From 2757bad050d6a50f125b1e553eec784be1257db6 Mon Sep 17 00:00:00 2001
From: Tony Cook <tony@develop=help.com>
Date: Tue, 1 Apr 2008 10:12:17 +0000
Subject: [PATCH]  - mixing qtype scaling now sets all channels of a pixel to
 zero if    the pixel has zero coverage (zero alpha).  This should produce
 more    compressible output files.   
 http://rt.cpan.org/Ticket/Display.html?id=32324

---
 Changes      |  5 +++++
 scale.im     | 23 +++++++++++++++++++++++
 t/t40scale.t |  8 ++++++--
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/Changes b/Changes
index 9b32bd52..0364f99e 100644
--- a/Changes
+++ b/Changes
@@ -62,6 +62,11 @@ Bug fixes:
    parameters.
    http://rt.cpan.org/Ticket/Display.html?id=29937
 
+ - mixing qtype scaling now sets all channels of a pixel to zero if
+   the pixel has zero coverage (zero alpha).  This should produce more
+   compressible output files.
+   http://rt.cpan.org/Ticket/Display.html?id=32324
+
 Imager 0.62 - 10 December 2007
 ===========
 
diff --git a/scale.im b/scale.im
index dbfb2d11..36ed2a56 100644
--- a/scale.im
+++ b/scale.im
@@ -178,6 +178,17 @@ i_scale_mixing(i_img *src, int x_out, int y_out) {
 	      xscale_row[x].channel[ch] = IM_LIMIT(val);
 	    }
 	  }
+	  else {
+	    /* rather than leaving any color data as whatever was
+	       originally in the buffer, set it to black.  This isn't
+	       any more correct, but it gives us more compressible
+	       image data.
+	       RT #32324
+	    */
+	    for (ch = 0; ch < alpha_chan; ++ch) {
+	      xscale_row[x].channel[ch] = 0;
+	    }
+	  }
 	  xscale_row[x].channel[alpha_chan] = IM_LIMIT(accum_row[x].channel[alpha_chan]+0.5);
 	}
       }
@@ -275,6 +286,12 @@ IM_SUFFIX(horizontal_scale)(IM_COLOR *out, int out_width,
 	    out[out_x].channel[ch] = IM_LIMIT(val);
 	  }
 	}
+	else {
+	  for (ch = 0; ch < alpha_chan; ++ch) {
+	    /* See RT #32324 (and mention above) */
+	    out[out_x].channel[ch] = 0;
+	  }
+	}
 	out[out_x].channel[alpha_chan] = IM_LIMIT(IM_ROUND(accum[alpha_chan]));
       }
       else {
@@ -315,6 +332,12 @@ IM_SUFFIX(horizontal_scale)(IM_COLOR *out, int out_width,
 	  out[out_x].channel[ch] = IM_LIMIT(val);
 	}
       }
+      else {
+	for (ch = 0; ch < alpha_chan; ++ch) {
+	  /* See RT #32324 (and mention above) */
+	  out[out_x].channel[ch] = 0;
+	}
+      }
       out[out_x].channel[alpha_chan] = IM_LIMIT(IM_ROUND(accum[alpha_chan]));
     }
     else {
diff --git a/t/t40scale.t b/t/t40scale.t
index 8bd0e8b0..f4de6408 100644
--- a/t/t40scale.t
+++ b/t/t40scale.t
@@ -1,9 +1,9 @@
 #!perl -w
 use strict;
-use Test::More tests => 227;
+use Test::More tests => 228;
 
 BEGIN { use_ok(Imager=>':all') }
-use Imager::Test qw(is_image);
+use Imager::Test qw(is_image is_color4);
 
 #require "t/testtools.pl";
 
@@ -194,6 +194,10 @@ SKIP:
   my $cmp = $out->copy;
   $out->rubthrough(src => $sc);
   is_image($out, $cmp, "check we get the right image after scaling");
+
+  # we now set alpha=0 pixels to zero on scaling
+  is_color4($sc->getpixel('x' => 39, 'y' => 39), 0, 0, 0, 0,
+	    "check we set alpha=0 pixels to zero on scaling");
 }
 
 { # scale_calculate
-- 
2.39.5