sequential can now skip ahead

vips_sequential() no longer bans all non-seq access, it just bans
rewinding --- if you ask for something some way ahead, it reads and
throws away pixels

this means VipsExtract and VipsInsert can now be seq: they will read and
throw away any stuff they don't need

ChangeLog

@@ -18,6 +18,7 @@
 - vips8 command-line interface uses this to turn sequential mode on
   automatically when possible
 - better handling of input files in vips7 command-line interface
+- sequential can skip ahead, so extract / insert are now seq
 
 18/6/12 started 7.28.9
 - slightly more memory debugging output

libvips/conversion/extract.c

@@ -180,6 +180,7 @@ vips_extract_area_class_init( VipsExtractAreaClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *vobject_class = VIPS_OBJECT_CLASS( class );
+	VipsOperationClass *operation_class = VIPS_OPERATION_CLASS( class );
 
 	VIPS_DEBUG_MSG( "vips_extract_area_class_init\n" );
 
@@ -190,8 +191,7 @@ vips_extract_area_class_init( VipsExtractAreaClass *class )
 	vobject_class->description = _( "extract an area from an image" );
 	vobject_class->build = vips_extract_area_build;
 
-	/* Not sequential, since we skip the top and bottom of the image.
-	 */
+	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
 
 	VIPS_ARG_IMAGE( class, "input", 0, 
 		_( "Input" ), 

libvips/conversion/insert.c

@@ -339,6 +339,7 @@ vips_insert_class_init( VipsInsertClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *vobject_class = VIPS_OBJECT_CLASS( class );
+	VipsOperationClass *operation_class = VIPS_OPERATION_CLASS( class );
 
 	VIPS_DEBUG_MSG( "vips_insert_class_init\n" );
 
@@ -349,9 +350,7 @@ vips_insert_class_init( VipsInsertClass *class )
 	vobject_class->description = _( "insert an image" );
 	vobject_class->build = vips_insert_build;
 
-	/* Not sequential, since the image being inserted may be clipped and
-	 * we may need to skip the top or bottom.
-	 */
+	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
 
 	VIPS_ARG_IMAGE( class, "main", -1, 
 		_( "Main" ), 

libvips/conversion/sequential.c

@@ -5,6 +5,8 @@
  *
  * 15/2/12
  * 	- from VipsForeignLoad
+ * 14/7/12
+ * 	- support skip forwards as well, so we can do extract/insert
  */
 
 /*
@@ -74,14 +76,9 @@ vips_sequential_generate( VipsRegion *or,
 
 	VIPS_DEBUG_MSG( "vips_sequential_generate %d\n", r->top );
 
-	/* The y pos of the request must be the same as our current file
-	 * position.
+	/* We can't go backwards, but we can skip forwards.
 	 */
-	if( r->top != sequential->y_pos ) {
-		printf( "vips_sequential_generate: error -- "
-			"at position %d in file, but position %d requested\n",
-			sequential->y_pos, r->top );
-
+	if( r->top < sequential->y_pos ) {
 		vips_error( "VipsSequential", 
 			_( "non-sequential read --- "
 			"at position %d in file, but position %d requested" ),
@@ -96,6 +93,24 @@ vips_sequential_generate( VipsRegion *or,
 	g_assert( r->width == or->im->Xsize );
 	g_assert( VIPS_RECT_BOTTOM( r ) <= or->im->Ysize );
 
+	/* Skip forwards, if necessary.
+	 */
+	while( sequential->y_pos < r->top ) {
+		VipsRect rect;
+
+		rect.top = sequential->y_pos;
+		rect.left = 0;
+		rect.width = or->im->Xsize;
+		rect.height = VIPS_MIN( r->top - sequential->y_pos, 
+				VIPS__FATSTRIP_HEIGHT );
+		if( vips_region_prepare( ir, &rect ) )
+			return( -1 );
+
+		sequential->y_pos += rect.height;
+	}
+
+	g_assert( sequential->y_pos == r->top );
+
 	/* Pointer copy.
 	 */
         if( vips_region_prepare( ir, r ) ||