From 10bada0161abf0cc8b33c8284b8173ba3dea3852 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Mon, 21 Sep 2020 09:38:11 +0100
Subject: [PATCH] tiny thumbnail speedup

thumbnail can skip premultiply/unpre if there's no residual resize
---
 libvips/resample/thumbnail.c | 38 +++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/libvips/resample/thumbnail.c b/libvips/resample/thumbnail.c
index b6ffe672..88b18d8a 100644
--- a/libvips/resample/thumbnail.c
+++ b/libvips/resample/thumbnail.c
@@ -736,24 +736,6 @@ vips_thumbnail_build( VipsObject *object )
 		return( -1 ); 
 	in = t[2];
 
-	/* If there's an alpha, we have to premultiply before shrinking. See
-	 * https://github.com/libvips/libvips/issues/291
-	 */
-	have_premultiplied = FALSE;
-	if( vips_image_hasalpha( in ) ) { 
-		g_info( "premultiplying alpha" ); 
-		if( vips_premultiply( in, &t[3], NULL ) ) 
-			return( -1 );
-		have_premultiplied = TRUE;
-
-		/* vips_premultiply() makes a float image. When we
-		 * vips_unpremultiply() below, we need to cast back to the
-		 * pre-premultiply format.
-		 */
-		unpremultiplied_format = in->BandFmt;
-		in = t[3];
-	}
-
 	/* Shrink to preshrunk_page_height, so we work for multi-page images.
 	 */
 	vips_thumbnail_calculate_shrink( thumbnail, 
@@ -771,6 +753,26 @@ vips_thumbnail_build( VipsObject *object )
 		vshrink = (double) in->Ysize / target_image_height;
 	}
 
+	/* If there's an alpha, we have to premultiply before shrinking. See
+	 * https://github.com/libvips/libvips/issues/291
+	 */
+	have_premultiplied = FALSE;
+	if( vips_image_hasalpha( in ) && 
+	 	hshrink != 1.0 &&
+		vshrink != 1.0  ) { 
+		g_info( "premultiplying alpha" ); 
+		if( vips_premultiply( in, &t[3], NULL ) ) 
+			return( -1 );
+		have_premultiplied = TRUE;
+
+		/* vips_premultiply() makes a float image. When we
+		 * vips_unpremultiply() below, we need to cast back to the
+		 * pre-premultiply format.
+		 */
+		unpremultiplied_format = in->BandFmt;
+		in = t[3];
+	}
+
 	if( vips_resize( in, &t[4], 1.0 / hshrink, 
 		"vscale", 1.0 / vshrink, 
 		NULL ) )