  A recent technology known as transparent screens is transforming windows into
displays. These smart windows are present in buses, airports and offices. They
can remain transparent, as a normal window, or display relevant information
that overlays their panoramic views. In this paper, we propose transforming
these windows not only into screens but also into wireless transmitters. To
achieve this goal, we build upon the research area of screen-to-camera
communication. In this area, videos are modified in a way that smartphone
cameras can decode data out of them, while this data remains invisible to the
viewers. A person sees a normal video, but the camera sees the video plus
additional information. In this communication method, one of the biggest
disadvantages is the traditional screens' power consumption, more than 80% of
which is used to generate light. To solve this, we employ novel transparent
screens relying on ambient light to display pictures, hence eliminating the
power source. However, this comes at the cost of a lower image quality, since
they use variable and out-of-control environment light, instead of generating a
constant and strong light by LED panels. Our work, dubbed PassiveCam, overcomes
the challenge of creating the first screen-to-camera communication link using
passive displays. This paper presents two main contributions. First, we analyze
and modify existing screens and encoding methods to embed information reliably
in ambient light. Second, we develop an Android App that optimizes the decoding
process, obtaining a real-time performance. Our evaluation, which considers a
musical application, shows a Packet Success Rate (PSR) of close to 90%. In
addition, our real-time application achieves response times of 530 ms and 1071
ms when the camera is static and when it is hand-held, respectively.
