fa 100644
--- a/libvips/convolution/im_spcor.c
+++ b/libvips/convolution/im_spcor.c
@@ -86,26 +86,34 @@ typedef struct {
 	double c1;		/* sqrt(sumij (ref(i,j)-mean(ref))^2) */
 } Spcor;
 
-#define LOOP(IN) { \
+#define LOOP( IN ) { \
 	IN *a = (IN *) p; \
 	IN *b = (IN *) ref->data; \
 	int in_lsk = lsk / sizeof( IN ); \
-	IN *a1, *b1; \
+	IN *a1; \
+	IN *b1; \
  	\
 	/* For each pel in or, loop over ref. First, \
 	 * calculate mean of area in ir corresponding to ref. \
 	 */ \
-	for( a1 = a, sum1 = 0, j = 0; j < ref->Ysize; j++, a1 += in_lsk )  \
+	a1 = a; \
+	sum1 = 0; \
+	for( j = 0; j < ref->Ysize; j++ ) { \
 		for( i = 0; i < ref->Xsize; i++ ) \
 			sum1 += a1[i]; \
-	imean = (double) sum1 / (ref->Xsize * ref->Ysize); \
+		a1 += in_lsk;  \
+	} \
+	imean = sum1 / (ref->Xsize * ref->Ysize); \
  	\
 	/* Loop over ir again, this time calculating  \
 	 * sum-of-squares-of-differences for this window on \
 	 * ir, and also sum-of-products-of-differences from mean. \
 	 */ \
-	for( a1 = a, b1 = b, sum2 = 0.0, sum3 = 0.0, j = 0; \
-		j < ref->Ysize; j++, a1 += in_lsk, b1 += ref->Xsize ) { \
+	a1 = a; \
+	b1 = b; \
+	sum2 = 0.0; \
+	sum3 = 0.0; \
+	for( j = 0; j < ref->Ysize; j++ ) { \
 		for( i = 0; i < ref->Xsize; i++ ) { \
 			/* Reference pel, and input pel. \
 			 */ \
@@ -122,6 +130,8 @@ typedef struct {
 			 */ \
 			sum3 += (rp - spcor->rmean) * (ip - imean); \
 		} \
+		a1 += in_lsk; \
+		b1 += ref->Xsize; \
 	} \
 }
 
@@ -137,8 +147,8 @@ spcor_gen( REGION *or, void *vseq, void *a, void *b )
 	Rect *r = &or->valid;
 	int le = r->left;
 	int to = r->top;
-	int bo = IM_RECT_BOTTOM(r);
-	int ri = IM_RECT_RIGHT(r);
+	int bo = IM_RECT_BOTTOM( r );
+	int ri = IM_RECT_RIGHT( r );
 
 	int x, y, i, j;
 	int lsk;
@@ -196,10 +206,10 @@ static Spcor *
 spcor_new( IMAGE *out, IMAGE *ref )
 {
 	Spcor *spcor;
-	int sz = ref->Xsize * ref->Ysize;
+	size_t sz = ref->Xsize * ref->Ysize;
 	PEL *p = (PEL *) ref->data;
 	double s;
-	int i;
+	size_t i;
 
 	if( !(spcor = IM_NEW( out, Spcor )) )
 		return( NULL );
@@ -212,7 +222,8 @@ spcor_new( IMAGE *out, IMAGE *ref )
 
 	/* Find sqrt-of-sum-of-squares-of-differences.
 	 */
-	for( s = 0.0, i = 0; i < sz; i++ ) {
+	s = 0.0;
+	for( i = 0; i < sz; i++ ) {
 		double t = (int) p[i] - spcor->rmean;
 		s += t * t;
 	}
@@ -245,18 +256,11 @@ im_spcor_raw( IMAGE *in, IMAGE *ref, IMAGE *out )
 	 */
 	if( im_check_uncoded( "im_spcor", in ) ||
 		im_check_mono( "im_spcor", in ) || 
-		im_check_uncoded( "im_spcor", ref ) ||
-		im_check_mono( "im_spcor", ref ) || 
+		im_check_8or16( "im_spcor", in ) ||
+		im_check_coding_same( "im_spcor", in, ref ) ||
+		im_check_bands_same( "im_spcor", in, ref ) || 
 		im_check_format_same( "im_spcor", in, ref ) )
 		return( -1 );
-	if( in->BandFmt != IM_BANDFMT_UCHAR && 
-		in->BandFmt != IM_BANDFMT_CHAR &&
-		in->BandFmt != IM_BANDFMT_SHORT &&
-		in->BandFmt != IM_BANDFMT_USHORT ) {
-		im_error( "im_spcor", 
-			"%s", _( "input not char/uchar/short/ushort" ) );
-		return( -1 );
-	}
 
 	/* Prepare the output image. 
 	 */
diff --git a/libvips/include/vips/check.h b/libvips/include/vips/check.h
index 4d70142e..dc9f28b4 100644
--- a/libvips/include/vips/check.h
+++ b/libvips/include/vips/check.h
@@ -60,6 +60,7 @@ int im_check_noncomplex( const char *domain, IMAGE *im );
 int im_check_complex( const char *domain, IMAGE *im );
 int im_check_format( const char *domain, IMAGE *im, VipsBandFmt fmt );
 int im_check_u8or16( const char *domain, IMAGE *im );
+int im_check_8or16( const char *domain, IMAGE *im );
 int im_check_format_same( const char *domain, IMAGE *im1, IMAGE *im2 );
 int im_check_size_same( const char *domain, IMAGE *im1, IMAGE *im2 );
 int im_check_vector( const char *domain, int n, IMAGE *im );
diff --git a/libvips/iofuncs/check.c b/libvips/iofuncs/check.c
index 181f4698..cd24c96b 100644
--- a/libvips/iofuncs/check.c
+++ b/libvips/iofuncs/check.c
@@ -865,6 +865,35 @@ im_check_uint( const char *domain, IMAGE *im )
 	return( 0 );
 }
 
+/**
+ * im_check_8or16:
+ * @domain: the originating domain for the error message
+ * @im: image to check
+ *
+ * Check that the image is 8 or 16-bit integer, signed or unsigned.
+ * Otherwise set an error message
+ * and return non-zero.
+ *
+ * See also: im_error().
+ *
+ * Returns: 0 if OK, -1 otherwise.
+ */
+int
+im_check_8or16( const char *domain, IMAGE *im )
+{
+	if( im->BandFmt != IM_BANDFMT_UCHAR &&
+		im->BandFmt != IM_BANDFMT_USHORT &&
+		im->BandFmt != IM_BANDFMT_CHAR &&
+		im->BandFmt != IM_BANDFMT_SHORT ) {
+		im_error( domain, "%s", 
+			_( "image must be 8- or 16-bit integer, "
+				"signed or unsigned" ) );
+		return( -1 );
+	}
+
+	return( 0 );
+}
+
 /**
  * im_check_u8or16:
  * @domain: the originating domain for the error message
diff --git a/libvips/resample/mbicubic.cpp b/libvips/resample/mbicubic.cpp
index 4394f4d1..7d06efa8 100644
--- a/libvips/resample/mbicubic.cpp
+++ b/libvips/resample/mbicubic.cpp
@@ -624,7 +624,6 @@ vips_interpolate_mbicubic_interpolate( VipsInterpolate* restrict interpolate,
 static void
 vips_interpolate_mbicubic_class_init( VipsInterpolateMbicubicClass *klass )
 {
-  GObjectClass *gobject_class = G_OBJECT_CLASS( klass );
   VipsObjectClass *object_class = VIPS_OBJECT_CLASS( klass );
   VipsInterpolateClass *interpolate_class =
     VIPS_INTERPOLATE_CLASS( klass );