From d8ed630d77c21a994e15939ccd148fcff9e423f2 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Sat, 16 Jan 2021 15:45:59 +0000
Subject: [PATCH] better 8/16 bit casting for pngsave

pngsave was just using the image format to decide whether to save as 8
or 16 bit PNG. With this patch it checks Type as well.

This means that eg. a uint32 image tagged as sRGB will save as png8, but
a uint32 image tagged as RGB16 will save as png16.
---
 ChangeLog                 |  1 +
 libvips/foreign/foreign.c |  9 ++++++---
 libvips/foreign/pngsave.c | 33 ++++++++++++++++++++++++++++-----
 3 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 8cb7f038..9e25c5be 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,6 +12,7 @@
 - add vips_source_g_input_stream_new() to load images from a GInputStream
 - add openslideload_source(), vipsload_source(), vipssave_target()
 - add hist path to rank for large windows on uchar images
+- better 8/16-bit choice for pngsave
 
 22/12/20 start 8.10.6
 - don't seek on bad file descriptors [kleisauke]
diff --git a/libvips/foreign/foreign.c b/libvips/foreign/foreign.c
index e9a8b31c..326b4507 100644
--- a/libvips/foreign/foreign.c
+++ b/libvips/foreign/foreign.c
@@ -1395,12 +1395,15 @@ vips__foreign_convert_saveable( VipsImage *in, VipsImage **ready,
 		in = out;
 	}
 
-	/* If this is something other than CMYK or RAD, eg. maybe a LAB image,
-	 * we need to transform to RGB.
+	/* If this is something other than CMYK or RAD, and it's not already
+	 * an RGB image, eg. maybe a LAB or scRGB image, we need to transform 
+	 * to RGB.
 	 */
 	if( !coding[VIPS_CODING_RAD] &&
 		in->Bands >= 3 &&
 		in->Type != VIPS_INTERPRETATION_CMYK &&
+		in->Type != VIPS_INTERPRETATION_sRGB &&
+		in->Type != VIPS_INTERPRETATION_RGB16 &&
 		vips_colourspace_issupported( in ) &&
 		(saveable == VIPS_SAVEABLE_RGB ||
 		 saveable == VIPS_SAVEABLE_RGBA ||
@@ -1426,7 +1429,7 @@ vips__foreign_convert_saveable( VipsImage *in, VipsImage **ready,
 		in = out;
 	}
 
-	/* VIPS_SAVEABLE_RGBA_ONLY does not support 1 or 2 bands ... convert 
+	/* VIPS_SAVEABLE_RGBA_ONLY does not support mono types ... convert 
 	 * to sRGB. 
 	 */
 	if( !coding[VIPS_CODING_RAD] &&
diff --git a/libvips/foreign/pngsave.c b/libvips/foreign/pngsave.c
index 7113d972..f188f3f9 100644
--- a/libvips/foreign/pngsave.c
+++ b/libvips/foreign/pngsave.c
@@ -102,10 +102,31 @@ vips_foreign_save_png_build( VipsObject *object )
 	VipsForeignSave *save = (VipsForeignSave *) object;
 	VipsForeignSavePng *png = (VipsForeignSavePng *) object;
 
+	VipsImage *in;
+
 	if( VIPS_OBJECT_CLASS( vips_foreign_save_png_parent_class )->
 		build( object ) )
 		return( -1 );
 
+	in = save->ready;
+	g_object_ref( in );
+
+	/* save->ready will have been converted to uint16 for high-bitdepth
+	 * 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 ||
+		in->Type == VIPS_INTERPRETATION_B_W ) {
+		VipsImage *x;
+
+		if( vips_cast( in, &x, VIPS_FORMAT_UCHAR, NULL ) ) {
+			g_object_unref( in );
+			return( -1 );
+		}
+		g_object_unref( in );
+		in = x;
+	}
+
 	/* Deprecated "colours" arg just sets bitdepth large enough to hold
 	 * that many colours.
 	 */
@@ -113,17 +134,16 @@ vips_foreign_save_png_build( VipsObject *object )
 		png->bitdepth = ceil( log2( png->colours ) );
 
         if( !vips_object_argument_isset( object, "bitdepth" ) ) 
-		png->bitdepth = 
-			save->ready->BandFmt == VIPS_FORMAT_UCHAR ? 8 : 16;
+		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.
 	 */
-        if( save->ready->Bands > 2 &&
+        if( in->Bands > 2 &&
 		png->bitdepth < 8 )
 		png->palette = TRUE;
 
-	if( vips__png_write_target( save->ready, png->target,
+	if( vips__png_write_target( in, png->target,
 		png->compression, png->interlace, png->profile, png->filter,
 		save->strip, png->palette, png->Q, png->dither,
 		png->bitdepth ) )
@@ -145,9 +165,12 @@ vips_foreign_save_png_build( VipsObject *object )
 #define D VIPS_FORMAT_DOUBLE
 #define DX VIPS_FORMAT_DPCOMPLEX
 
+/* Except for 8-bit inputs, we send everything else to 16. We decide on png8
+ * vs. png16 based on Type in_build(), see above.
+ */
 static int bandfmt_png[10] = {
 /* UC  C   US  S   UI  I   F   X   D   DX */
-   UC, UC, US, US, US, US, UC, UC, UC, UC
+   UC, UC, US, US, US, US, US, US, US, US
 };
 
 static void