[tools 2/3] winetest/build-patterns: Use the Luv colorspace for interpolating colors.

[Francois Gouget]

Interpolation works best in a colorspace where the distance between
colors is representative of their perceptual difference. That's
roughly the case in the Luv colorspace and much less so in the RGB
one.

Signed-off-by: Francois Gouget 
---
This introduces a dependency on the Graphics::ColorObject perl module
which is provided by the libgraphics-colorobject-perl Debian package.
Note that on Debian 11 this package is missing a dependency on
libgraphics-colornames-www-perl.
---
 winetest/build-patterns | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/winetest/build-patterns b/winetest/build-patterns
index ff72516f6..72291401e 100755
--- a/winetest/build-patterns
+++ b/winetest/build-patterns
@@ -22,6 +22,7 @@ use open ':utf8';
 use CGI qw(:standard);
 use POSIX; # ceil()

+use Graphics::ColorObject;
 use Text::CSV::Encoded;
 use Time::Piece;

@@ -860,15 +861,15 @@ if (open(my $fh, "-|", $cmd))
 sub color2html($)
 {
     my ($c) = @_;
-    return sprintf "#%02x%02x%02x", $c->[0], $c->[1], $c->[2];
+    return "#". $c->as_RGBhex();
 }

-sub blend($$$)
+sub interpolate($$$)
 {
     my ($p, $start, $end) = @_;
-    my $r = [int($start->[0] * (1 - $p) + $end->[0] * $p + 0.5),
-             int($start->[1] * (1 - $p) + $end->[1] * $p + 0.5),
-             int($start->[2] * (1 - $p) + $end->[2] * $p + 0.5)];
+    my $r = [$start->[0] * (1 - $p) + $end->[0] * $p,
+             $start->[1] * (1 - $p) + $end->[1] * $p,
+             $start->[2] * (1 - $p) + $end->[2] * $p];
     return $r;
 }

@@ -896,7 +897,7 @@ sub compute_set_colors($)
     my $count = @values;
     if ($count == 1)
     {
-        $set->{$values[0]} = color2html($keycolors[0]);
+        $set->{$values[0]} = color2html(Graphics::ColorObject->new_RGB255($keycolors[0]));
     }
     else
     {
@@ -904,6 +905,12 @@ sub compute_set_colors($)
         # when many colors are needed.
         $keycolors[0] = [0, 179, 179] if ($count > 10);

+        # Convert the key colors to the Luv colorspace where the distance
+        # between colors is roughly representative of the perceptual difference
+        # between them. This allows linear interpolation to produce relatively
+        # uniform gradients (which is not the case in the RGB colorspace).
+        map { $_ = Graphics::ColorObject->new_RGB255($_)->as_Luv() } @keycolors;
+
         # Compute the total length of the path traced by the key colors through
         # the colorspace. This is necessary so the gradient remains uniform
         # even if two key colors are close to each other. This does assume that
@@ -929,8 +936,8 @@ sub compute_set_colors($)
             }

# And blend
-            my $Rgb = blend(($pos - $keypos[$k]) / ($keypos[$k+1] - $keypos[$k]), $keycolors[$k], $keycolors[$k+1]);
-            $set->{$values[$_]} = color2html($Rgb);
+            my $Luv = interpolate(($pos - $keypos[$k]) / ($keypos[$k+1] - $keypos[$k]), $keycolors[$k], $keycolors[$k+1]);
+            $set->{$values[$_]} = color2html(Graphics::ColorObject->new_Luv($Luv));
         }
     }
 }