Testing graphics generation with JUnit

Question

I'm using Java's Graphics2D to generate a graphical representation of a graph. I'm also using ImageIO to write a PNG file. (ImageIO.write(image, "png", out);)

I'm wondering how should I write JUnit tests to test whether the generated graphics is what is expected. I could pre-generate the PNG files but what if the font is a bit different on a different machine?

Answer 1

You could try testing for specific, known features of the output e.g.:

• It there a white pixel at (100,100)?
• It the border completely black?
• Is the image the expected size?

And/or you could write tests for some "aggregate properties" that allow for some fuzziness in the results:

• Does the image at least 90% match the reference image? (to allow for different fonts, antialiasing differences etc.)
• Is the most common colour in the image equal to the background colour?

Answer 2

You could read all the RGB values of the generated images into an array and compare that 2D-array against one representing a pre-generated image, if you are really specific about the complete image.

If you wish to ignore the fonts, you could do the same for the same for regions of the image that do not contain any variable data depending on the environment where the images are generated. Building in correction and normalization routines for unit tests would be a waste of time, unless the application is expected to generate images of such high accuracy as warranted.