Generate a preview without print margins

When a PDF is used for both printing and displaying. It will most likely
contain a crop box in order to hide print margins when displaying the PDF.

Use Poppler's parameter to automatically use the crop box (visible box)
rather than the media box (printable box) in order to remove those margins
when drawing the PDF.

See https://manpages.debian.org/testing/poppler-utils/pdftoppm.1.en.html

activestorage/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   The Poppler PDF previewer renders a preview image using the original
+    document's crop box rather than its media box, hiding print margins. This
+    matches the behavior of the MuPDF previewer.
+
+    *Vincent Robert*
+
+
 ## Rails 5.2.4.4 (September 09, 2020) ##
 
 *   No changes.

activestorage/lib/active_storage/previewer/poppler_pdf_previewer.rb

@@ -28,8 +28,8 @@ def preview
 
     private
       def draw_first_page_from(file, &block)
-        # use 72 dpi to match thumbnail  dimesions of the PDF
-        draw self.class.pdftoppm_path, "-singlefile", "-r", "72", "-png", file.path, &block
+        # use 72 dpi to match thumbnail dimensions of the PDF
+        draw self.class.pdftoppm_path, "-singlefile", "-cropbox", "-r", "72", "-png", file.path, &block
       end
   end
 end

activestorage/test/models/preview_test.rb

@@ -17,6 +17,19 @@ class ActiveStorage::PreviewTest < ActiveSupport::TestCase
     assert_equal 792, image.height
   end
 
+  test "previewing a cropped PDF" do
+    blob = create_file_blob(filename: "cropped.pdf", content_type: "application/pdf")
+    preview = blob.preview(resize: "640x280").processed
+
+    assert_predicate preview.image, :attached?
+    assert_equal "cropped.png", preview.image.filename.to_s
+    assert_equal "image/png", preview.image.content_type
+
+    image = read_image(preview.image)
+    assert_equal 430, image.width
+    assert_equal 145, image.height
+  end
+
   test "previewing an MP4 video" do
     blob = create_file_blob(filename: "video.mp4", content_type: "video/mp4")
     preview = blob.preview(resize: "640x280").processed

activestorage/test/previewer/mupdf_previewer_test.rb

@@ -6,12 +6,10 @@
 require "active_storage/previewer/mupdf_previewer"
 
 class ActiveStorage::Previewer::MuPDFPreviewerTest < ActiveSupport::TestCase
-  setup do
-    @blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
-  end
-
   test "previewing a PDF document" do
-    ActiveStorage::Previewer::MuPDFPreviewer.new(@blob).preview do |attachable|
+    blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
+
+    ActiveStorage::Previewer::MuPDFPreviewer.new(blob).preview do |attachable|
       assert_equal "image/png", attachable[:content_type]
       assert_equal "report.png", attachable[:filename]
 
@@ -20,4 +18,17 @@ class ActiveStorage::Previewer::MuPDFPreviewerTest < ActiveSupport::TestCase
       assert_equal 792, image.height
     end
   end
+
+  test "previewing a cropped PDF document" do
+    blob = create_file_blob(filename: "cropped.pdf", content_type: "application/pdf")
+
+    ActiveStorage::Previewer::MuPDFPreviewer.new(blob).preview do |attachable|
+      assert_equal "image/png", attachable[:content_type]
+      assert_equal "cropped.png", attachable[:filename]
+
+      image = MiniMagick::Image.read(attachable[:io])
+      assert_equal 430, image.width
+      assert_equal 145, image.height
+    end
+  end
 end

activestorage/test/previewer/poppler_pdf_previewer_test.rb

@@ -6,12 +6,10 @@
 require "active_storage/previewer/poppler_pdf_previewer"
 
 class ActiveStorage::Previewer::PopplerPDFPreviewerTest < ActiveSupport::TestCase
-  setup do
-    @blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
-  end
-
   test "previewing a PDF document" do
-    ActiveStorage::Previewer::PopplerPDFPreviewer.new(@blob).preview do |attachable|
+    blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
+
+    ActiveStorage::Previewer::PopplerPDFPreviewer.new(blob).preview do |attachable|
       assert_equal "image/png", attachable[:content_type]
       assert_equal "report.png", attachable[:filename]
 
@@ -20,4 +18,17 @@ class ActiveStorage::Previewer::PopplerPDFPreviewerTest < ActiveSupport::TestCas
       assert_equal 792, image.height
     end
   end
+
+  test "previewing a cropped PDF document" do
+    blob = create_file_blob(filename: "cropped.pdf", content_type: "application/pdf")
+
+    ActiveStorage::Previewer::PopplerPDFPreviewer.new(blob).preview do |attachable|
+      assert_equal "image/png", attachable[:content_type]
+      assert_equal "cropped.png", attachable[:filename]
+
+      image = MiniMagick::Image.read(attachable[:io])
+      assert_equal 430, image.width
+      assert_equal 145, image.height
+    end
+  end
 end