  We propose a simple yet effective reflection-free cue for robust reflection
removal from a pair of flash and ambient (no-flash) images. The reflection-free
cue exploits a flash-only image obtained by subtracting the ambient image from
the corresponding flash image in raw data space. The flash-only image is
equivalent to an image taken in a dark environment with only a flash on. We
observe that this flash-only image is visually reflection-free, and thus it can
provide robust cues to infer the reflection in the ambient image. Since the
flash-only image usually has artifacts, we further propose a dedicated model
that not only utilizes the reflection-free cue but also avoids introducing
artifacts, which helps accurately estimate reflection and transmission. Our
experiments on real-world images with various types of reflection demonstrate
the effectiveness of our model with reflection-free flash-only cues: our model
outperforms state-of-the-art reflection removal approaches by more than 5.23dB
in PSNR, 0.04 in SSIM, and 0.068 in LPIPS. Our source code and dataset are
publicly available at {github.com/ChenyangLEI/flash-reflection-removal}.
