Is there a simple way to compare BufferedImage instances?

Question

I am working on part of a Java application that takes an image as a byte array, reads it into a java.awt.image.BufferedImage instance and passes it to a third-party library for processing.

For a unit test, I want to take an image (from a file on disk) and assert that it is equal to the same image that has been processed by the code.

• My expected BufferedImage is read from a PNG file on disk using ImageIO.read(URL).
• My test code reads the same file into a BufferedImage and writes that to a byte array as PNG to provide to the system under test.

When the system under test writes the byte array to a new BufferedImage I want to assert that the two images are equal in a meaningful way. Using equals() (inherited from Object) doesn’t work (of course). Comparing BufferedImage.toString() values also doesn’t work because the output string includes object reference information.

Does anybody know any shortcuts? I would prefer not to bring in a third-party library for a single unit test in a small part of a large application.

Answer 1

This is the best approach. No need to keep a variable to tell whether the image is still equal. Simply return false immediately when the condition if false. Short-circuit evaluation helps save time looping over pixels after the comparison fails as is the case in trumpetlick's answer.

/**
 * Compares two images pixel by pixel.
 *
 * @param imgA the first image.
 * @param imgB the second image.
 * @return whether the images are both the same or not.
 */
public static boolean compareImages(BufferedImage imgA, BufferedImage imgB) {
  // The images must be the same size.
  if (imgA.getWidth() != imgB.getWidth() || imgA.getHeight() != imgB.getHeight()) {
    return false;
  }

  int width  = imgA.getWidth();
  int height = imgA.getHeight();

  // Loop over every pixel.
  for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
      // Compare the pixels for equality.
      if (imgA.getRGB(x, y) != imgB.getRGB(x, y)) {
        return false;
      }
    }
  }

  return true;
}

Answer 2

If speed is an issue, and both BufferedImages are of the same bit-depth, arrangement, etc. (which seems like it must be true here) you can do this:

DataBuffer dbActual = myBufferedImage.getRaster().getDataBuffer();
DataBuffer dbExpected = bufferImageReadFromAFile.getRaster().getDataBuffer();

figure out which type it is, e.g. a DataBufferInt

DataBufferInt actualDBAsDBInt = (DataBufferInt) dbActual ;
DataBufferInt expectedDBAsDBInt = (DataBufferInt) dbExpected ;

do a few "sanity checks" for equals on the sizes and banks of the DataBuffers, then loop

for (int bank = 0; bank < actualDBAsDBInt.getNumBanks(); bank++) {
   int[] actual = actualDBAsDBInt.getData(bank);
   int[] expected = expectedDBAsDBInt.getData(bank);

   // this line may vary depending on your test framework
   assertTrue(Arrays.equals(actual, expected));
}

This is close to as fast as you can get cause you are grabbing a chunk of the data at a time, not one at a time.

Answer 3

You could write your own routine for comparison!

int width;
int height;
boolean imagesEqual = true;

if( image1.getWidth()  == ( width  = image2.getWidth() ) && 
    image1.getHeight() == ( height = image2.getHeight() ) ){

    for(int x = 0;imagesEqual == true && x < width; x++){
        for(int y = 0;imagesEqual == true && y < height; y++){
            if( image1.getRGB(x, y) != image2.getRGB(x, y) ){
                imagesEqual = false;
            }
        }
    }
}else{
    imagesEqual = false;
}

This would be one way!!!

Answer 4

I changed function that equals by pixels in Groovy, may be helpful:

boolean imagesAreEqual(BufferedImage image1, BufferedImage image2) {
    if (image1.width != image2.width || image1.height != image2.height) {
         return false
    }
    for (int x = 1; x < image2.width; x++) {
        for (int y = 1; y < image2.height; y++) {
             if (image1.getRGB(x, y) != image2.getRGB(x, y)) {
                 return false
             }
        }
    }
    return true
}

Answer 5

I can't think of anything besides a brute force "do loop":

  BufferedImage bi1, bi2, ...
   ...
  Raster r1 = bi1.getData();
  DataBuffer db1 = r1.getDataBuffer();
  if (db1.getSize() != db2.getSize ())
     ...
  for (int i = 0; i < db1.getSize(); i++) {  
    int px = db1.getElem(i);
  }

Answer 6

You can write that image via imageio through an OutputStream to a byte[]. In my code, it looks more or less like this:

byte[] encodeJpegLossless(BufferedImage img){...using imageio...}
...
Assert.assertTrue(Arrays.equals(encodeJpegLossless(img1)
                               ,encodeJpegLossless(img2)
                               )
                 );

Answer 7

working well but not efficient

public static boolean compareImage(File fileA, File fileB) {        
    try {
        // take buffer data from botm image files //
        BufferedImage biA = ImageIO.read(fileA);
        DataBuffer dbA = biA.getData().getDataBuffer();
        int sizeA = dbA.getSize();                      
        BufferedImage biB = ImageIO.read(fileB);
        DataBuffer dbB = biB.getData().getDataBuffer();
        int sizeB = dbB.getSize();
        // compare data-buffer objects //
        if(sizeA == sizeB) {
            for(int i=0; i<sizeA; i++) { 
                if(dbA.getElem(i) != dbB.getElem(i)) {
                    return false;
                }
            }
            return true;
        }
        else {
            return false;
        }
    } 
    catch (Exception e) { 
        e.printStackTrace();
        return  false;
    }
}

Answer 8

If you want to use Mockito, then you could write a Hamcrest Matcher

import org.mockito.ArgumentMatcher;

public class BufferedImageMatcher extends ArgumentMatcher<BufferedImage> {

  private final BufferedImage expected;

  public BufferedImageMatcher(BufferedImage expected) {
    this.expected = expected;
  }

  @Override
  public boolean matches(Object argument) {
    BufferedImage actual = (BufferedImage) argument;

    assertEquals(expected.getWidth(), actual.getWidth());
    assertEquals(expected.getHeight(), actual.getHeight());

    for (int x = 0; x < actual.getWidth(); x++) {
      for (int y = 0; y < actual.getHeight(); y++) {
        assertEquals(expected.getRGB(x, y), actual.getRGB(x, y));
      }
    }

    return true;
  }
}

and use it like this

assertThat(actual, new BufferedImageMatcher(expected));