diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..af70a54
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        ruby: ['3.0', '3.1', '3.2', '3.3']
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install package dependencies
+        run: "[ -e $APT_DEPS ] || sudo apt-get install -y --no-install-recommends $APT_DEPS"
+        env:
+          APT_DEPS: "imagemagick"
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{matrix.ruby}}
+          bundler-cache: true
+
+      - name: Run tests
+        run: bundle exec rspec
diff --git a/lib/mork/mimage.rb b/lib/mork/mimage.rb
index 2f71d1e..dc00e49 100644
--- a/lib/mork/mimage.rb
+++ b/lib/mork/mimage.rb
@@ -51,12 +51,14 @@ def choice_mean_darkness
     end
 
     def marked
-      @marked_choices ||= begin
-        choice_mean_darkness.map do |cho|
-          [].tap do |choices|
-            cho.map.with_index do |drk, c|
-              choices << c if drk < choice_threshold
-            end
+      @marked_choices ||=
+        choice_mean_darkness
+          # We use the unmarked X cells to compute the reference darkness for each row
+          .zip(@grom.calibration_cell_areas.map { |c| reg_pixels.average c }.cycle)
+          .map do |cho, ref|
+        [].tap do |choices|
+          cho.map.with_index do |drk, c|
+            choices << c if drk < [ref * 0.95, choice_threshold].min
           end
         end
       end
diff --git a/lib/mork/version.rb b/lib/mork/version.rb
index 3ad696b..e164042 100644
--- a/lib/mork/version.rb
+++ b/lib/mork/version.rb
@@ -1,3 +1,3 @@
 module Mork
-  VERSION = '0.15.1'
+  VERSION = '0.15.2'
 end
diff --git a/spec/mork/sheet_omr_spec.rb b/spec/mork/sheet_omr_spec.rb
index c2b9aa9..7bc5ae5 100644
--- a/spec/mork/sheet_omr_spec.rb
+++ b/spec/mork/sheet_omr_spec.rb
@@ -13,7 +13,7 @@ module Mork
       describe 'trying to process a corrupted file' do
         it 'throws an IO error' do
           fn = sample_img('corrupted-pdf').image_path
-          expect { SheetOMR.new fn}.to raise_error(IOError, 'Invalid image. File may have been damaged')
+          expect { SheetOMR.new fn }.to raise_error(IOError)
         end
       end
     end
@@ -190,6 +190,7 @@ module Mork
       let(:barr0) { SheetOMR.new 'spec/samples/syst/barr0.jpg', choices: [5]*100, layout: 'spec/samples/syst/layout.yml'}
       let(:barr1) { SheetOMR.new 'spec/samples/syst/barr1.jpg', choices: [5]*100, layout: 'spec/samples/syst/layout.yml'}
       let(:barr2) { SheetOMR.new 'spec/samples/syst/barr2.jpg', choices: [5]*100, layout: 'spec/samples/syst/layout.yml'}
+      let(:default) {SheetOMR.new 'spec/samples/standard.png', choices: [5] * 120, layout: 'spec/samples/syst/layout.yml'}
 
       it 'checks bila' do
         expect(bila0.marked_letters.flatten).to eq(bila)
@@ -229,6 +230,10 @@ module Mork
         expect(barr1.marked_letters.flatten).to eq(barr)
         expect(barr2.marked_letters.flatten).to eq(barr)
       end
+
+      it 'checks default is unchecked' do
+        expect(default.marked_letters).to eq(Array.new(120, []))
+      end
     end
     # context "multi-page pdf" do
     #   before(:all) do