How do I read half precision floating point numbers from a Metal texture with Swift?

Question

My Metal compute kernel writes to a texture with format MTLPixelFormat.RG16Float, half precision floating point numbers. This is because Metal does not support writing to 32 bits floating point textures.

I need to read these half precision numbers in my swift program? I have moved the texture into a Swift UInt8 array, but I cannot figure out how to convert the half precision floats into Swift floats.

Answer 1

Actually @Muzza's answer is not correct. You could have read them from a float16_t pointer and cast them to a normal float32_t. No need to use external libraries. Just import the arm_neon header.

Answer 2

EDIT: the below answer was written for Swift v1. Later versions of Swift added support via the float16_t type. Manual conversion may still be useful in some cases.

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There is no built in method to interpret halfs as floats, you have to convert it yourself. The half datatype is a floating point format following IEEE 754. You can read the values in your swift program using UInt16 and then convert from this to a float value. I don't know of any conversion routines written in Swift, but here are two libraries written in C++ that could be converted easily enough:

http://mrob.com/pub/math/s10e5.h.txt

http://half.sourceforge.net/

In the first library it is the function operator float() that you need to convert, that takes the UInt16 member variable _h and outputs a float.