auto rshift to 8 bits for 8-bit save

ChangeLog

@@ -2,6 +2,7 @@
 - vipsthumbnail lets you specify the sharpening mask
 - turn off caching for im_copy()/vips_copy(), we use copy to stop sharing, and
 it's cheap so caching doesn't help anyway
+- auto rshift down to 8 bits on save, if necessary
 
 14/5/13 started 7.32.4
 - icc import and export could segv on very wide images

TODO

@@ -1,6 +1,4 @@
 
-- save 16bit RGB as jpg, does not drop bottom 8 bits, should it?
-
 - look at
 	
 	There is an order 1 algorithm for doing medians over boxes (truly O(1)

libvips/foreign/foreign.c

@@ -4,6 +4,8 @@
  * 	- add support for sequential reads
  * 18/6/12
  * 	- flatten alpha with vips_flatten()
+ * 28/5/13
+ * 	- auto rshift down to 8 bits during save
  */
 
 /*
@@ -1258,6 +1260,27 @@ vips_foreign_convert_saveable( VipsForeignSave *save )
 		in = out;
 	}
 
+	/* Shift down to 8 bits. Handy for 8-bit-only formats like jpeg.
+	 *
+	 * If the operation wants to write 8 bits for this format and this
+	 * image is 16 bits or more and Type is RGB16 or GREY16 ...
+	 * automatically shift down.
+	 */
+	if( vips_band_format_is8bit( class->format_table[in->BandFmt] ) &&
+		!vips_band_format_is8bit( in->BandFmt ) &&
+		(in->Type == VIPS_INTERPRETATION_RGB16 ||
+			in->Type == VIPS_INTERPRETATION_GREY16) ) {
+		VipsImage *out;
+
+		if( vips_rshift_const1( in, &out, 8, NULL ) ) { 
+			g_object_unref( in );
+			return( -1 );
+		}
+		g_object_unref( in );
+
+		in = out;
+	}
+
 	/* Cast to the output format.
 	 */
 	{

libvips/include/vips/image.h

@@ -526,6 +526,7 @@ int vips_image_write_line( VipsImage *image, int ypos, VipsPel *linebuffer );
 
 gboolean vips_band_format_isint( VipsBandFormat format );
 gboolean vips_band_format_isuint( VipsBandFormat format );
+gboolean vips_band_format_is8bit( VipsBandFormat format );
 gboolean vips_band_format_isfloat( VipsBandFormat format );
 gboolean vips_band_format_iscomplex( VipsBandFormat format );
 

libvips/iofuncs/image.c

@@ -2447,6 +2447,36 @@ vips_band_format_isuint( VipsBandFormat format )
 	}
 }
 
+/**
+ * vips_band_format_is8bit:
+ * @format: format to test
+ *
+ * Return %TRUE if @format is uchar or schar.
+ */
+gboolean
+vips_band_format_is8bit( VipsBandFormat format )
+{
+	switch( format ) {
+	case VIPS_FORMAT_UCHAR:
+	case VIPS_FORMAT_CHAR:
+		return( TRUE );
+
+	case VIPS_FORMAT_USHORT:
+	case VIPS_FORMAT_SHORT:
+	case VIPS_FORMAT_UINT:
+	case VIPS_FORMAT_INT:
+	case VIPS_FORMAT_FLOAT:
+	case VIPS_FORMAT_DOUBLE:
+	case VIPS_FORMAT_COMPLEX:
+	case VIPS_FORMAT_DPCOMPLEX:
+		return( FALSE );
+
+	default:
+		g_assert( 0 );
+		return( -1 );
+	}
+}
+
 /**
  * vips_band_format_isfloat:
  * @format: format to test