Metamorphic Testing of Image Captioning Systems via Image-Level Reduction

The Image Captioning (IC) technique is widely used to describe images in natural language. However, even state-of-the-art IC systems can still produce incorrect captions and lead to misunderstandings. Recently, some IC system testing methods have been proposed. However, these methods still rely on pre-annotated information and hence cannot really alleviate the difficulty in identifying the test oracle. Furthermore, their methods artificially manipulate objects, which may generate unreal images as test cases and thus lead to less meaningful testing results. Thirdly, existing methods have various requirements on the eligibility of source test cases, and hence cannot fully utilize the given images to perform testing. To tackle these issues, in this paper, we propose ReIC to perform metamorphic testing for the IC systems with some image-level reduction transformations like image cropping and stretching. Instead of relying on the pre-annotated information, ReIC uses a localization method to align objects in the caption with corresponding objects in the image, and checks whether each object is correctly described or deleted in the caption after transformation. With the image-level reduction transformations, ReIC does not artificially manipulate any objects and hence can avoid generating unreal follow-up images. Additionally, it eliminates the requirement on the eligibility of source test cases during the metamorphic transformation process, as well as decreases the ambiguity and boosts the diversity among the follow-up test cases, which consequently enables testing to be performed on any test image and reveals more distinct valid violations. We employ ReIC to test five popular IC systems. The results demonstrate that ReIC can sufficiently leverage the provided test images to generate follow-up cases of good realism, and effectively detect a great number of distinct violations, without the need for any pre-annotated information.