In rare cases, games are released as open-source by the publishers themselves, allowing developers to perform a source port of the code.Some engines come about simply because they were inspired by the original game, and the programmer felt confident enough that no reverse engineering was necessary to make an engine that does the same thing. Alternatively, they can be remade based on a clean room design, in which the project implements the abstract features without having to disassemble the original, going by how components are expected to be used rather than how the game uses them. How the developers go about this process depends on their philosophy they may opt to decompile the original executable and have their own program rely on the original until all of its functions have been remade, at which point the original binary is no longer needed. When the developer only has a binary to work with (and not the source code), they recreate the engine rather than port it, hence the term game engine recreations (or alternatively game engine re-implementations).These kinds of projects are common when abandonware has large communities when the original developer has disbanded and can no longer support or update it, an effort is then made to get it running natively on newer versions of- and/or entirely different- operating systems and platforms. In that case, reimplementing that game alone would save more time than implementing the platform it runs on. This is especially true when a developer only wants to get one game (or a number of games that use the same engine) working. Sometimes a full system emulator is overkill.

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