Some of the most popular engines have already announced support for metal, meaning developers can get the benefits without massive recoding.
Taking a step back from the technical details Metal also has interesting political implications. Apple is, as it often does when it can, taking its destiny into its own hands. Rather than be tied up with The Khronos Group (the body responsible for OpenGL and OpenCL), Apple is free to make changes to Metal as quickly as it would like, in order to continue to best address its hardware.
This is a big strategic move, because it signals that the company believes it has the developer base and enough devices in the field to forge its own path. It'd be hard to argue that it's wrong about that. It also signals that Apple's taking gaming more seriously than ever before. Apple has famously never had a great platform for gaming, but with iOS that has changed—and it appears that Apple is changing too. Metal and the investment Apple's made in its development and support shows that the company's now taking gaming very seriously, indeed.
None of this is to say that OpenGL (or OpenCL) will be going away or won't serve their own purposes. There are still many, many applications for which they're perfectly well suited. But Metal is a lower level, more accurate, model of modern chips like the A7. And once we start seeing apps that use Metal we'll start to see what the A7 is really capable of.
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