It is a fundamental feature of copyright law that it protects only original expression, not ideas. Applied to software, the law of copyright tells us that certain elements of a computer program are not protectable by copyright. For example, purely functional elements such as the structure of a library, or database, or elements dictated by the operating system, can fall outside the scope of copyright protection, since those elements lack the originality necessary for copyright.
In Tetris Holding LLC v. Xio Interactive, Inc., an app developer was found to have infringed copyright in the famous Tetris game. The puzzle game Mino was, by the defendant’s own admission, inspired by Tetris. However, the defendant maintained that it only copied unprotected elements, a conclusion that the developer reached after researching copyright law. The court did not agree. After an exhaustive review of the idea-expression dichotomy (40-pages worth of anaylsis, if you want to read more), the court decided that Mino did infringe the protectable “look and feel” of Tetris. According to this case, you shouldn’t assume that expression is unprotectible merely because it is related to a game rule or game function.
UPDATE: In the Oracle v Google case (see our earlier coverage here), the judge decided that APIs in this case were not eligible for copyright protection. This amounts to a complete loss for Oracle in its suit against Google for infringement of the Java APIs used in Google’s Android software. This case is expected to go up to appeal.