Add dist test

core/src/main/java/org/mapfish/print/test/util/ImageSimilarity.java

@@ -217,7 +217,7 @@ private File getRelatedFile(final String name) {
    *
    * @return a number between 0 and 10000 or Double.MAX_VALUE on images format error.
    */
-  private double calcDistance(final BufferedImage actual) {
+  double calcDistance(final BufferedImage actual) {
     // There are several ways to calculate distances between two vectors,
     // we will calculate the sum of the distances between the RGB values of
     // pixels in the same positions.

core/src/test/java/org/mapfish/print/AbstractMapfishSpringTest.java

@@ -57,7 +57,8 @@ public abstract class AbstractMapfishSpringTest {
   public static File getFile(Class<?> testClass, String fileName) {
     final URL resource = testClass.getResource(fileName);
     if (resource == null) {
-      throw new AssertionError("Unable to find test resource: " + fileName);
+      throw new AssertionError(
+          "Unable to find test resource: " + fileName + ", on: " + testClass.getName());
     }
 
     return new File(resource.getFile());

core/src/test/java/org/mapfish/print/test/util/ImageSimilarityTest.java

@@ -0,0 +1,27 @@
+package org.mapfish.print.test.util;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import javax.imageio.ImageIO;
+import org.junit.Test;
+import org.mapfish.print.AbstractMapfishSpringTest;
+
+public class ImageSimilarityTest extends AbstractMapfishSpringTest {
+
+  /** Ensure that we ar not regenerating the expected images in the CI. */
+  @Test
+  public void testNotRegenerateImage() throws Exception {
+    assertFalse(
+        "This flag should not be committed as true", ImageSimilarity.REGENERATE_EXPECTED_IMAGES);
+  }
+
+  /** Test that we get a distance in images with small differences. */
+  @Test
+  public void testSmallDist() throws Exception {
+    assertTrue(
+        new ImageSimilarity(getFile("imageSimilarity/with.png"))
+                .calcDistance(ImageIO.read(getFile("imageSimilarity/without.png")))
+            > 0);
+  }
+}