MacOS – is QuickTime available on OSX 10.9.5

QuickTime Player should be located directly in /Applications/, that's its default location. QuickTime Player X is part of the OS installation, and Apple provides no separate downloads of the app.

Reinstalling the OS is of course a huge step just to get QuickTime back. If you have access to the Mountain Lion installer, you can use an app like Pacifist to extract QuickTime Player from it.

Mountain Lion, as it was a paid upgrade, also cannot simply be downloaded. You may have a copy of the installer on an usb drive or a dvd-r. Originally it was an App Store download, and when bought an app file called "Install OS X Mountain Lion" was placed in /Applications/. After the installation this file, which was more than 4 GB in size, was deleted. You can get it back from the Purchases tab in the App Store to re-download the file. If it shows as "Installed," you should try holding down the option key while clicking on the Purchases tab. You might also be able to locate the installer on a Time Machine backup in the /Applications/ folder.

In short, you (often) need to include the argument -pix_fmt yuv420p when using ffmpeg to generate H.264 content for Apple software/devices, and a bunch of other decoders that don't handle yuv444p.

# ffmpeg -i input.avi -pix_fmt yuv420p output.mp4

This is not mentioned in the output when using the defaults, but can be found in their Encode/H.264 guide.

It turns out that ffmpeg have chosen a default pixel format of yuv444p for libx264 encoding (under some circumstances), which is not compatible with Apple's software/hardware H.264 decoders (and perhaps not very widely compatible in general?). Unfortunately this seems unlikely to change, and there's no warning about it, so just something you have to know if using ffmpeg to make H.264 files.

As far as I can tell, if converting a video that's already using yuv420p it'll maintain that, but if converting from something a bit more exotic (e.g. images or a GIF or whatever) the pixel format must be specified for general Apple compatibility.