fix: GdDriver resizeCanvas save alpha channel; (#266)

* fix: GdDriver resizeCanvas save alpha channel;

* test: GD resizeCanvas for pngs with transparent bg;

* test: fix failing GD resizeCanvas for pngs with transparent bg;

* chore: delete loose use in ResizeTest;

src/Drivers/Gd/GdDriver.php

@@ -394,6 +394,7 @@ public function resizeCanvas(
         // even if background-color is set
         $transparent = imagecolorallocatealpha($canvas->image, 255, 255, 255, 127);
         imagealphablending($canvas->image, false); // do not blend / just overwrite
+        imagesavealpha($canvas->image, true); // save alpha channel
         imagefilledrectangle($canvas->image, $destinationX, $destinationY, $destinationX + $sourceWidth - 1, $destinationY + $sourceHeight - 1, $transparent);
 
         // copy image into new canvas

tests/Manipulations/ResizeTest.php

@@ -14,3 +14,23 @@
 
     assertMatchesImageSnapshot($targetFile);
 })->with('drivers');
+
+it('can resize canvas of transparent PNGs without loosing transparency when GD is used', function (ImageDriver $driver) {
+    if ($driver->driverName() !== 'gd') {
+        return $this->expectNotToPerformAssertions();
+    }
+
+    $targetFile = $this->tempDir->path("{$driver->driverName()}/resize-with-transparent-png.png");
+    $image = $driver->loadFile(getTestFile('transparent.png')); // 1640x923
+    $transparentColor = imagecolorallocatealpha($image->image(), 255, 255, 255, 127);
+
+    expect($image->getHeight())->toEqual(923);
+    expect($image->pickColor(0, 0, \Spatie\Image\Enums\ColorFormat::Int))->toEqual($transparentColor);
+
+    // make it square, so height is added top and bottom, bg transparent
+    $image->resizeCanvas(1640, 1640, \Spatie\Image\Enums\AlignPosition::Center, false, $image->pickColor(0, 0, \Spatie\Image\Enums\ColorFormat::Rgba))->save($targetFile);
+
+    $targetImage = $driver->loadFile($targetFile);
+    expect($targetImage->getHeight())->toEqual(1640);
+    expect($targetImage->pickColor(0, 0, \Spatie\Image\Enums\ColorFormat::Int))->toEqual($transparentColor);
+})->with('drivers');