fix low bitdepth PNG save of high bitdepth images

SOme combinations of high bitdepth images with low bitdepth PNG save could
produce incorrect images, for example saving a 16-bit fourier image as 1-bit.

ChangeLog

@@ -5,6 +5,7 @@
 - add "unlimited" to jpegload
 - better 0 detection in unpremultiply
 - fix low bitdepth spng save [jeffska]
+- fix PNG low bitdepth save of high bitdepth images
 
 21/11/21 started 8.13
 - configure fails for requested but unmet dependencies [remicollet]

libvips/foreign/pngsave.c

@@ -112,12 +112,22 @@ vips_foreign_save_png_build( VipsObject *object )
 	in = save->ready;
 	g_object_ref( in );
 
-	/* save->ready will have been converted to uint16 for high-bitdepth
+	/* If no output bitdepth has been specified, use input Type to pick.
-	 * formats (eg. float) ... we need to check Type to see if we want 
-	 * to save as 8 or 16-bits. Eg. imagine a float image tagged as sRGB.
 	 */
-	if( in->Type == VIPS_INTERPRETATION_sRGB ||
+        if( !vips_object_argument_isset( object, "bitdepth" ) ) 
-		in->Type == VIPS_INTERPRETATION_B_W ) {
+		png->bitdepth = 
+                        in->Type == VIPS_INTERPRETATION_RGB16 ||
+                        in->Type == VIPS_INTERPRETATION_GREY16 ? 16 : 8;
+
+	/* Deprecated "colours" arg just sets bitdepth large enough to hold
+	 * that many colours.
+	 */
+        if( vips_object_argument_isset( object, "colours" ) ) 
+		png->bitdepth = ceil( log2( png->colours ) );
+
+	/* Cast in down to 8 bit if we can.
+	 */
+	if( png->bitdepth <= 8 ) { 
 		VipsImage *x;
 
 		if( vips_cast( in, &x, VIPS_FORMAT_UCHAR, NULL ) ) {
@@ -128,15 +138,6 @@ vips_foreign_save_png_build( VipsObject *object )
 		in = x;
 	}
 
-	/* Deprecated "colours" arg just sets bitdepth large enough to hold
-	 * that many colours.
-	 */
-        if( vips_object_argument_isset( object, "colours" ) ) 
-		png->bitdepth = ceil( log2( png->colours ) );
-
-        if( !vips_object_argument_isset( object, "bitdepth" ) ) 
-		png->bitdepth = in->BandFmt == VIPS_FORMAT_UCHAR ? 8 : 16;
-
 	/* If this is a RGB or RGBA image and a low bit depth has been
 	 * requested, enable palettization.
 	 */
@@ -144,6 +145,11 @@ vips_foreign_save_png_build( VipsObject *object )
 		png->bitdepth < 8 )
 		png->palette = TRUE;
 
+        /* Disable palettization for >8 bit save.
+         */
+        if( png->bitdepth >= 8 )
+		png->palette = FALSE;
+
 	if( vips__png_write_target( in, png->target,
 		png->compression, png->interlace, png->profile, png->filter,
 		save->strip, png->palette, png->Q, png->dither,

libvips/foreign/spngsave.c

@@ -42,9 +42,9 @@
  */
 
 /*
+ */
 #define DEBUG_VERBOSE
 #define DEBUG
- */
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
@@ -485,7 +485,7 @@ vips_foreign_save_spng_write( VipsForeignSaveSpng *spng, VipsImage *in )
 	spng_set_option( spng->ctx, 
 		SPNG_FILTER_CHOICE, spng->filter );
 
-	/* Set resolution. libpng uses pixels per meter.
+	/* Set resolution. png uses pixels per meter.
 	 */
 	phys.unit_specifier = 1;
 	phys.ppu_x = VIPS_RINT( in->Xres * 1000.0 );
@@ -580,12 +580,22 @@ vips_foreign_save_spng_build( VipsObject *object )
 	in = save->ready;
 	g_object_ref( in );
 
-	/* in will have been converted to uint16 for high-bitdepth
+	/* If no output bitdepth has been specified, use input Type to pick.
-	 * formats (eg. float) ... we need to check Type to see if we want 
-	 * to save as 8 or 16-bits. Eg. imagine a float image tagged as sRGB.
 	 */
-	if( in->Type == VIPS_INTERPRETATION_sRGB ||
+        if( !vips_object_argument_isset( object, "bitdepth" ) ) 
-		in->Type == VIPS_INTERPRETATION_B_W ) {
+		spng->bitdepth = 
+                        in->Type == VIPS_INTERPRETATION_RGB16 ||
+                        in->Type == VIPS_INTERPRETATION_GREY16 ? 16 : 8;
+
+	/* Deprecated "colours" arg just sets bitdepth large enough to hold
+	 * that many colours.
+	 */
+        if( vips_object_argument_isset( object, "colours" ) ) 
+		spng->bitdepth = ceil( log2( spng->colours ) );
+
+	/* Cast in down to 8 bit if we can.
+	 */
+	if( spng->bitdepth <= 8 ) { 
 		VipsImage *x;
 
 		if( vips_cast( in, &x, VIPS_FORMAT_UCHAR, NULL ) ) {
@@ -596,19 +606,6 @@ vips_foreign_save_spng_build( VipsObject *object )
 		in = x;
 	}
 
-	/* If no output bitdepth has been specified, use input Type to pick.
-	 * We only go for 16 bits for the types where we know there's a
-	 * 0-65535 range.
-	 */
-        if( !vips_object_argument_isset( object, "bitdepth" ) ) 
-		spng->bitdepth = in->BandFmt == VIPS_FORMAT_UCHAR ? 8 : 16;
-
-	/* Deprecated "colours" arg just sets bitdepth large enough to hold
-	 * that many colours.
-	 */
-        if( vips_object_argument_isset( object, "colours" ) ) 
-		spng->bitdepth = ceil( log2( spng->colours ) );
-
 	/* If this is a RGB or RGBA image and a low bit depth has been
 	 * requested, enable palettisation.
 	 */
@@ -616,6 +613,11 @@ vips_foreign_save_spng_build( VipsObject *object )
 		spng->bitdepth < 8 )
 		spng->palette = TRUE;
 
+        /* Disable palettization for >8 bit save.
+         */
+        if( spng->bitdepth >= 8 )
+		spng->palette = FALSE;
+
 	if( vips_foreign_save_spng_write( spng, in ) ) {
 		g_object_unref( in );
 		return( -1 );