diff --git a/ChangeLog b/ChangeLog
index a97c1924..7a6adf91 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,5 @@
 21/1/14 started 7.39.0
-- auto-decode for (almost) all operations
+- auto-decode for (almost) all operations, see vips_image_decode()
 - background render thread cleans up and quits neatly
 - colourspace has a source_space option
 - operations can be tagged as "deprecated"
diff --git a/TODO b/TODO
index 3831bfd4..2bf0c0a2 100644
--- a/TODO
+++ b/TODO
@@ -1,21 +1,4 @@
 
-- check_uncoded() left to do:
-
-	resample/quadratic.c
-
-- move vips__image_decode() into the public API
-
-- we vips_image_write() a lot, could we do this by 
-
-	int
-	vips_image_write( in, out )
-	{
-		g_object_ref( in)
-		vips_object_local( out,in )
-	}
-
-  i.e. do everything at pipe build time
-
 - inplace
 
 	can we set a region for the pixels we will modify?
diff --git a/libvips/arithmetic/arithmetic.c b/libvips/arithmetic/arithmetic.c
index a8aea1de..e273cea0 100644
--- a/libvips/arithmetic/arithmetic.c
+++ b/libvips/arithmetic/arithmetic.c
@@ -539,7 +539,7 @@ vips_arithmetic_build( VipsObject *object )
 	/* Decode RAD/LABQ etc.
 	 */
 	for( i = 0; i < arithmetic->n; i++ )
-		if( vips__image_decode( arithmetic->in[i], &decode[i] ) )
+		if( vips_image_decode( arithmetic->in[i], &decode[i] ) )
 			return( -1 );
 
 	/* Cast our input images up to a common format, bands and size.
diff --git a/libvips/arithmetic/statistic.c b/libvips/arithmetic/statistic.c
index 1ab1cc2f..e4553c5f 100644
--- a/libvips/arithmetic/statistic.c
+++ b/libvips/arithmetic/statistic.c
@@ -123,7 +123,7 @@ vips_statistic_build( VipsObject *object )
 
 	statistic->ready = statistic->in;
 
-	if( vips__image_decode( statistic->ready, &t[0] ) )
+	if( vips_image_decode( statistic->ready, &t[0] ) )
 		return( -1 );
 	statistic->ready = t[0];
 
diff --git a/libvips/colour/colour.c b/libvips/colour/colour.c
index 3645dc88..0ea7e6ee 100644
--- a/libvips/colour/colour.c
+++ b/libvips/colour/colour.c
@@ -628,13 +628,13 @@ vips_colour_difference_build( VipsObject *object )
 	extra = NULL;
 
 	if( left ) {
-		if( vips__image_decode( left, &t[0] ) )
+		if( vips_image_decode( left, &t[0] ) )
 			return( -1 );
 		left = t[0];
 	}
 
 	if( right ) {
-		if( vips__image_decode( right, &t[1] ) )
+		if( vips_image_decode( right, &t[1] ) )
 			return( -1 );
 		right = t[1];
 	}
diff --git a/libvips/conversion/bandary.c b/libvips/conversion/bandary.c
index 06ede11a..2bda6da4 100644
--- a/libvips/conversion/bandary.c
+++ b/libvips/conversion/bandary.c
@@ -146,7 +146,7 @@ vips_bandary_build( VipsObject *object )
 	size = (VipsImage **) vips_object_local_array( object, bandary->n );
 
 	for( i = 0; i < bandary->n; i++ )
-		if( vips__image_decode( bandary->in[i], &decode[i] ) )
+		if( vips_image_decode( bandary->in[i], &decode[i] ) )
 			return( -1 );
 	if( vips__formatalike_vec( decode, format, bandary->n ) ||
 		vips__sizealike_vec( format, size, bandary->n ) )
diff --git a/libvips/conversion/cast.c b/libvips/conversion/cast.c
index 9a080fdb..eadcefaa 100644
--- a/libvips/conversion/cast.c
+++ b/libvips/conversion/cast.c
@@ -446,7 +446,7 @@ vips_cast_build( VipsObject *object )
 	if( in->BandFmt == cast->format ) 
 		return( vips_image_write( in, conversion->out ) );
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/conversion/falsecolour.c b/libvips/conversion/falsecolour.c
index 867e6fe2..630b5b2c 100644
--- a/libvips/conversion/falsecolour.c
+++ b/libvips/conversion/falsecolour.c
@@ -349,7 +349,7 @@ vips_falsecolour_build( VipsObject *object )
 	 * want to work for images which aren't in a recognised space, like
 	 * MULTIBAND.
 	 */
-	if( vips__image_decode( falsecolour->in, &t[1] ) ||
+	if( vips_image_decode( falsecolour->in, &t[1] ) ||
 		vips_extract_band( t[1], &t[2], 0, NULL ) ||
 		vips_cast( t[2], &t[3], VIPS_FORMAT_UCHAR, NULL ) ||
 		vips_maplut( t[3], &t[4], t[0], NULL ) ||
diff --git a/libvips/conversion/flatten.c b/libvips/conversion/flatten.c
index 1bf4626e..85baee21 100644
--- a/libvips/conversion/flatten.c
+++ b/libvips/conversion/flatten.c
@@ -311,7 +311,7 @@ vips_flatten_build( VipsObject *object )
 
 	in = flatten->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/conversion/recomb.c b/libvips/conversion/recomb.c
index 0e39beb9..6adee2dc 100644
--- a/libvips/conversion/recomb.c
+++ b/libvips/conversion/recomb.c
@@ -150,7 +150,7 @@ vips_recomb_build( VipsObject *object )
 
 	in = recomb->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/convolution/conv.c b/libvips/convolution/conv.c
index 151a23bb..48ebe4ec 100644
--- a/libvips/convolution/conv.c
+++ b/libvips/convolution/conv.c
@@ -94,7 +94,7 @@ vips_conv_build( VipsObject *object )
 
 	/* Unpack for processing.
 	 */
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0];
 
diff --git a/libvips/freqfilt/fwfft.c b/libvips/freqfilt/fwfft.c
index 813a5cc1..618f8db2 100644
--- a/libvips/freqfilt/fwfft.c
+++ b/libvips/freqfilt/fwfft.c
@@ -303,7 +303,7 @@ vips_fwfft_build( VipsObject *object )
 
 	in = freqfilt->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/freqfilt/invfft.c b/libvips/freqfilt/invfft.c
index 8b8611ee..e20e5822 100644
--- a/libvips/freqfilt/invfft.c
+++ b/libvips/freqfilt/invfft.c
@@ -216,7 +216,7 @@ vips_invfft_build( VipsObject *object )
 
 	in = freqfilt->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/histogram/hist_local.c b/libvips/histogram/hist_local.c
index 9243ec05..15c518f7 100644
--- a/libvips/histogram/hist_local.c
+++ b/libvips/histogram/hist_local.c
@@ -236,7 +236,7 @@ vips_hist_local_build( VipsObject *object )
 
 	in = local->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/histogram/histogram.c b/libvips/histogram/histogram.c
index 8064dfa7..e4edf5b2 100644
--- a/libvips/histogram/histogram.c
+++ b/libvips/histogram/histogram.c
@@ -143,7 +143,7 @@ vips_histogram_build( VipsObject *object )
 	g_object_set( histogram, "out", vips_image_new(), NULL ); 
 
 	for( i = 0; i < histogram->n; i++ ) 
-		if( vips__image_decode( histogram->in[i], &decode[i] ) ||
+		if( vips_image_decode( histogram->in[i], &decode[i] ) ||
 			vips_check_hist( class->nickname, decode[i] ) )
 			return( -1 ); 
 
diff --git a/libvips/histogram/stdif.c b/libvips/histogram/stdif.c
index 0406bbd2..86521614 100644
--- a/libvips/histogram/stdif.c
+++ b/libvips/histogram/stdif.c
@@ -224,7 +224,7 @@ vips_stdif_build( VipsObject *object )
 
 	in = stdif->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0]; 
 
diff --git a/libvips/include/vips/image.h b/libvips/include/vips/image.h
index 5a59cc86..34f8a617 100644
--- a/libvips/include/vips/image.h
+++ b/libvips/include/vips/image.h
@@ -430,6 +430,8 @@ VipsImage *vips_image_new_temp_file( const char *format );
 int vips_image_write( VipsImage *image, VipsImage *out );
 int vips_image_write_to_file( VipsImage *image, const char *filename );
 
+int vips_image_decode( VipsImage *in, VipsImage **out );
+
 gboolean vips_image_isMSBfirst( VipsImage *image );
 gboolean vips_image_isfile( VipsImage *image );
 gboolean vips_image_ispartial( VipsImage *image );
diff --git a/libvips/include/vips/internal.h b/libvips/include/vips/internal.h
index e29d64df..39c233b6 100644
--- a/libvips/include/vips/internal.h
+++ b/libvips/include/vips/internal.h
@@ -297,8 +297,6 @@ IMAGE *vips__deprecated_open_write( const char *filename );
 
 int vips__input_interpolate_init( im_object *obj, char *str );
 
-int vips__image_decode( VipsImage *in, VipsImage **out );
-
 #ifdef __cplusplus
 }
 #endif /*__cplusplus*/
diff --git a/libvips/iofuncs/image.c b/libvips/iofuncs/image.c
index e97bcdef..0b9cef78 100644
--- a/libvips/iofuncs/image.c
+++ b/libvips/iofuncs/image.c
@@ -1997,6 +1997,37 @@ vips_image_write_to_file( VipsImage *image, const char *filename )
 	return( 0 );
 }
 
+/**
+ * vips_image_decode:
+ * @in: image to decode
+ * @out: write to this image
+ *
+ * A convenience function to unpack to a format that we can compute with. 
+ * @out->Coding is always VIPS_CODING_NONE. 
+ *
+ * See also: vips_LabQ2LabS(), vips_rad2float(). 
+ *
+ * Returns: 0 on success, or -1 on error.
+ */
+int
+vips_image_decode( VipsImage *in, VipsImage **out )
+{
+	if( in->Coding == VIPS_CODING_LABQ ) {
+		if( vips_LabQ2LabS( in, out, NULL ) )
+			return( -1 );
+	} 
+	else if( in->Coding == VIPS_CODING_RAD ) {
+		if( vips_rad2float( in, out, NULL ) )
+			return( -1 );
+	}
+	else {
+		if( vips_copy( in, out, NULL ) )
+			return( -1 );
+	}
+
+	return( 0 );
+}
+
 /**
  * vips_image_isMSBfirst:
  * @image: image to test
diff --git a/libvips/morphology/morph.c b/libvips/morphology/morph.c
index 8ce26671..e8e037c5 100644
--- a/libvips/morphology/morph.c
+++ b/libvips/morphology/morph.c
@@ -93,7 +93,7 @@ vips_morph_build( VipsObject *object )
 
 	in = morphology->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0];
 
diff --git a/libvips/morphology/rank.c b/libvips/morphology/rank.c
index b88154a5..a6388918 100644
--- a/libvips/morphology/rank.c
+++ b/libvips/morphology/rank.c
@@ -341,7 +341,7 @@ vips_rank_build( VipsObject *object )
 
 	in = morphology->in; 
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0];
 
diff --git a/libvips/resample/affine.c b/libvips/resample/affine.c
index 7d17a06e..13659405 100644
--- a/libvips/resample/affine.c
+++ b/libvips/resample/affine.c
@@ -375,27 +375,6 @@ vips_affine_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop )
 	return( 0 );
 }
 
-/* Unpack to a format that we can compute with.
- */
-int
-vips__image_decode( VipsImage *in, VipsImage **out )
-{
-	if( in->Coding == VIPS_CODING_LABQ ) {
-		if( vips_LabQ2LabS( in, out, NULL ) )
-			return( -1 );
-	} 
-	else if( in->Coding == VIPS_CODING_RAD ) {
-		if( vips_rad2float( in, out, NULL ) )
-			return( -1 );
-	}
-	else {
-		if( vips_copy( in, out, NULL ) )
-			return( -1 );
-	}
-
-	return( 0 );
-}
-
 static int
 vips_affine_build( VipsObject *object )
 {
@@ -485,7 +464,7 @@ vips_affine_build( VipsObject *object )
 		return( -1 );
 	}
 
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0];
 
diff --git a/libvips/resample/shrink.c b/libvips/resample/shrink.c
index f27e4bf9..0604f4d5 100644
--- a/libvips/resample/shrink.c
+++ b/libvips/resample/shrink.c
@@ -347,7 +347,7 @@ vips_shrink_build( VipsObject *object )
 
 	/* Unpack for processing.
 	 */
-	if( vips__image_decode( in, &t[0] ) )
+	if( vips_image_decode( in, &t[0] ) )
 		return( -1 );
 	in = t[0];