add sniffing framework to magickload

though it only spots ICO for now

see https://github.com/jcupitt/pyvips/issues/39

ChangeLog

@@ -35,6 +35,7 @@
 - paste in the test suite from pyvips
 - get EXIF tag names from tag plus ifd [@Nan619]
 - escape ASCII control characters in XML
+- magickload now sniffs some file types itself
 
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges [Adrià]

libvips/foreign/magick.c

@@ -1,6 +1,9 @@
 /* Common functions for interfacing with ImageMagick.
  *
  * 22/12/17 dlemstra 
+ *
+ * 24/7/18
+ * 	- add the sniffer
  */
 
 /*
@@ -238,6 +241,46 @@ magick_set_image_option( ImageInfo *image_info,
 #endif /*HAVE_SETIMAGEOPTION*/
 }
 
+/* ImageMagick can't detect some formats, like ICO, by examining the contents --
+ * ico.c simply does not have a recogniser.
+ *
+ * For these formats, do the detection ourselves.
+ *
+ * Set image_info->magick if we spot one of the things we can spot. 
+ */
+static const char *
+magick_sniff( const unsigned char *bytes, size_t length )
+{
+	if( length >= 4 &&
+		bytes[0] == 0 &&
+		bytes[1] == 0 &&
+		bytes[2] == 1 &&
+		bytes[3] == 0 )
+		return( "ICO" );
+
+	return( NULL );
+}
+
+void
+magick_sniff_bytes( ImageInfo *image_info, 
+	const unsigned char *bytes, size_t length )
+{
+	const char *format;
+
+	if( (format = magick_sniff( bytes, length )) )
+		vips_strncpy( image_info->magick, format, MaxTextExtent );
+}
+
+void
+magick_sniff_file( ImageInfo *image_info, const char *filename )
+{
+	unsigned char bytes[256];
+	size_t length;
+
+	if( (length = vips__get_bytes( filename, bytes, 256 )) >= 4 )
+		magick_sniff_bytes( image_info, bytes, 256 );
+}
+
 void
 magick_vips_error( const char *domain, ExceptionInfo *exception )
 {

libvips/foreign/magick.h

@@ -59,6 +59,9 @@ void magick_set_number_scenes( ImageInfo *image_info,
 	int scene, int number_scenes );
 
 void magick_inherit_exception( ExceptionInfo *exception, Image *image );
+void magick_sniff_bytes( ImageInfo *image_info, 
+		const unsigned char *bytes, size_t length );
+void magick_sniff_file( ImageInfo *image_info, const char *filename );
 void magick_vips_error( const char *domain, ExceptionInfo *exception );
 
 void magick_genesis( void );

libvips/foreign/magick2vips.c

@@ -57,6 +57,8 @@
  * 	- try using GetImageChannelDepth() instead of ->depth
  * 25/5/18
  * 	- don't use Ping, it's too unreliable
+ * 24/7/18
+ * 	- sniff extra filetypes
  */
 
 /*
@@ -128,6 +130,8 @@
 typedef struct _Read {
 	char *filename;
 	VipsImage *im;
+	const void *buf;
+	size_t len; 
 	int page;
 	int n;
 
@@ -182,7 +186,8 @@ read_close( VipsImage *im, Read *read )
 
 static Read *
 read_new( const char *filename, VipsImage *im, 
-	const char *density, int page, int n )
+	const void *buf, const size_t len, 
+	const char *density, int page, int n ) 
 {
 	Read *read;
 
@@ -197,6 +202,8 @@ read_new( const char *filename, VipsImage *im,
 	if( !(read = VIPS_NEW( im, Read )) )
 		return( NULL );
 	read->filename = filename ? g_strdup( filename ) : NULL;
+	read->buf = buf;
+	read->len = len;
 	read->page = page;
 	read->n = n;
 	read->im = im;
@@ -218,15 +225,12 @@ read_new( const char *filename, VipsImage *im,
 		vips_strncpy( read->image_info->filename, 
 			filename, MaxTextExtent );
 
-	/* The file format hint, eg. "ICO".
+	/* Any extra file format detection.
-	 *
-	if( format ) 
-		vips_strncpy( read->image_info->magick, 
-			format, MaxTextExtent );
-	 *
 	 */
-	printf( "magick2vips: insert format stuff here\n" ); 
+	if( filename ) 
-
+		magick_sniff_file( read->image_info, filename );
+	if( buf ) 
+		magick_sniff_bytes( read->image_info, buf, len );
 
 	/* Canvas resolution for rendering vector formats like SVG.
 	 */
@@ -754,7 +758,7 @@ vips__magick_read( const char *filename,
 	printf( "magick2vips: vips__magick_read: %s\n", filename );
 #endif /*DEBUG*/
 
-	if( !(read = read_new( filename, out, density, page, n )) )
+	if( !(read = read_new( filename, out, NULL, n, density, page, n )) )
 		return( -1 );
 
 #ifdef DEBUG
@@ -788,7 +792,7 @@ vips__magick_read_header( const char *filename,
 	printf( "vips__magick_read_header: %s\n", filename );
 #endif /*DEBUG*/
 
-	if( !(read = read_new( filename, out, density, page, n )) )
+	if( !(read = read_new( filename, out, NULL, 0, density, page, n )) )
 		return( -1 );
 
 #ifdef DEBUG
@@ -833,7 +837,7 @@ vips__magick_read_buffer( const void *buf, const size_t len,
 	printf( "magick2vips: vips__magick_read_buffer: %p %zu\n", buf, len );
 #endif /*DEBUG*/
 
-	if( !(read = read_new( NULL, out, density, page, n )) )
+	if( !(read = read_new( NULL, out, buf, len, density, page, n )) )
 		return( -1 );
 
 #ifdef DEBUG
@@ -867,7 +871,7 @@ vips__magick_read_buffer_header( const void *buf, const size_t len,
 	printf( "vips__magick_read_buffer_header: %p %zu\n", buf, len );
 #endif /*DEBUG*/
 
-	if( !(read = read_new( NULL, out, density, page, n )) )
+	if( !(read = read_new( NULL, out, buf, len, density, page, n )) )
 		return( -1 );
 
 #ifdef DEBUG

libvips/foreign/magick7load.c

@@ -4,6 +4,8 @@
  * 	- from magickload
  * 25/11/16
  * 	- add @n, deprecate @all_frames (just sets n = -1)
+ * 24/7/18
+ * 	- sniff extra filetypes
  */
 
 /*
@@ -305,14 +307,6 @@ vips_foreign_load_magick7_build( VipsObject *object )
 	if( magick7->all_frames )
 		magick7->n = -1;
 
-	/* The file format hint, eg. "ICO".
-	 *
-	if( magick7->format ) 
-		vips_strncpy( magick7->image_info->magick, 
-			magick7->format, MaxTextExtent );
-	 *
-	 */
-
 	/* Canvas resolution for rendering vector formats like SVG.
 	 */
 	VIPS_SETSTR( magick7->image_info->density, magick7->density );
@@ -768,6 +762,7 @@ ismagick7( const char *filename )
 	image_info = CloneImageInfo( NULL );
 	exception = AcquireExceptionInfo();
 	vips_strncpy( image_info->filename, filename, MagickPathExtent );
+	magick_sniff_file( image_info, filename );
 	image = PingImage( image_info, exception );
 	result = image != NULL;
 	VIPS_FREEF( DestroyImageList, image );
@@ -790,6 +785,8 @@ vips_foreign_load_magick7_file_header( VipsForeignLoad *load )
 	vips_strncpy( magick7->image_info->filename, file->filename, 
 		MagickPathExtent );
 
+	magick_sniff_file( magick7->image_info, file->filename );
+
 	/* It would be great if we could PingImage and just read the header,
 	 * but sadly many IM coders do not support ping. The critical one for
 	 * us is DICOM.
@@ -867,6 +864,7 @@ vips_foreign_load_magick7_buffer_is_a_buffer( const void *buf, size_t len )
 	 */
 	image_info = CloneImageInfo( NULL );
 	exception = AcquireExceptionInfo();
+	magick_sniff_bytes( image_info, buf, len );
 	image = PingBlob( image_info, buf, len, exception );
 	result = image != NULL;
 	VIPS_FREEF( DestroyImageList, image );
@@ -893,6 +891,8 @@ vips_foreign_load_magick7_buffer_header( VipsForeignLoad *load )
 	 * 
 	 * We have to read the whole image in _header.
 	 */
+	magick_sniff_bytes( magick7->image_info, 
+		magick7_buffer->buf->data, magick7_buffer->buf->length );
 	magick7->image = BlobToImage( magick7->image_info, 
 		magick7_buffer->buf->data, magick7_buffer->buf->length,
 		magick7->exception );