feat: add backgound to previews

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

lib/Preview/Provider.php

@@ -76,6 +76,19 @@ final class Provider implements PreviewProviderInterface
             }
             $image->setResolution(96, 96);
             $image->readImage('pdf:' . stream_get_meta_data($file)['uri'] . '[0]');
+            // Treat transparent PDF areas as paper in every output format.
+            $image->setImageBackgroundColor('white');
+            $image->setImageAlphaChannel(Imagick::ALPHACHANNEL_REMOVE);
+            // Scale the inset outline so it remains about one pixel after resizing.
+            $stroke = max($image->getImageWidth() / $request->maxWidth, $image->getImageHeight() / $request->maxHeight, 1);
+            $outline = new \ImagickDraw();
+            $outline->setFillColor('transparent');
+            $outline->setStrokeColor('#b8b8b8');
+            $outline->setStrokeWidth($stroke);
+            $outline->rectangle($stroke / 2, $stroke / 2,
+                $image->getImageWidth() - $stroke / 2, $image->getImageHeight() - $stroke / 2);
+            $image->drawImage($outline);
+            $outline->clear();
             $image->setImageFormat('png');
             $png = $image->getImageBlob();
             return $this->images->generate(new PreviewSource(

tests/php/Unit/PreviewTest.php

@@ -68,7 +68,12 @@ final class PreviewTest extends TestCase
         $pdf = new Imagick();
         foreach ([[400, 200, 'red'], [200, 400, 'blue']] as [$width, $height, $color]) {
             $page = new Imagick();
-            $page->newImage($width, $height, $color, 'pdf');
+            $page->newImage($width, $height, 'transparent', 'pdf');
+            $drawing = new ImagickDraw();
+            $drawing->setFillColor($color);
+            $drawing->rectangle($width * 0.15, $height * 0.2, $width * 0.75, $height * 0.8);
+            $page->drawImage($drawing);
+            $drawing->clear();
             $pdf->addImage($page);
             $page->clear();
         }
@@ -89,6 +94,14 @@ final class PreviewTest extends TestCase
         $color = $image->getImagePixelColor(20, 20)->getColor();
         self::assertGreaterThan(200, $color['r']);
         self::assertLessThan(30, $color['b']);
+        $paper = $image->getImagePixelColor(5, 5)->getColor();
+        self::assertGreaterThan(245, $paper['r']);
+        self::assertGreaterThan(245, $paper['g']);
+        self::assertGreaterThan(245, $paper['b']);
+        self::assertSame(1.0, $image->getImagePixelColor(5, 5)->getColorValue(Imagick::COLOR_ALPHA));
+        $edge = $image->getImagePixelColor(0, 25)->getColor();
+        self::assertLessThan(230, $edge['r']);
+        self::assertGreaterThan(130, $edge['r']);
         $image->clear();
     }