magicksave: Add bitdepth option (#2819)

* flag to save bmp with bitdepth 1

* magicksave: add bitdepth option

* update comments

* update documentation

* revised based on review comments

Co-authored-by: Thomas Barton <thomasryanbarton@gmail.com>

ChangeLog

@@ -28,6 +28,7 @@
 - add vips_target_end(), deprecate vips_target_finish()
 - add "mixed" to webpsave [dloebl]
 - add "reoptimise" to gifsave [dloebl]
+- add "bitdepth" to magicksave [dloebl]
 
 26/11/21 started 8.12.3
 - better arg checking for vips_hist_find_ndim() [travisbell]

libvips/foreign/magick.c

@@ -286,6 +286,19 @@ magick_ismagick( const unsigned char *bytes, size_t length )
 		GetImageMagick( bytes, length, format ) );
 }
 
+int
+magick_quantize_images( Image *images,
+	const size_t depth, ExceptionInfo *exception )
+{
+	QuantizeInfo info;
+
+	GetQuantizeInfo( &info );
+	info.number_colors = 1 << depth;
+	QuantizeImages( &info, images, exception );
+
+	return( exception->severity == UndefinedException );
+}
+
 #endif /*HAVE_MAGICK7*/
 
 #ifdef HAVE_MAGICK6
@@ -616,6 +629,17 @@ magick_ismagick( const unsigned char *bytes, size_t length )
 #endif
 }
 
+int
+magick_quantize_images( Image *images,
+	const size_t depth, ExceptionInfo *exception )
+{
+	QuantizeInfo info;
+
+	GetQuantizeInfo( &info );
+	info.number_colors = (1 << depth);
+	return QuantizeImages( &info, images );
+}
+
 #endif /*HAVE_MAGICK6*/
 
 #if defined(HAVE_MAGICK6) || defined(HAVE_MAGICK7)

libvips/foreign/magick.h

@@ -88,6 +88,8 @@ int magick_optimize_image_layers( Image **images, ExceptionInfo *exception );
 int magick_optimize_image_transparency( const Image *images,
     ExceptionInfo *exception );
 
+int magick_quantize_images( Image *images, const size_t depth, ExceptionInfo *exception  );
+
 gboolean magick_ismagick( const unsigned char *bytes, size_t length );
 
 #endif /*HAVE_MAGICK6*/

libvips/foreign/magicksave.c

@@ -66,6 +66,7 @@
  * * @format: %gchararray, format to save as
  * * @optimize_gif_frames: %gboolean, apply GIF frames optimization
  * * @optimize_gif_transparency: %gboolean, apply GIF transparency optimization
+ * * @bitdepth: %gint, number of bits per pixel
  *
  * Write an image using libMagick.
  *
@@ -83,6 +84,9 @@
  * through animation are made transparent. This takes some time for computation
  * but saves some time on encoding and produces smaller files in some cases.
  *
+ * @bitdepth specifies the number of bits per pixel. The image will be quantized
+ * and dithered if the value is within the valid range (1 to 8).
+ *
  * See also: vips_magicksave_buffer(), vips_magickload().
  *
  * Returns: 0 on success, -1 on error.
@@ -113,6 +117,7 @@ vips_magicksave( VipsImage *in, const char *filename, ... )
  * * @format: %gchararray, format to save as
  * * @optimize_gif_frames: %gboolean, apply GIF frames optimization
  * * @optimize_gif_transparency: %gboolean, apply GIF transparency optimization
+ * * @bitdepth: %gint, number of bits per pixel
  *
  * As vips_magicksave(), but save to a memory buffer. 
  *

libvips/foreign/vips2magick.c

@@ -69,6 +69,7 @@ typedef struct _VipsForeignSaveMagick {
 	char *filename;		/* NULL during buffer output */
 	char *format;
 	int quality;
+	int bitdepth;
 	gboolean optimize_gif_frames;
 	gboolean optimize_gif_transparency;
 
@@ -406,6 +407,21 @@ vips_foreign_save_magick_build( VipsObject *object )
 			return( -1 );
 		}
 	}
+	
+	/* Bitdepth <= 8 requested? Quantize/Dither images.
+	 * ImageMagick then selects the appropriate bit depth when writing
+	 * the actual image (e.g. BMP or GIF).
+	 */
+	if( magick->bitdepth ) {
+		if ( !magick_quantize_images( magick->images,
+			magick->bitdepth, magick->exception ) ) {
+			magick_inherit_exception( magick->exception,
+				magick->images );
+			magick_vips_error( class->nickname, magick->exception );
+
+			return( -1 );
+		}
+	}
 
 	return( 0 );
 }
@@ -493,6 +509,13 @@ vips_foreign_save_magick_class_init( VipsForeignSaveMagickClass *class )
 		G_STRUCT_OFFSET( VipsForeignSaveMagick, 
 			optimize_gif_transparency ),
 		FALSE );
+		
+	VIPS_ARG_INT( class, "bitdepth", 6,
+		_( "Image bitdepth" ),
+		_( "Image bitdepth. Default is auto bitdepth." ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsForeignSaveMagick, bitdepth ),
+		0, 8, 0);
 }
 
 static void
@@ -502,6 +525,7 @@ vips_foreign_save_magick_init( VipsForeignSaveMagick *magick )
 	 * int array later.
 	 */
 	g_value_init( &magick->delay_gvalue, G_TYPE_INT );
+	magick->bitdepth = 0;
 }
 
 typedef struct _VipsForeignSaveMagickFile {